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Mustang S197 and S550 Wheels + Why "Big Tires Matter"

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  • Mustang S197 and S550 Wheels + Why "Big Tires Matter"

    Most people know that Vorshlag is all about "maximizing wheel and tire" widths for a given car. We try to use the most tire width under a given fender "and beyond" for the S197 and S550 Mustang chassis - and many more. This discussion below is one we have often with customers that have S197 (2005-14) or S550 (2015-up) Mustangs and we will point customers here to read up on our theories, experiences, and data derived from testing wide wheels and tires under these cars. Hopefully it helps some people learn from our mistakes and progress over the years, and shortcut that line from stock to wide wheels on their track or autocross car.


    To explain why we push "Big Tires" on modern Mustangs we have to go back to the beginning of the Coyote S197 Mustang era...

    Vorshlag purchased a 2011 Mustang GT as soon as it debuted with the Coyote V8 in 2010 (2011 model year). We campaigned this car in Autocross and Time Trial events until 2015 and have worked with many S197 racers ever since. The S197 Mustang GT came from the factory with tiny 18x8" or 19x9" wheels, and we started racing on these 18x9" Enkei wheels above with 265mm wide tires - because of autocross and Time Trial class tire width limit rules.

    We also started rounding up and/or buying wheels and test fitting them to our 2011 GT in the summer of 2010, only to find that NOBODY made a 10" or wider wheel that fit this chassis at both ends. Most wheels that were sold for this car had too little offset and "poked" past the fenders (see above left), or had too little barrel diameter or spoke clearance and ran into the 14" Brembo calipers. Even many 18" diameter wheels back then didn't clear this brake package (now there are 18" wheels that clear factory S550 15" diameter rotor and 6 piston Brembo brakes).

    We found a few existing 18x10" wheels that would fit the rear but not the front, or vice-versa. This Enkei 18x10.5" wheel was a popular wheel choice among S197 autocrossers, and it fit up front - but poked past the fenders by a huge amount on the rear. We actually had people try to tell us "tHiS wHeEl FiTs GrEaT!" but that is just not factual. You had to run a very tall ride height in the rear to avoid CUTTING THE TIRE INTO THE FENDERS. Or worse - had to cut and flare the car to fit these somewhat small 10.5" wide wheels...

    Meanwhile we were racing in classes that were needlessly limiting these big, heavy, powerful cars to a ridiculously narrow tire, so we spent a lot of time sideways (above left). At a dedicated tire test in April 2011 (above right) we realized that in a low speed autocross test this Mustang was faster on all 3 sets of 265mm tires we tested with 4 different Nationally competitive drivers in 3rd gear vs 2nd gear. The engine made too much power (430 whp) for 2nd gear to be useful - it all turned to wheelspin - so we effectively reduced torque by a considerable amount in the higher gear, and went a full second faster on a 30 second course. That was when we knew it was a lost cause... we needed more tire width!


    After that tire test we set out to make a wider wheel for this chassis, and moved to better autocross and TT classes that allowed a wider tire package.

    We came up with a spec for an 18x10" wheel built for us D-Force Wheels, using the same flow formed wheel style we had been selling to BMW folks since 2007 except we made it stronger for the heavier Mustang chassis - it gained 1 pound in weight. Every wheel maker that supported this chassis quickly copied this 18x10 size and our ET43 offset, but we still sold hundreds of these wheels over the next couple of years. We learned another valuable lesson - don't publish the wheel offset specs, to make the copy cat wheel makers to actually have to do their own homework to figure out how to fit wider wheels onto various cars.

    This 18x10" wheel was ideal for a 275-295mm wide tire, and we ran many sets of tires in these widths and various compounds. We had the best success with a 295/35/18 tire - which happened to be the biggest tire that effectively fit this wheel size. The car went from a total nightmare to drive at low speed events (autocross) or on track (Time Trial / HPDE) on the 265mm tires to much more manageable on the 275-295mm sizes. Our car was instantly faster and our class results got better.

    Over time the available tire options in 18" diameters that fit this car (it needs a 26-28" tall tire) shrank drastically, and you cannot sell a wheel size if there are few or no tires to choose from. So we moved street tire customers to 19" wheels on these cars. These days it is very rare for us to sell 10" wide wheels for either the S197 or S550 Mustangs anymore, after the popular 285-295mm wide tire sizes went away - and the car is so much faster on wider tires. We start at 11...

    There is always a point of diminishing returns on tire width for a given wheel width. And remember: not all tires measure the same, even with the same size printed on the sidewall. Hoosiers tend to "run big" and this 295/40/18 was a bit too wide for the 18x10" wheel we made. That tire needed an 18x11 wheel, and it was also a bit tall - which affected both final gearing and ride height.

    As you can see above, the 315/35/18 race tire was an even more massive squeeze onto the same 18x10" wheel. The only people that squeeze fat tires onto narrow wheels are doing so due to: poorly written class rules, shear ignorance, or poverty budgets that limit them to a wheel they already have. This is a BAD IDEA. As much as we preach "Big Tires Matter" mantra, you have to couple wheel width with tire width. More on that below.


    In late 2011 we found out about a wheel company called Forgestar - the first "flow formed" wheel company that made bespoke wheels to order. We gathered a lot of measurements, sent them an order, and in March 2011 we got our first set of Big Wheels ever to go onto an S197.

    We made this 18x11" front and 18x12" rear wheel in their F14 wheel style and mounted that 315/35/18 Kumho race tire under the stock fenders. Nothing like this had been done before, not that we ever saw, and images of this wheel and tire package "broke the internet". Lots of haters argued this "wasn't possible" but the pictures don't lie...

    Neither did the results. We burst into the NASA Time Trial TTS class then TT3 scene with a lot of wins and track records over the 2012-15 seasons. We also improved our autocross class results dramatically in SCCA ESP class, until we felt that the SCCA class restrictions were holding back our NASA Time Trial setup.

    I want to warn folks here that running the 18x12" rear wheel is trick on these cars and requires cutting the upper bump stop mount off the car to allow the wheel to droop out fully when on a lift.

    For 99% of our S197 customers we instead sold them 18x11" wheels for both ends. Now the offsets are over an inch different so the wheels do not swap front to rear. This way you don't have to buy some massive 30mm thick spacer + longer wheel studs, or a bolt on spacer. We instead help our customers maximize tire wear by getting proper suspension components and alignment settings rather than let them fret about rotating tires front to back.

    We don't mention that to be flippant - we mean it, and we have proven data points on dozens of sets of tires that show you can get good, even wear with proper dampers + spring rates, plus proven camber and toe settings. Over the life of one set of track tires we tend to flip them inside/out once to get the best outer shoulder wear. This might be a once a year task for racers - not a huge expense. Flipping wheels front to back will not add additional life to a tire like this inside/out trick.

    Since 2012 we have sold a LOT of flow formed Forgestar wheels, and now their new parent company MOMO wheels as well. D-Force Wheels went away a few years back, so these are our mainstay wheel company. There just isn't another company making flow formed wheels (light + strong) in bespoke wheel width/color/offsets, anywhere near this price point. It is easy to literally spend 200-500% more for the same sized wheels from other custom wheel makers.

    Often these higher end wheels are no lighter or stronger - they just cost a lot more money. We tell our customers that "the clocks don't care" how much you spend on wheels. If they are straight and true, are built to the offsets we order, clear the brakes and fit your car just go with it. If it makes you feel better spending more money, you can always buy 3 sets for the price of one set of fancier wheels.


    Between 2010 through 2015 we progressed from 265mm to 275mm to 285mm to 305mm to 315mm to 335mm to 345mm tires. The first year we ran with our first "dedicated" Forgestar wheel set for use with 315mm tires was the 18x11" front and 18x12" rear. What wheel width you should run for a given tire size is a big subject and we can get lost in the weeds really quickly. I will summarize this in a couple of points then show some examples.
    • Running a wider wheel for a given tire size puts more "tire on the ground", up to a point. It is a smaller gain than you might think.
      • This is a trick that can maximize tire width restricted classes, but most rules makers figured out the "wide wheel" trick
      • There are usually space constraints on the chassis and/or bodywork that limit wheel width, so its not a "free" mod.
      • Sometimes people get overly focused on maximizing wheel width and ignore the tires. These two need to go hand-in-hand.
    • The tires are the only thing connecting the car to the track.
      • Nothing else is more important to track performance than maximizing tire on the ground. NOTHING.
      • Everything we do on the suspension is done to "make the tires happy", to keep them on the ground and maximizing the usable contact patches.

    Early in 2010 when we were testing wheels and tires for the S197 we experimented with tires mounted to differing wheel widths. The old "go to" wheel size for a 285/30/18 Hoosier used to be an 18x10" wheel, but when we mounted it to an 18x10.5" wheel it fit even better.

    Some autocrossers and track drivers will run this 285/30/18 tire on an 18x11" wheel - which we did on the LS V8 powered BMW E30 above - just don't lose site of the fact that running more tire width helps more than just maximizing wheel to tire ratios. The diminishing returns start to kick in for this tire after 10.5"... the car above got a free set of 285 Hoosiers, but it would have been faster on a 315/30/18 tire on these 18x11 wheels. If speed matters to you, bias towards more tire width over the "perfect" wheel width.

    After seeing better wear on the 18x12" rear wheel, we felt it was time to I got over my hesitance to "cut the car", so we bought another set of front fenders. In May of 2013 we moved the same 315/30/18 Hoosier tires onto 18x12" wheels front and rear, as shown above. We ran like this for 9 events over the next 6 months and saw a marginal lap time drop of roughly 1/2 second, and better front tire wear. Again - just a little more tire on the ground helps.

    This was our S197 Mustang on 18x12" wheels front and rear with a 315/30/18 Hoosier at NASA Nationals in 2013 - we had added a custom fender flare at the front to clear the wider wheel setup (all of the added inch of width was outboard), and on an S550 chassis you would have to cut and flare BOTH ends to fit this wheel package. As good as this 315mm/18x12" package looks - if you have to cut the car to do it, just stick with the 18x11" wheel and 315mm tire instead. The gains are too small for the costs of flares / fab work to cut fenders.


    In early 2012 I was tempted by the gains we made going from 265 all the way up to 315mm tires, and ordered a pair of these 345/35/18 Hoosiers... we call it the #GodTire.

    This tire measures an astounding 13.9" wide for section width and 13.2" for tread width. That is a big mother! The biggest DOT tire Hoosier makes. It was designed for use behind Rear Wheel Drive V8 and V10 powered cars for the rear drive tires. It isn't typically used as a "steering tire" for the front. 26.8" tall, also comes in 19" diameter.

    We ran this tire on our car with 315mm fronts using the existing 18x12" rear wheels - briefly, at 3 autocross events. It needed a spacer to clear the inner fenders, but by the time we got it to stop smoking from massive tire rub inboard it was rubbing on the fender outboard. The 345mm Hoosier simply will not fit under S197 Mustang fenders. These tires were faster - the car had AMAZING forward bite, right up until you had to turn. The sidewalls rubbed so badly it would make the car hop and bounce and smoke like mad. Corner workers kept calling in that the car was on fire. DON'T TRY THIS without flares...

    After seeing the gains by going to a 12" wide front wheel on the same 315mm rears and the brief testing we did with the 345s out back, we decided to cut and flare the rear fenders. After a few weeks of fabrication and bodywork we had a flared rear fender, to match our over-flare on the front.

    We immediately moved to a wider 335/30/18 front tire and the previously tested 345/35/18 rear Hoosiers. Now ideally we would have moved to an 18x13" wheel all the way around, but at the time Forgestar didn't make anything wider.

    Starting in 2015 Forgestar added this 18x13" size for their F14 wheel style. And yes, this "would have been better" - but... with this wider tire package we still dropped 1.5 to 2 seconds per lap in 2014-15. Even on a "less than ideal" wheel size.

    Admittedly this was a less than ideal wheel size for these 335/345 Hoosiers - but - we set more track records and had bigger win margins in NASA TT3 class on these tires vs the 18x11 / 315mm or 18x12 / 315mm Hoosiers. Lap times were better, car was easier to drive, tires lasted longer. We could get on the throttle SO MUCH SOONER exiting a corner than the 315mm tires. #ResultsMatter

    We aren't saying you should try to use the massive 345 Hoosier on a 12" wide wheel, especially now that an affordable 18x13" wheel exists from Forgestar, but we did see big gains going up from a 315mm tire on the same 18x12. That's the 345 Hoosier mounted to our 18x12" wheel setup, above left. It isn't a a hideous squeeze, but the picture above right (courtesy of Chet Wayne) shows the 345mm Hoosier on a proper 18x13" wheel. This pic has two of each wheels/tires stacked up and the wider wheel IS giving a more tire on the ground. If you are going to cut and flare anyway, might as well get the wider wheel.

    continued below
    Last edited by Fair!; 11-21-2019, 12:42 PM.
    Terry Fair -
    2018 GT / S550 Dev + 2013 FR-S / 86 Dev + 2011 GT / S197 Dev + C4 Corvette Dev
    EVO X Dev + 2007 Z06 / C6 Dev + BMW E46 Dev + C5 Corvette Dev

  • #2
    continued from above

    The big flares allowed us to also run 18x12" wheels and 335/30/18 BFG Rival tires for "200 treadwear" events, like the Optima Ultimate street Car events. We had more success running the 315mm tires in this series than we did on 295mm tires a year earlier. "Bigger tires make bigger grip"

    This is another shot of our 2011 Mustang on the 18x11" front / 18x12" rear wheel and 315mm Hoosier package. Again, that extra little bit of rear tire on the ground ensured we could maximize rear grip with the power this car made - It was worth as much as 1/2 second per lap time drop in testing. We set about ~10 class track records in TT competition on this setup from 2012-2013. But going to a wider tire on the same wheel made a bigger gain - at the cost of fender fabrication and flares. Bigger tires + wider wheels would be even better. Choose your battles and don't forget - TIRES MATTER MOST for lap times..


    In the Fall of 2014 - right after these cars came out - we measured for wheel fitments on the new 2015 Mustang S550 chassis. We learned a lot after testing a number of wheels we had on hand, as well as this special Forgestar CF5 18x10" wheel shown below. This had the new "high brake clearance" barrel made specifically to clear the 15" diameter rotor and 6 piston Brembo caliper that came on the optional Performance Pack S550 brakes.

    This barrel has since been migrated to all of the 18" Forgestar wheel designs, like the F14 shown in gold above.

    These 18x10" wheels could be rotated front to back, but they needed a different offset than the same size for the S197 chassis. We sold a handful of these 18x10" wheels but by then the "good tire choices" in 18" diameter that would work on the S197/S550 Mustangs started to disappear. Customers wanted to see 19x10" wheels instead, but more often than not they wanted 19x11" wheels.

    We helped specify the 18x11" wheels used on Aaron's 2015 Mustang GT, which he campaigned in Optima series for a few years. We also used his car to develop the S550 camber-caster plate and the first MCS coilover dampers for this chassis. We made a 19x11" wheel using these same specs and that was a popular seller.

    In February of 2018 we purchased our own S550, this 2018 Mustang GT shown above. This was a "base" model and came with 18x8" wheels and 235mm all season tires. After performing our first track test on this car in factory stock condition we immediately began testing wheels again.

    We quickly moved to our existing 19x11" S550 wheel specs (that don't rotate front to back) with an order of Forgestar F14s. We used these with 305mm tires in TT and autocross competition immediately.

    The 305/30/19 tire size had grown in popularity over the previous few years and there were dozens of choices in this size to pick from. The costs for both the wheels and tires are higher than their 18" counterparts - as are the weights - but the tire choices were so much better that 90% of our S550 wheel sales moved to this size.

    Much of this push to 19" diameter tires comes down to the tire height - the S550 and S197 both "fill out the fenders" better with the 26.2" tall 305/30/19 vs the (now much harder to source) 315/30/18 tires, which are 25.5" tall. The wider 18" tire options dwindled down to only 2 or 3 choices, too.

    The 305mm vs 315mm options above are pretty close in tread width - with 0.3" separating them. We feel this this picture above shows that difference well.

    When looking at "200 treadwear" classing, these are probably your best two options - we have listed the tire data from the manufacturers, as well as show these two with our own measuring stick. The height is the biggest difference, but the 18" wheels and tires will generally be less costly and weigh less. If you want to get the most tire, use one of the 200 treadwear tires, and "stay within the fender confines" the 18" option is probably more competitive - but LOOK AT THE TIRE options first. These are limited to two choices, whereas the 305/30/19 has a lot selection. A lot more.

    This 19x11" wheel and 305mm tire fitments is as wide as you are going to fit under the fenders of the S550 chassis. There simply is no way to use a 12" wide wheel with either the 305/30/19 or 315/30/18 tires and keep the stock fender contours. We tried. The 1" wider 335mm tires are just not possible without cutting/flares, either.

    We ran our 2018 GT on Hoosiers as well - and quickly ordered a set of these MOMO Heritage 6 wheels in 18x11" to fit the 315/30/18 Hoosier shown above. These are a good bit wider than the 200 treadwear "315" tires, so be prepared to run a little spacer out back and you might need to adjust the parking brake cable brackets.

    We actually had a short period where we ran the 315/30/19 on a 19x11" wheel set, too. The move to Hoosiers was worth 1-2 seconds per lap over the RE-71R - which we tested extensively, back to back, on many race weekends.

    We tried to fit an 18x12" wheel on the front or back of this S550 chassis, but came up short. Not possible without massive tire poke, cutting, or flares... 11" is the maximum and the 315mm Hoosier is pushing the limits of what "fits" - as you can see above..

    All of the 11" wide front wheel/tire combos will rub the inner frame rail at full steering lock (see above left) but the 315mm Hoosier was a pretty snug fit. We experimented with some small spacers on this 19x11" wheel set to keep the tire from deforming under cornering load and touching the strut. You will have to run about -3° camber to fit these big Hoosiers, and the tires will want it. The "strut to spindle" interface has a bit of slop in it - this will need to be adjusted to keep the most inboard wheel room for the tire to clear. This is a MAX FITMENT tire on the S550 - until you are ready to cut fenders.

    Out back the big 315mm Hoosiers were rubbing on the parking brake cable, as shown. We had to adjust the rear parking brake cable brackets for more clearance. A couple of hammer whacks against a block of wood will push the bracket inboard and move the cable out of the way.

    This is the 315/30/19 Hoosier R7 next to a 305/30/19 Bridgestone (above left) then mounted to a MOMO 18x11" wheel. wouldn't say this tire is "squeezed" or "pinched" in any way, but yes - moving this tire to a 12" wide wheel would help... a small amount. Not enough to cut the fenders or add flares, in my opinion. If you are going to cut and flare, it is worth moving to a wider tire at the same time.

    After two seasons competing in our own car, have accumulated a lot of track time on S550s with 305mm to 315mm tires - and now we won't recommend anything narrower for these cars. They are just too heavy and too powerful of a car to be used for serious track use on anything smaller. Maybe if your class rules require it? We occasionally get the 19x10" wheel and 285mm tire customer, but its rare.


    The latest batch of "200 treadwear" tires are brutally fast, including the Bridgestone RE-71R, BFG Rival S 1.5, Yokohama A052, Kumho V720 ACR, and even the Hankook RS-4. The wear rate has shot up to match their track speed, and they can all overheat pretty easily. We treat these tires all like proper race tires, and do not recommend them for daily driver use.

    In 2019 we did a number of NASA TT weekends where we ran the RE-71R street tire on day 1 and the Hoosier R7 on day 2. The Hoosier R7 was always faster (rain or shine), had slightly better heat resistance, and slightly worse wear rates. Even in the wet the "slick" tread of the Hoosier was faster, up until there was heavy rainfall. After many weekends we saw at least 1 second improvement on the Hoosier R7 tire over the RE-71R, with an occasional weekend as much as 2 seconds. Not a huge difference, but big enough to matter

    We got to a point where we were racing in classes against cars that weighed 300-800 pounds less, had more power, and wider tires fitted. It was an uphill battle in a car we didn't want to "cut up", and we knew the car would improve with a wider wheel and tire setup. Instead of cutting up this new 2018 Mustang, we bought another S550 chassis...

    In September of this year we retired our 2018 GT and started work on this 2015 Mustang chassis. It will debut with the 18x11" wheel / 315mm tire from our other car but we will quickly move to the 18x12" / 335mm Hoosier front and 18x13" / 345mm Hoosier rear tire, using cut fenders and some sort of flares. We will show the development of this car in our S550 build thread, located elsewhere on this forum.

    This car will receive a large displacement LS V8, which we have started to swap in. Anything north of 600 whp really benefits a LOT from the 18x13" wheel and 345mm Hoosier rear tire. We will update this page with the wider S550 wheel and tire package and possibly the flare kit we will use at a later date.

    We hope this has been a fun, informative read about tires and wheels for the S197 and S550 Mustang. Remember: you can never have too much tire width for these heavy, powerful Mustangs. We recommend getting as much tire width as the fenders allow - and if you can afford to cut and flare and add more, it will only go faster.

    Last edited by Fair!; 11-21-2019, 12:43 PM.
    Terry Fair -
    2018 GT / S550 Dev + 2013 FR-S / 86 Dev + 2011 GT / S197 Dev + C4 Corvette Dev
    EVO X Dev + 2007 Z06 / C6 Dev + BMW E46 Dev + C5 Corvette Dev