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"When are tires TOO wide for track or autox use?"

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  • "When are tires TOO wide for track or autox use?"

    This was a reply I made to the question in the subject line above on another forum


    We have NEVER seen a situation where you can make the "the tires are too wide" on a road course car. This is something we have been preaching - and testing - on track/autox situations for over 30 years. I've tested this theory on many different cars, but I will share the S197 an S550 tests we've done since they are more relevant.

    You can ignore our 3 decades of testing and proving that "wider is better" at your own peril. We have had online "arguments" with folks about since the 1990s, and we have heard the tired old wive's tales of "too much rolling resistance" and "can't get enough heat into the tires" before. This line of thinking is broken, and just not factual.

    If giant tires can make even a 4 cylinder Miata faster in a 45 second autocross, they damn sure can get enough heat in them in a 15-20 minute track session. People that think otherwise are either A) very, very slow or B) very misinformed and C) likely don't pay attention to lap times. The results sheets and the timers don't lie...

    2011 Mustang GT S197 Test

    On cars as big/heavy/powerful as S197 or S550 Mustangs, there is literally not a "too wide" size available! We have tested over 5 years of autocross and time trial competition on our S197 Mustang GT, from 2010 - 2015 seasons. We started with 255mm stock tires and moved up to 18x9" wheels with 265mm 140 treadwear tires - which was the widest limit for the class we ran for 1.5 years (SCCA STX).

    It was painful to drive with tires this narrow - we were constantly fighting rear tire spin even with stock power levels (380 whp), but it was laughable with 430 whp on the 265s. Autocross and track testing with multiple National level drivers and it was a hot mess. We tested with FOUR brands of 265/40/18 tires, all of the "hot" tire models, raced it dozens of times. Futile!

    Then we switched classes briefly (STU) when we went up to 275mm street tires, the car got faster and easier to drive. By a lot. Could we still overpower the rears? Sure, but in the same SCCA region our "PAX" finishes went from "top 75" (out of 150 cars) to "top 10" on 275s. Same 140-200 treadwear, just a bump up to 18x10" wheels and 275/40/18 Re-11s.

    Then we tried 295s, and the car kept dropping time and won more races than before. We started winning at Goodguys and even some portions of Optima series qualifier events on this size 295/35/18 street tire.

    We switched to 315mm 200 treadwear tires and the car WOKE UP. This was the max width we could fit under stock fenders and the car won an entire event at an Optima series qualifier, with only 430 whp. Some of the cars we raced against had 1000 hp...

    Meanwhile the car was going from "sucking" in NASA TTB class on 265 tires to KILLING IT on 315mm Hoosier A6 rubber in TT3 class. The car was unstoppable on the 315 Hoosier A6 DOT tires, and lap times dropped significantly from the 295mm street tires at the same track (from 2:03 to 1:50). TIRES MATTER. A LOT.

    The next season we cut fenders, added flares, and switched to 335F/R sizes for 200 treadwear competition, and 335F/345R Hoosier A7s for NASA TT3 class. Our previous lap records fell by 2-3 seconds from the 315mm Hoosiers.

    At this point the 345/35/18 Hoosier was and still is the BIGGEST TIRE THIS COMPANY MAKES, and few other tires are even made that wide. We maxed out the width available and ran the fastest times in the last 2 years we owned that car. If Hoosier had made a 355mm A7 we would have bought it.

    2018 Mustang GT S550 Test

    On our 2018 GT we went from the skinny 235mm tires on 18x8" wheels to 305/30R19 RE-71Rs on 19x11" wheels and dropped TEN SECONDS at the same test track (MSR-C) with the same driver, in just one week's time. We proved the "wider is better" game on the S197, so why screw around on this car? We made one big jump right off the bat.

    We will test 315mm A7 tires on this car next and watch the lap times drop drop drop. When the wife allows me to cut fenders I will go straight to 335F/345R sizes - because that's the biggest we can get.

    Of course with increasing levels of grip, the spring rates and dampers needed to control them need to go up. I've got examples of where we ignored this and it bit us in the ass, too. Real R-compounds in real widths need REAL shocks - not the rubbish that Ford installs, but real monotube adjustable dampers. The tires aren't EVERYTHING, but they are MOST of the game. We do all of our suspension tricks just to keep the tires happy and working efficiently. So yes, as width/grip/compound increase, so must suspension spring/damper/bar/bushing upgrades.

    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