NT01 225/45 or 205/50 best on 15x7s for track

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NT01 225/45 or 205/50 best on 15x7s for track

Postby Aruba » Wed Nov 13, 2013 1:16 pm

I have a '95 track-prepped HPDE/street Miata with a relatively low-mileage and lightly modified '97 engine which on a Mustang dyno shows SAE corrected (air temp and altitude) peak power of 148 rwhp at 7000 rpm and 124 ft-lbs torque at 5000 rpm. The car has a 5-spd trans and a 4.30 Torsen diff vs the stock 4.10 diff, and the suspension uses the original (and somewhat soft) FM height-adjustable V-Maxx springs and shocks. I am running 15x7 35mm offset Enkei RP-F1 wheels and the suspension and alignment settings have been tweaked and optimized over three years with those wheels and 205/50/15 NT01 tires. The car is now fairly quick on the track and I am able to keep up with or actually catch most higher powered cars in the twisty parts of the tracks I frequent, especially in the tighter corners. It's time for a new set of NT01s and I am considering upgrading to 225/45/15 NT01s for better grip and car control, but for budget reasons I must stick with my 15x7 Enkei wheels for the time being rather than upgrading to 15x8 wheels which I am sure would be preferable with the 225s. My question is: what are the chances that using 225 NT01s vs the 205s with the 15x7 Enkei wheels would measurably improve lap times with my Miata on the track at advanced HPDE events?
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Re: NT01 225/45 or 205/50 best on 15x7s for track

Postby Brian » Wed Nov 13, 2013 4:38 pm

Sorry, it will feel like a sloppy mess. My suggestion is stay on 205/50 with those 7 inch rims. Eventually retire your 15x7 to street duty (great daily ride quality with wheels that light), and get wider wheels for your track rubber. Ideally we like 205/50 on 15x8, and 225/45 on 15x9. The difference that comes with having the sidewalls properly supported will amaze you, the car is not just faster because the tire is not rolling over, the car is also faster because it feels MUCH better with the tire not rolling over. The result is much more confidence inspiring at the limit.
Brian Goodwin
Good-Win Racing
www.good-win-racing.com
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Re: NT01 225/45 or 205/50 best on 15x7s for track

Postby Aruba » Thu Nov 14, 2013 10:10 am

Thanks for the quick and frank response to my tire question, Brian. I suspected that mounting the half-inch wider NT01s on my 7-inch Enkei wheels was probably not the best idea but it's always good to get unambiguous confirmation from the more experienced Miata track folks.
1995 Miata 97 Miata engine with head shave, RoadsterSport-3 CAT-back, custom CAI = Mustang Dyno SAE numbers-148 whp at 7000 rpm, 124 ft-lbs torque at 5200 rpm.
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