Cary North Carolina Autocross - Tarheel Sports Car Club

J, I hope you had fun! I haven't run an a-x since '09. Have you run where you can "let it out", like an airfield? Faster is funner, imo.

Yep, they hold 1 or 2 events every year at the Danville airport in VA. Those are usually more spacious faster courses. The event this weekend was in a mall parking lot, however, so it was very short and tight. Had a great time. Didn't turn competitive times but I'm still pretty new to autox and running 560tw all-seasons so I didn't expect to be. I did, however, learn a ton. Each time I do one of these event my car control skills get significantly better

Your setup is similar to mine...but you really should be running the 15x9 Avanti Storms.  Your 16" tires are a little cheaper, but you're giving up a lot of grip with only 215 mm width.  I haven't calculated the tire diameter torque loss for a 16" tire, but you lose 9% torque going from a 15" to a 17...I'm guessing you're right in the middle of that.  The only benefit is a slightly faster top end for your gearing.

You are really not loosing torque when you change tire sizes, you are modifying the power curve and area under the curve which also occurs when you change gearing.

Power (W) = Torque (N-m) x Angular Velocity (rad/s) or HP = Torque (lb-ft) x RPM/5252

So tweaking the power curve with tire sizes or gearing may increase the area under the curve and improve overall performance within a given RPM range.

Just some things to think about.

Posted by: neomr2

You are really not loosing torque when you change tire sizes, you are modifying the power curve and area under the curve which also occurs when you change gearing.

Power (W) = Torque (N-m) x Angular Velocity (rad/s) or HP = Torque (lb-ft) x RPM/5252

I can't confirm the math here but I can tell you that it feels like my acceleration curve has changed. Feels like less acceleration at lower RPMs but once you get into the higher rev ranges it seems to wind up faster. May just be placebo but it certainly feels that way. That being said, I'm willing to bet I'd turn better times on the 15's

The engine makes a certain amount of torque at the shaft and the cars weight, transmission gearing, and tire/wheel diameter, inertia, and rolling resistance all affect overall performance.  

So changing tire size and mass will definitely change the power/torque curves but torque is not really lost, it is primarily displaced from its original curves just like gearing will do.