I was thinking about the fact that everyone gets caught up in the plus 1 and plus 2 game and neglects what I consider the number one all-around performance enhancer out there. Lower profile tires on the SAME diameter rim.
Back in 1982 I had a Mustang GT that I drove on the autobahn for 2 1/2 years. It came stock with lowly 185/70 tires and weighed about 2900 lbs if I recall. When those tires wore out, I replaced them with 185/60 tires and noticed an instant acceleration increase and better handling.
I've done the opposite and put 195/60 tires to replace 185/60 on my old TR-7 and 245/60 tires to replace 225/60 on my 85 Mustang GT and noticed slower acceleration in both.
I also used 225/50 (vs. 225/60 stock)BFG Comp TA R tires, one of the original autocross tire compounds, and noticed much better acceleration.
The bottom line is that you know the lower tire diameter of low profile tires on the same size rim will lower your gearing giving you faster acceleration. At the same time it lowers your car and gives you a wider contact patch and less rollover from a shorter sidewall for much better handling.
Of course it induces speedometer error, reduces gas mileage and top speed. But I constantly see folks making all kinds of minor mods, often EXPENSIVE, to get maybe 1-5 horsepower, when much greater acceleration gains are easily attainable simply by changing your tires to a lower profile.
I am contemplating buying 235/45 tires almost immediately to replace stock 215/55 tires if they fit the wheel well. Tire Rack says I need a half inch wider rim than stock to do that, but they said the same about my current 215/40 tires on the SVT Contour. These new tires will be one inch lower in diameter inducing 3.5% odometer/speedometer error. So at at an actual 96,500 miles it will read 100,000 miles. Whoopeedoo. That is really gonna make a huge difference in resale value.
If you can multiply you can solve the speedo problem. My current Contour tires are a whopping 7% too small in diameter. So I simply multiply in my head my speed by .7 and subtract that from my indicated speed to know my true speed. It ain't rocket science and it yields HUGE bang for the buck in performance. I'm convinced it was a major factor for my autocross time at SZ.
