Originally Posted by totalimmortal
I pretty sure we all know that larger wheels change the speedo, at least if you do a search you will. Basically what your saying is that due to the larger wheels, your actual speed will differ from what is shown on the tach and speedo. I still don't see where gear ratio's are coming in, your just explaining the speedo concept again.
edit: Going from stock 15's to 17's with a 215/45 tires will yield a 1.7% change. At 60mph on the speedo, I'll be going 61. And as you go faster, the difference will increase.
Like I said, the gears in your transmission will not be affected. However, a larger diameter tire than stock will have the same affect as longer gears for the same reason it affects the speedometer. At the same engine rpm and in the same gear, your car will be going faster with larger diameter tires. It thinks it's going the same speed because the tire is spilling spinning at the same rpm, but because the tire is larger, 1 rotation of a larger tire = more ground covered = a higher speed than a smaller tire assuming the gear and engine rpm's stay the same.