Hi guys. New to this forum but I'm gonna ask a common question. I've had seven mustangs since I've been driving. And I've upgraded all of them to a certain point with bolt on parts and as many power and torque adders as I could. The most common change I've always made during the first part was gear changes with different results. Currently I own a 2010 GT. Just a JLT CIA Hurst shifter flowmasters and I recently added 4.10's. I'm in the process of getting eventually a programmer. I'm not used to just how much the cars computer is affected by performance changes so here's my question. Without a program to adjust the speedo, how much is the speedo usually off with 4.10's in the 2010 GT? Also what rpm would the car be pulling at 65 mph. I have a set of aftermarket wheels 18 inch . I'm thinking they ar 18 by nine front 18 by 9 1/2 or 10 out back. I bought it used so not sure. I def know they are18inch. Any help is greatly appreciated. Glad to be back in a pony its been a while!