right now I have 235/75/15's and after searching on here I see that I can fit 30's stock, so that being said right now my speedo is off with the 235's on, the speedo will say 62ish when going 60 mph according to my gps. to my understanding if i put on 30's when the speedo say's 60 mph I will be going 60 mph on the gps, right or am I backwards?

 

if your 235's are the stock tire size im not sure on how to explain the difference between the speedo and gps reading, but if you speedo is reading 60mph on 235/75's it should read about 57mph with 30x9.50's. the bigger the tire the slower the speedo will read.

 

The stock speedometers are not calibrated all that well. Showing 62mph while actually going 60mph is not all that bad.

Larger tires will cause an increase in ground speed for a given indicated speed on the speedometer. You are correct in your assessment of the speed change when going from the ~29" tall tire (235/75R15) to the 30" tire. This should match up closer to the speedometer.

 

so was my statement about the speedo reading slower with a larger tire incorrect?

 

thats what i thought but i was unsure cause if you think about it, a bigger tire will "roll" further with one revelation to the smaller tire, so that difference would be you would be going faster than what your speedo reads, so lets say if stock 235's read 60 mph on speedo and gps, put on 30's and go 60 on the speedo, you would be going 62 or 63 on the gps....right?

 

Correct on both accounts. Just depends on what you are using as your comparison point. If you are holding ground speed constant, then the speedometer will read lower when increasing the tire OD. If holding speedometer reading constant, then your ground speed will increase when increasing the tire OD.

Your speedometer will read 3% slower than actual ground speed when going from a 235/75R15 (29" OD) to a 30" OD tire. So at 60MPH ground speed, the speedometer should read 62MPH. Since your speedometer already reads 62MPH...