I have a1991 F150 that originally had 215/75R15 tires that I changed to 235/75R15. The tire size calculator shows a speed variance of 4.09% (too slow) and a diameter variance of 4.26% with this larger tire. Is this going to affect my computer and mpg? I am concerned as most of the threads deal with '92 and newer trucks with electronic speedometers and much larger tire size changes. These trucks all need to have the speedo gears changed too recalibrate their computers for improved mpg. I am not sure if I need to do the same stuff? I have a mechanical speedometer (cable) that I actually changed the needle to match my GPS at 25 mph. It took a few... pull the needle off and on tries, but I got it within a 1/2 mph.

 

According to your figures of a little over 4%, your speedometer would be off about two and one-half MPH at 60 MPH. If you can't live with that, you might try going with a speedometer gear with one less tooth in the transmission, transfer case if you have a 4X4.

 

Your odometer and speedometer will be off a little (they'll say you haven't gone as far as you really have and that you're going slower than you really are). If you can live with that, it's not going to affect anything else.

 

235/75R15 should have been the original tire size anyway.. what size is listed on the decal on your drivers door pillar?