So how does a regeared rear end (which I know the op has, as do I) affect this? I know mine reads faster than I am actually moving because on the 65mph highway I drive at 70mph on my speedo and cars pass me like I'm standing still lol.

Rob

 

Because the speedometer is reading the transfer case output shaft revolutions, anything that changes after that point affects the "ratio" of it's reading to the actual vehicle speed. It is easier to think of the speedometer as a "drive shaft tachometer" instead of a speedometer. So if you have exactly stock tires and gears then you'll find that the speedometer gives a pretty good speed reading. Make the tires 20% taller than stock and now the speedometer will be reading 20% slower than the actual speed of the vehicle. So while you may see the speedo needle at the same 60 that you did with the smaller tires, you are really going 72. Re-gear the axles with a 20% larger (numerical) gear and now the driveshaft needs to turn 20% more to make the wheels turn the same as before, so now you need to spin the driveshaft 20% faster and the speedometer reads 20% faster and that makes up for the taller tire's error.

 

Wow ok thanks for explaining that to me. I always thought big tires made the speedometer read faster, not the regearing.

Rob

 

I am geared at 5.29 with 35s, my speedo is about 7MPH off when I go 70, are there more gears or is 5.29 the best for 35s? sorry for the newb question, but you have to start somewhere, alos, do I have a low-fuel light?

 

With my '85, I find 4.88s give almost correct speedometer readings with 35s. However, 5.29s are probably a better fit for 35s in terms of performance.

Not sure on the low fuel light, look on the instrument cluster and see if there is one there. My '85 does not have one.