In my car I originally had 2.73's I believe. Now my rear is a 3.42 posi 4th gen rear. I never changed my speedo gear, obviously my speedo is off, more the faster I go.

I know my milage is off to cause it thinks i am going faster, therfore further. But I watched it click off the tenths and it seems rite on mile for mile.

About how far off do you think it is per/100 miles

No exact answer needed, Just wanna try and figure out my gas milage some time soon.

 

It should be about 25% high.

 

really that much higher?

Is there anyway to explain this, my school is 15.4 miles away, got gas it was like xxx,466, went to school, came home, went to Girlfreinds house and walmart, basically all on the way home from school, and now my odometer reads xxx,498, which suggests that its accurate, how could that be if the speedo is off?

 

I come up with 25.2% error, assuming it was right before.

Drive along a measured section of road, like freeway with mileage markers. See what your odometer says compared to a no-BS fact measurement. That will tell you how far off your speedo gears are.

Speedo gears are only available in increments, not a smooth continuum; so for example if this is a stick car you might need a 7 tooth drive gear and a 20 tooth driven gear, but that might be 3% high; on the other hand a 21 tooth driven gear might be 4% low. You're never going to get it dead on, even your tire pressure will affect it if you try to calibrate it to that level of precision.

If you find that your odometer is off by a specific amount, say you drive a known 10 miles and your odo shows 11.8, then you're (duh) 18% high. Once you know how far off you are, you can look at your gears, and see if you can get a driven gear with 18% more teeth; for instance, if it is an auto car, and the driven gear in there now has 35 teeth, you'd want one with 35 x 1.18 teeth, or 41.3. They don't make fractional teeth on gears (duh again) so you'd need to look for a 41 or 42 tooth gear, either of which is within 2% of dead-on. Then check the speedo by driving the measured distance with the speedo showing exactly 60 mph; it should go exactly one mile per minute, or 10 miles in 10 minutes. If the speedo is off, you can send it to a speedo shop to get it calibrated; the odometer is the gold standard, once it's right, the gears are right, and any speedo error is in the speedo itself, not the gears.