speedometer off by 5 miles at 60mph
 

according to a gps app that shows mph, i'm doing 60,while the speedometer shows 65.if i got ,say 300 miles to a tank,what is my actual mileage?

...which do you "believe" is more correct, GPS or car speedometer? Both have intrinsic errors, the question though, is which is LEAST wrong?

...what vehicle? GM products are "known" for speedometers displaying 2 mph ABOVE their true value...but, the odometer is (supposedly) not affected by this difference.

1987 toyota truck.

...1987 will almost certainly still be using direct cable drive from the transmission to a mechanical speedometer "head"--for these, the aftermarket sells "gears" the can be put "in-line" with the cable to compensate for tires, transmissions and axles affects on the speedometer readings.

Can you find a nearby section of freeway with odometer calibration markings? They have several near me in Washington, not sure how common they are elsewhere. But, you can compare your odometer to a 5 mile long section of road that is marked at precise, 1 mile increments. Use that to determine what percentage your odometer is off (which may or may not reflect the error in your speedometer).

Assuming the 60 actual miles = 65 miles indicated, you would need to multiply your indicated miles by (60/65) or .92

300 x (60/65) = 277 actual.

Not where I live,near L.A

i used my GPS to determine odometer error. I recorded the odometer readings at 20, 40, 60, and 80 miles, v. the GPS miles. Now, when I fill a tank, I multiply the odometer reading by the % error from my testing.

It could also be tires. If your tires are worn down about 1/4 inch which is 1/2 inch in diameter it will show a ~2% error compaired to a new tire. Another thing is make sure you have stock size tires on if you want the speedo to be accurate

My mileage from odo to gps was 2.8% different on many tanks of over 600 miles distance.

Speedo reads 1 high though.

On my xB with OEM tires, the speedometer was 5% optimistic, but the odometer was 3.3% pessimistic. Now I'm running tires that are 5.31% larger diameter, and my speedometer is accurate, but my odometer is 9.9% pessimistic.