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.

My car reads 4 -5 mph faster than it is, according to the radar speed alert signs and my Garmin GPS although my odometer reads true. It is my understanding that all cars from the factory are preset to read faster than actual to insure any speeding tickets are not caused by speedo.
Any non stock tire size will effect the speedometer and odometer.

...for the numerically-inclined:

MPH = [ 60 / (G×A) ]×[ RPM/rpm ]

...where:

MPH = vehicle speed, miles-per-hour
RPM = engine speed, revolutions-per-minute
rpm = tire speed, revolutions-per-mile <--get this from your tire dealership
60 = conversion constant, minutes-per-hour
G = transmission GEAR ratio, ie: 1.00:1 <--usually highest gear
A = transmission AXLE ratio, ie: 3.91:1

I made my weekly long trip again. Nor. Cal. does have mile markers.For every 10 miles, i gain 4/10ths.

At the rate of gaining 4/10ths of a mile for every ten miles your odometer will be off 4,000 miles per 100,000 miles. Your car has less mileage on it then you thought and your mpg is not quite as good as you thought, you should be subtracting 00.4 off of your trip meter before calculating its fuel economy.Assuming the mile marker signs are correct..

same as minusing 4 miles per hundred?

My speedo is only off by a small amount (Ive calculated a .054 difference between stock tire size and the taller tires I have now) but my odometer seems to be nearly an entire 1/10th off when I use mile markers on the hwy. I use my odometer to roughly figure out my MPG over an entire tank but will prolly look into a more accurate measure soon.
Matt

I use a GPS over long (50-100 mile) distances.

...a tires' revolutions-per-mile (rpm) number are measured at 45 mph test speed and its rated load capacity, but that number varies with changes in speed and/or load.

...for example, below are the speedometer-vs.-actual results for my '70 AAR 'Cuda (S=speedometer; A = actual):

S = 100.0 mph, A = 102.0 mph, or 2.00% high.
S = _90.0 mph, A = _91.5 mph, or 1.67% high.
S = _80.0 mph, A = _81.2 mph, or 1.50% high.
S = _70.0 mph, A = _71.0 mph, or 1.43% high.
S = _60.0 mph, A = _60.5 mph, or 0.83% high.
S = _50.0 mph, A = _50.0 mph, or 0.00% no error.
S = _40.0 mph, A = _40.0 mph, or 0.00% no error.
S = _30.0 mph, A = _29.0 mph, or -3.33% low.

...thus, tire "error" is not static, it gets bigger above & below 45 mph!

to answer the question....
the gps is accurate.
Everything else is just for looks.
So what if the speedo is off....or the odometer is off.....
If you have a gps you have an accurate measure of distance and speed.
Think about it.
Is there an adjustment option on a gps?
no.
Is there an adjustment option on scangauge (which reads obdII ie: the enternal car numbers) yes.

I don't think the speedometers are purposely set a fixed mph high. I think it has more to do with there being a manufacturing tolerance on the accuracy of the speedometers. Instead of being specified accurate +/- 5 %, I think they are accurate with a tolerance of + 10 % and - 0 %, which could also be interpreted as being 5 % high with a tolerance of +/- 5 %. The tolerance keeps costs down by allowing more speedometers to be accepted without costly individual tweaks. So everyone's speedometer will read a little different, though all will probably purposely read high to avoid speeding tickets and lawsuits, as mentioned earlier. Speedometers that read low are rejected and (hopefully) never shipped.