Odometer off by 2 miles

Speedometer? Yes that's normal.

 

I think the GPS is the more accurate one. We have grid-roads here, and section-lines are reasonably close for an exact mile-measurement. The GPS is never very far off, but the car odometer can be off by quite a lot.

 

If we are talking about the speedometer, believe the GPS. Car speedometers almost always read fast. Among the lore about the reason why this is that auto manufacturers don't want to be sued by owners who get at ticket for speeding but claim that the speedometer said that they were not speeding.

 

Is it the odometer, the readout that tells how far you have gone? Do you have stock size wheels/tires?

If it is the odometer, then you have a problem that should be looked at under warranty. Odometers are supposed to be accurate lest they give you false MPG numbers which would get them in trouble with the feds.

 

i know the computer mpg's are off, not sure about odo. is it the trip odo, or lifetime?

 

I'm sure I've griped about this before, but I don't see why they don't use something more accurate like ground-radar. I know it sounds expensive, but I'm sure it can be mass-produced at a cost insignificant to the price of the vehicle.

 

OP, the difference between your GPS and the speedometer, if that's in fact your concern, is by legislation. Car manufacturers are obligated on the side of caution, show your speed a bit lower than actual. On the other hand, the odometer should be reasonably accurate, with stock tires.

 

it will be down to ~1mph at 60 after you get different tires.

 

You're saying that it's off by two miles over what distance?
Two miles "faster" over 100,000 miles isn't much of an error, and attributing all of the error to either the GPS or the MDF would presume that the other one is absolutely accurate.
(Hint: It's probably not.)

This is all pretty easy.
The next time that you're in the interstate measure both the car and the GPS against the mile markers.
The signposts are fairly accurate...especially if you use 100 of them which will pretty much identify any irregularities in the GPS and MDF readouts and as a bonus it makes the math really easy. You don't even have to take off the shoes.
I did this the last time I had to drive on the interstate for a 100 mile run (car was about 10,000 miles old) and my MDF and GPS were pretty much in agreement.....like within 0.5miles of each other. I could give a rats about speed because....well....it's a Prius.
GPS isn't always my go-to instrument for accurate velocity measurements anyway and although there will be a bias in the MDF reading, it should be more repeatable, since tires tend to take (most people) 50,000-80,000 miles to wear down enough to matter.


I dang sure don't trust ANY Prius to tell me what their fuel efficiency is!!
However (comma!) my particular car is within 1 mile on the odometer.
Once you've verified that the odometer is accurate to within a percentage point, then fuel burn rates are pretty to figure out as well.

Good Luck!

 

One could imagine that the odometer might be off compared to GPS on curvy roads based on the sampling rate of the position... or not.

 

Until the OP clarifies some basics, we're all just barking in the dark. I've got a strong hunch he's just noticing the speedo vs GPS diff.

 

Also keep in mind it is normal for a car to under-report its odometer and over-report its speedometer. The car will always tell you that you are going the actual physical speed or faster than the actual speed. The odometer will always show the actual physical distance travelled or less than the physical distance travelled.

Speedometer is so that the car companies don't get sued by you going 71mph in a 70mph zone with the car indicating 70mph. They fudge it.

Likewise, they don't want to get in battles when a major warranty item pops up at 60,001 miles instead of 59,999 miles on a 60K warranty. If the car says 60,001 miles, it sure as heck went 60,001 miles and most like a fraction of a percent more.

 

my guess is speedometer. mine reads 30 when the radar sign says 28.

 

But we are asking you if it is the SPEED that is off or the DISTANCE.
To have the speed off a bit toward the high side is quite normal.

 

How does it work when you have larger rims? Is there something programmed into it?

 

Nope. If you change the overall wheel + tire diameter, the reported speed changes. Another reason they read high. Even just the wear on tires changes the diameter and changes the speedometer readings.

 

No, it depends on the outside diameter of the TIRE, not the rim.
Most tires 15" and above have about the same diameter......about. (or is that 16 and above??)

 

The easiest way to figure out is to go to tirerack and compare tires by revolutions per mile. Higher number more "optimistic" speedo is.
The 17" Prius tires have very similar outside diameter so the readings will be the same.