Odometer error

Using a GPS device and the highway mile markers, I’ve concluded that my odometer '01 Subaru Forester increases mileage by about 3%. For example, when I’ve actually gone 100 miles (GPS/mile markers) my odometer will read 103 miles.

Here’s the question: I’m due for a timing belt change @ 100,000 miles. When my odometer reads 100,000 miles I’ll have actually only gone about 97,000. Can I go by the real mileage, or does the engine think it’s gone the 100,000 miles (# of revolutions) as the odometer reads? Thanks

Three percent error on an automotive speedometer is acceptable. Most vehicles have this this margin of error and more!

My brother found this out when he got his Tom-Tom. And just for yucks and giggles, we drove my vehicle with his Tom-Tom. And my speedo/odo was off by 3.8%.


to let you know about GPS though… the accuracy is not that good, because the refresh rate (at 60 mph) is not really good to get MPG figures. if you go on long trips it will average out, and then the GPS will be more accurate than the odometer. GPS is superbly accurate at slow, low speed navigation. fast navigation (like in your car) is blurred.

just to share some personal experience with you; the GPS signal was intentionally degraded. when GPS first came out there was an additional signal available (DGPS) which corrected GPS to something like a three meter (12’) accuracy. this was removed, and replaced with the “new” better gps signal. now accuracy is around 7 meter (25’.)

remember that repetitive errors make ANY location system inaccurate. think missile, terrorist, and WMD here for the implications.

i doubt the garmin gps are really filtered to be THAT accurate.

i have found that the only way a gps is realy accurate is when you are almost stoppped or completely stopped. i believe that the military has a whole lot better GPS equipment (ala cruise missile etc) but you and i aren’t going to get that kind of neat stuff.

The mile markers are accurate. If you drive at 60 MPH for that mile and you get to the next 1 mile marker in 60 seconds, your speedometer is accurate. That is the only way to check your speedometer. Drive 65 and you get there in 55 seconds.

The odometer is not hoooked to the engine; it takes readings from the driveshaft, or transmissionfinal drive. Having said that, I drive the same exact distance twice per month to a city 185 miles away, in different rental cars. The recorded mileage is never the same; often 3 miles more than my own Toyota with new tires. 3 miles in 185 is about 1.6%. Years ago a car rental company was convicted of fraud when they put tires on the drive wheels one size smaller than standard, resulting in a 4% overcharge on mileage to the customers. I would not worry about the discrepancy, since car service intervals have a large safety margins built into them. Go by the odometer reading to keep the dealer happy.

Nobody would even question you getting 100,000 mile service with 97,000 miles on the odometer or 103,000 for that matter. I’d go by the odometer because it’s easier and well within reasonable leeway for timing belt changes and the like.