When my CD player got real finicky during a recent trip, I thought I'd occupy some time checking my odometer against mile markers on I-40. In four different 100-mile stretches, in four different states, my odometer measured 96.2-96.9 miles. Has anyone else ever experienced this? I expected some discrepancies, but not such consistency in the difference. I wonder if this could be caused because the odometer was calibrated for the 18" wheels and I have the 19" (don't know the outside circumference difference of the two)?

I don't have a serious problem with this. If correct, it means my mileage is 3-4% better than I thought and the displayed mileage will be 3-4% less than actual. Any thoughts?

 

if its showing less than actual... then its good... why worry
however the circimference of the 18" or 19" is the same because the tire size reduces from 18 to 19 and the overall circumferance does not change.

 

I noticed some discrepancies between the odo and the road markers, but nothing too major.

Plus you gotta figure that driving in a certain lane on a curvy road may affect the actual overall distance you travel. i.e. while taking a left curve from the outside lane you will be traveling more than if you would to take the same curve in the inner lane and vice versa. These small differences add up eventually to the discrepancy you see between the odo and the road markers.

 

That does seem a little too much different than actual, but I wouldn't trust the markers. You need to measure your miles against know measured mile markers, or borrow someone's GPS for exact measurements and speed.

The tire's circumferences would have a direct baring on indicated distance travelled and speed. You need to know the difference between replacements and OEM.
C.

 

I've had 4 Nissans, against GPS on a flat, staight road, they have all read 2-3 mph fast at 70mph. I've been in rental cars (domestic) that read 3-4 too slow. Thats not fun if youre trying to push the limit and not get a ticket :/

 

I have 18" wheels (performance package) and it shows 4.5% difference. It's normal due to the upgrated tires (245/45/18). Just watch your speed. When it shows 50mph, you're actually doing 52mph. This could mean extra 2 insurance points.

 

Mile markers are estimates they are not exact!

 

They are estimats, but they are estimates by a matter of feet. Not a matter of miles. Either way, its a good thing when it comes to resale value, but watch out for your speed, and oil changes etc.

My mothers Subabru Legacy 2.5GT sometimes doesnt even register as moving. I'll be driving along and all of the sudden the speedometer just drops. The car reads about 90,000 miles, but it really has about 105,000. We donate our cars though, so it wont really matter that much to us.

 

Are you really checking against your odometer or the trip functions?

There are two trip functions that if you are driving you could easily be switching between the "A" and "B." This could explain the differences.

I find it very difficult to believe the odometer is faulty.

 

Go to this site:

http://researchyourcar.com/tirechange.php

Enter your original OEM wheel and tire sizes and then your aftermarket info and it will tell you what the difference in odometer reading will be. You will have to do front and rears separately but you'll get the idea and the probable reason for your discrepancy.

 

If every mile marker was 52.8' shy of a mile it would be off one mile at the hundred mile pole.

 

If the tire circumference is diff than stock, it absolutely effects it.
The norm on the stock 18" is 45 series, the 19"40 series.... but even that is effected if you went with say 19" 265/40 rather than 245/40

I know mine is pretty damn accurate, when I travel - like last night when I have to shoot to vegas, the nave said it would be 238 miles door to door, and guess what - the odometer said the same - it always does.

 

The mile markers are never 100% accurate. They are placed as accurately as possible, as well as the best possible location for visibly, so if its a few off thats most likely why.