Many people mistakenly assume their speedometer and odometer are calibrated to each other. A quick glance at a GPS will show a discrepancy between the GPS's speed and the indicated speed on the speedometer. You'll need to do additional tests to determine if your odometer is high or low compared to your speedometer.

Your odometer readings will affect your fuel economy calculations. People just assume that their odometer and speedometer will agree with each other.

As a data point, in my old BMW 328, my speedo indicated 5% high, while my odometer indicated 4% low. It's interesting that this (possibly unintentional) miscalibration will result in a reduced chance of getting a speeding ticket, and a better calculated fuel economy that actual, and fewer miles shown on the clock than actual, thereby possibly slowing down depreciation a bit.

yes thats what i have learnt since reading the posts around this subject over the last day or so.I have a 4x4 with 245/75/15 tires which have been changed from the original 235/85/16 .This gives me a 7% decrease in my speedo and will this tallies with a gps check .Will check this morning to seeif the odometer tallies with this

hi again just checked my odometer reading this morning and seems a different %out than the speedo can someone who's good at maths please confirm I have done my maths correctly
I have 245/75/15 tyres on ( a land rover defender) from the original standard tyres of 235/85/16 this gives a 7 % under readings on the SPEEDO-This corresponds to the GPS reading (although the speedo jumps about more than a modern car so hard to be exact-Hey someone commented I should use a calendar instead :) )

The odometer this morning was showing 9.7km reading compared to the 8.6 km reading from the GPS.
This I take as 11.34% increase on the odometer worked out by:

9.7-8.6 = 1.1

Then 1.1 divided by 9.7 = 0.1134

then 0.1134 x 100 = 11.34

thats a 11.34% increase in the oddometer reading

am i right?

Firstly, I recommend you do your test over 100 km instead of just 9.7. Your odometer has 1 decimal precision, so 0.1 / 9.7 means you have a margin of error of about + or - 1.03%. If you do your test over 100 km, then that 0.1 km precision works out to + or - 0.1%, which is a lot better. This also helps minimize any GPS error in distance calculations, too. Your call.

So assuming your GPS is giving you a really accurate distance:

1. Your odometer is registering 12.79% high.
   (9.7 / 8.6 = 1.1279, or odometer is 112.79% of GPS reading)
   .
2. To convert odometer to real km (as per GPS): Odometer x 0.8865979381443299 = GPS distance.
   or Odometer distance x (test_GPS / test_odometer)
   or Odometer distance x (8.6 / 9.7)

For those who travel roads with mile markers, like most of the US Interstate system.

Set your trip odometer at the beginning of your measurement, travel X miles (I like 100 for simplicity). When you get to the 100 mile marker, check the trip odometer.
Say you go 100 and your trip says 104.

Divide your indicate mileage by 1.04 to get the actual distance travelled.

Enter that amount in your fuel log as the TRUE distance travelled.

You must NOT do any Engine OFF coasting or touch the ignition key since any operation of the key COULD stop your odometer from working temporarily. Engine ON coasting is fine.

While GPS is one way to track miles, I don't think it calculates the added distance due to changes in elevation. If elevations changes are not significant then it would be accurate.

In the list of many "but are you sures", I have no idea how authorities determined the position of mile markers. Did they use a 5,280 foot tape measure? Simply use the odometer on the truck and assume it's right? A calibrated odometer, with accuracy of +/- X%? But what if the tires were changed and the odometer was not re-calibrated? And how diligent and precise were the operators that day? For example, if the mile marker's precise position was at an off-ramp, the operators felt it was unsafe to put the marker there, and so they planted it a few hundred feet later.

All these factors and more suggest mile markers might be just a "pert near" reference for traffic accidents, as opposed to a precise measure of distance.

I don't know what the answer is. I've done zero research to learn it. I'm just saying that we might assume, "well, of course they're accurate", when they might not be. Just like with an odometer. Just like assuming that a speedometer and odometer must agree with each other when this forum reveals they don't necessarily. Caveat emptor.