Good info,
I wasn't aware of the difference between speedometer error and odometer
error... my speedo needle bounces too much for me to tell what it's real
error is ;)

In any case, it's the error on the odometer that matters for calculating
mpg.

-Wes

Kim Brennan wrote:
> You haven't even begun to touch on this topic with this little sentence.
>
> On Sep 3, 2007, at 6:43 PM, Wesley Pegden wrote:
>
>> One thing to keep in mind is that many speedometers read a little fast.
>> You should compare your trip odometer to highway mileage markers to see
>> what factor it's off by to correct your numbers.
>
> The US Standard is that speedometers read 3-5% below actual speed. But
> odometers should be 1% or less from actual. But all that is moot once
> you start going away from stock tires. And if you have gone with
> larger wheels your figures can be all over the map.
>
> In my case. I have one syncro, where the SPEEDOMETER reads pretty much
> 10% low with the tires on it (when it says you are at 50 mph, you are
> actually at 55mph.) The ODOMETER, on the other paw is 7% off. When it
> says you have traveled 200 miles you have actually traveled 214 miles.
>
> In another van, the numbers are different. And the tire sizes are
> different. In that van the speedometer is 12% off, and the odometer is
> 8% off.
>
> Just because you have calculated the margin of error for your ODOMETER
> doesn't mean you have calculated the margin of error for your
> SPEEDOMETER.
>
> My calculations of margin of error are based on GPS values for speed
> and distance.
>