I'll admit that all this math is making my head hurt. So here's my stupid question. I assume the speedometer error affects the odometer. How, then, is gas mileage calculation affected?

 

I wouldn't be sure the odometer is affected as much. The manufacturer wants to make sure the speedometer never will read low, so adds some fudge factor for variance. It would look bad if a certain car started getting lots of tickets because the speedometers err in the wrong direction. The odometer doesn't have the same problem.

Throw in mileage calculations then you have to have fuel flow. There'd be some variability in that too. Not just measuring fuel, but things like temperature/expansion. It's amazing they can end up with a calculation within a few percent.

 

Yes, it's been documented that even when the speedo reads high, the odometer is usually more accurate. This is attributable to the regulations that require the speedo never read too low, so the mfgs build-in a design bias which seems to average about 5-7% high. The odometer, OTOH, is spec'd as a +/- couple percent tolerance, so it is usually pretty close.

Honda just lost a class action suit on odometers that read too high. They were within regulations, but Honda was deliberately biasing them a couple percent high to have warranties and leases expire prematurely. Soapbox: Of course, the lawyers got about $9M in the settlement, while the car owners got small extensions to their warranties and leases. All paid for, of course, by the folks buying new Hondas.

- Mark