Are the speedometer and odometer fed from the same source? or do they get their data from gearbox/wheel revolutions, i.e. independent(ish) sources?

Reason I ask is that I had a play with a GPS at the weekend and the speedo consistently over-read by 10%, i.e. at indicated 66mph was doing 60mph. I trust the GPS more than the speedo

If the odometer feed is from the same source then the indicated mileage is also +10%, i.e. when I take the car for 15000 mile service it has actually only done 13500 miles, so my servicing will cost 10% more than it should and when I come to sell the car it will show 10% higher mileage than actual.

Anyone know?

 

Anyone?

 

Interesting theory.

I'm watching with baited breath

Paul

 

The GPS is correct. The speedo and odometer are almost certainly fed from the same source, but that doesn't mean that your odometer is accumulating 10% extra miles. The speedometer is deliberately designed to over-read, beause it would be a legislative failure for it to under-read. legal limits are -0%, -0 MPH, and +10%, +2MPH, so at 60MPH your speedo can read anything between 60 and 68. With correctly inflated new tyres, you would expect an over-reading somewhere around 5% (depends on speed, over 100MPH, the accuracy usually becomes worse), obviously, as the tyres wear the over-reading would increase. The odometer is usually accurate for 25-50% worn tyres, with small errors either side as the tyres wear.