speedking

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?

speedking

Anyone?

Scooby Dooby Blue

Interesting theory.

I'm watching with baited breath

Paul

dr_ming

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.