Genuine mileage

[swamp]

My new sat nav tells me my car is traveling significantly slower than indicated on its speedometer. 77 mph is actually 70 mph, if the GPS is to be believed. I've heard this is commonplace: car manufacturers build in a good safety margin to allow for things like different tyre sizes.

Assuming this 10% over-read is linear, the odometer also must over-read by 10%. So in such a car with an indicated 55000 miles, it has actually only traveled 50000 'genuine' miles.

Just a thought.

[cmackay81]

swamp said:

My new sat nav tells me my car is traveling significantly slower than indicated on its speedometer. 77 mph is actually 70 mph, if the GPS is to be believed. I've heard this is commonplace: car manufacturers build in a good safety margin to allow for things like different tyre sizes.

Assuming this 10% over-read is linear, the odometer also must over-read by 10%. So in such a car with an indicated 55000 miles, it has actually only traveled 50000 'genuine' miles.

Just a thought.

aftermarket wheels?
correct tyres?
correct psi?

[mnkiboy]

In my old Focus you could get a speed readout on the dash. It turned out that it was only the needle that displayed roughly 4mph over. The readout from the ECU was much more accurate. Therefore I don't think the odometer will be as far out as you suspect.

Obviously it can't compensate for tyre wear, but I don't think it will be too far out.

[HellDiver]

Unless you've some old car which has a mechanical odomoeter, it's not likely that speedo and odometer errors are the same.

Just because the needle displays 70, it doesn't mean the car isn't fully aware you're actually doing 77mph.

[zeppelin101]

There is a discrepancy between the ECU measurement and the speedo measurement afaik, it is of course safer to be told you're travelling faster than you actually are.

[*Al*]

Which explains why you get dirty looks when passing people slowly in a speed limited area of road works, say 50mph as i go by the GPS.Most people are infact doing only 45mph but think they're doing the 50 limit.

[RB Will]

I think modern cars know the speedo shows too much as in my mums corolla the speedo says 34mph, GPS says 30 and if you look at the trip computer that says 30mph.

[Arese]

*Al* said:

Which explains why you get dirty looks when passing people slowly in a speed limited area of road works, say 50mph as i go by the GPS.Most people are infact doing only 45mph but think they're doing the 50 limit.

I always set my cruise control to 55mph in such situations, as I figure there would be a bit of leeway with the cameras as well as the speedo reading slightly over.

[swamp]

Some interesting reading:

http://en.wikipedia.org/wiki/Speedometer

[sinizter]

The Crack Fox said:

Take care though, my BMW is 100% spot on from 10MPH to 1 lepton, verified against GPS. My other cars are a 5% out typically.

Is this the E46 ?

The three BMWs I have managed to spend any time in (E46 330Ci, E9x 335i and Z4) all show are out by about 3mph on the speedo over most of the range, maybe a little more accurate at the slow end.