That's called driving in Vegas last time I looked.

As for the over-reading thing, I've only had one car that under-read (speed and distance), and that was because it had 75 profile tyres fitted instead of the stock 70 series. None of my cars have ever been spot on and I think a Volvo S60 was the worst for over-reading.

For those of you relying on GPS, what's the likely +/- error with those apps? I'm a measurement engineer, so I have a healthy skepticism about devices that claim to be super accurate, and GPS is just one of them.

I wonder if the mile markers at the roadside are placed per GPS or some other surveying method. Timing them accurately I would tend to believe a bit more over some Dumbphone app.

 

Some dealers are a bit useless at knowing they can (*) set things in the car (such as the tyre details).

Commonly the speed over-reads on the dash as per Mikey's wiki link but the OBD will give the data the car actually uses. It will calculate miles based on that, not on the over-read value. (Test over more than 1 mile!)

It should be obvious that fitting non-OE-size rims etc will affect the values if you don't reprogram the car. And again obviously and as has been posted, as tyres wear the size varies (as it does with tyre pressure).

(*) well, should be able to

 

I'll just leave this here...

http://media.caranddriver.com/ez/original/application/1e1b33f85f31df61e4830f32f82a5a71.pdf

 

New owner, also getting a speedo being high about 4mph at highway speeds.

Has anyone confirmed the ODO is tracking high or accurate vs with the Speedo being high?

Since I have a lease this is concerning to think of all the extra miles being put on the vehicle that I am not getting to actually drive.