An odd question

[tyranical]

As we all know speedo's are inaccurate etc.

If I drove up to 70mph indicated on the speedo (so could be around 65 really), set cruise control on, reset the mileage counter and didn't slow down for an hour would my cars mileage show that it had done another 70 miles or does it account for the speedo being inaccurate?

[Mr E]

tyranical said:

As we all know speedo's are inaccurate etc.

If I drove up to 70mph indicated on the speedo (so could be around 65 really), set cruise control on, reset the mileage counter and didn't slow down for an hour would my cars mileage show that it had done another 70 miles or does it account for the speedo being inaccurate?

Driven off the speedo, so I guess most cars show more miles than they've really done.

[Urban Sports]

Correct, very odd

[Astra Dan]

Depends on the age of the car. Mine has a direct drive from the gearbox via the cable to a mechanical counter in the speedo. The speedo works by a spinning magnetic disc inside a metal cup, so two very different methods of drive transfer, with different wear rates etc.
Also, with modern cars, maybe it's designed to very accurately measure the distance, but deliberately shows less speed.

[blank]

Modern cars the speedo will overread but the odo, trip and usually the average speed on the trip computer will be pretty much bang on.


So if you set cruise at an indicated 70mph (say real 67), and drive for 1 hour, your odo will go up by 67 rather than 70.

[spikeyhead]

Astra Dan said:

....
Also, with modern cars, maybe it's designed to very accurately measure the distance, but deliberately shows less speed.

This.

It would be easy to make the speedo read nominally correct, but then it wouldn't meet the regs half the time. So the odometer is usually about right and putting the numbers in a slightly different place on the speedo would make that nomially right, but they've been tweaked slightly to make them meet the regulations.

[Mr E]

spikeyhead said:

This.

It would be easy to make the speedo read nominally correct, but then it wouldn't meet the regs half the time. So the odometer is usually about right and putting the numbers in a slightly different place on the speedo would make that nomially right, but they've been tweaked slightly to make them meet the regulations.

Last time I ran the alfa a couple of hundred miles, the trip and the satnav disagreed by about 3%. I also know that the speedo over reads by 3%.
It *is* an alfa, so it's entirely possible all of the electrics are insane.