Sorry for the delay. It took me a while to find out the tire size on the now-gone 17 ACS wheels.
Anyway, on the original BMW 16in, tire size: 225/55R-16
On the 17in ACS, tire size: 255/40R-17

 

Rolling circumferece of the 17" wheels is 5.1cm shorter ie 2.5% difference so this shouldn't explain the big difference, I think the profile was 1 step too small on the 17's ......should have been 45 profile tyres

DD

Apologies for the maths!

 

Thanks.
I shall leave you to the calculations.
The tire size were 'recommended' according to the little sticker on the driver's side door rim.

 

You can see all the calculations for yourself using the following calculators:

This one gives a clear explanation of speedometer error:
Speedometer error calculator

This one (using javascript) gives a whole bunch of information:
Javascript tire size calculator

It seems that the error was closer to 2.8% and not 2.5%.

Have fun!

 

Ah well rudy, I worked it out mentally so to be 0.3% out restores my faith in my grey matter!

Still doesn't account for the big mileage difference....bit of a mystery there!

I see you're nearly at 400 posts yourself....You'll overtake me bigtime soon!

DD