We own an 2007 MCSa with optional 17" wheels factory installed.
I believe the speedo is off by as much as 10mph at highway speeds. As in showing 75mph when I am actually travelling at 65mph.

The MA says this is within tolerance????

I feel the computer wasn't updated for the different wheel/tire size.

Anyone else run into this?

Any suggestions on how I can guide the service department to correct it?

Thank you all in advance.

 

Mine is off 2mph which is pretty normal from my experience but 10 mph is way too much...thats putting that many extra miles on the odo. than your really traveling..do keep us posted

 

AAA used to do Speedo calibrations via a mobile trailer set-up. I don't know if they are still doing them, but it would be worth a call to their office.

Once you have their calibration in hand, you'd know for sure and if it is off as you suspect, you'd have something concrete to show the dealer.

 

Check it against a GPS.

 

Mines off by 4mph according to GPS. Traveled with a group to MITM this year and out of 6 cars I only matched 1 of them. Everyone elses was a bit different. However, my GPS and another cars GPS matched to the "mph". So I too am wondering if this can be adjusted.
It does however keep the kids from speeding. So there is a plus.

Batman

 

Mine is within one or two MPH according my Garmin Nuvi 350.

 

No way 10mph is 'within tolerance'. Standard is 2-3mph
Note that the 17" wheels have roughly the same diameter (24.4" vs 24.3") as the 16", as the aspect ratio (sidewall height) is lower. They should give a reading .7% higher than the 16".
See details with the nifty tire size calculator.

 

Cool tool! Thanks.

 

Yea, sounds lile the factory settings for the spedo are set to the wrong wheel size. Should not be any more than 2-3 miles lower than actual.

 

Does the getrag have a traditional worm gear to feed the speedo, or is it electronic??

 

Electronic.

 

Someone here posted a technical bulletin from MINI regarding speedometer accuracy. I haven't found the thread again, but the gist of it is that the speedometer is allowed to read up to 10% high, PLUS an additional 2 MPH. However, the speedometer is not allowed to read even 1 MPH lower than the actual speed.
So, if you're traveling 70 MPH, the speedometer can read out anywhere from 70MPH to 79 MPH and still be within spec.
And speedometer inaccuracies don't necessarily affect the odometer reading. Both the speedometer and odometer take their input from the same sensor, but each unit processes the data independently. The inaccuracy is almost certainly in the speedometer itself, and not in the sensor that feeds the two systems.

 

Mile post to mile post @ 60 M.P.H. in 1 minute and 2 seconds is what my car does, I agree + /- 10 M.P.H sounds like to much. I would think the dealer could recalibrate either the speedometer or the speed sensor output.