I have a question for the wheel and tire experts in the forum: If the wheel/ tire dimensions are changed from its original configuration, does the onboard computer (i.e. odometer, etc.) need to be adjusted? If so, how easy/ complicated is it to change these settings?

 

You can't adjust it. It will be off by the difference in outside circumference. The tire/wheel combination you have on the car may very well already be off.

 

Mini-AGR,

Just a note for you. On a 205/45R17 with that standard 11/32" new tread depth there is a change of approximately 1.3% from new to fully used (tread down to tread wear bars) so even if you were to be able to adjust it it would then vary over time.

These cars are set up so that the average diameter of the rim and tire combination should be approximately 24.2" to 24.5".

Also of note to you is that with the OEM tires of any size on the MINI the speedo is manufactured to have approximately 3MPH faster reading than what the car is actually doing when going more than 30 MPH. I.E if the speedo says 60 you are actually doing 57. This is because of European Union laws that state the speedometer can be off by +10% and -0% of actual speed.

You can test this theory with a standard aftermarket GPS system.

Also of note is that the OBC has a more acurate reading if you read directly form the OBDII port.

EDIT: As ran-to-matic stated there is no way to adjust it.

 

The dealer can adjust it for large changes in overall tire diameter, but as schatzy62 states, unless it's greater than a 10% variance, then why bother? The R56 is programmed with a 5% error (actual speed is always lower than speedometer readout). OBC is about 2% off versus my GPS. Just for reference some approximate speedometer error values for various tire sizes all else being equal:

205/45-17 = 24.53" tire = 5% error <-- stock Cooper S
215/35-17 = 23.14" tire = ~10% error
215/40-17 = 24.02" tire = ~7% error
215/45-17 = 24.90" tire = ~3% error
205/35-18 = 23.86" tire = ~7% error

 

Use this calculator to give you an idea of tire size and how it effects speed.

http://www.miata.net/garage/tirecalc.html

 

Good calculator...been using it for several years myself!!

 

+1!!

 

Thank all for the very elaborated responses!

 

Yup according to my GPS, my stock 205/45/17s are about 2-4 mph higher depending on speed (25-75mph)

The 'oversized' 205/55/16s (should be 205/50) read about 0-1 mph higher... almost right on.

Thus i am never going as fast as the car says I am...

Like the others said, no need to recalibrate really... only on trucks with 35's, ect. does this sort of thing come to play

 

No matter how fast I'm driving, my car reads a constant 4-5 mph faster than my GPS says. I have 17x7s with 205/45/17 tires.

 

This is the first I have ever heard of the dealer being able to adjust a MINI speedo. The only thing I have heard of is a dealer replacing parts it the car is outside of the allowed tolerance.

 

The real truth here is that the dealer CAN NOT ADJUST the accuracy of the Speedometer or OBC/ODBII.

They can however change parts out if they feel the reading are incorrect. The problem there is that due to the European Laws the spec is so wide the following would have to be meet to make it a problem.

1. Vehicle true speed higher than the ODBII/OBC/Speedo reading
2. Vehicle true speed less than 10% +2.4KPH (1.49mph) slower

or at 60 miles per hour the car can not be going more than 60mph or less than 52.5mph. With this wide a range it is doubtful that MINI will really have to even replace parts.

BTW at 30mph the limits would be mo more than 30mph and no less than 25.5mph

 

And BTW the real error is not 5% on all cars. The error is anywhere from approximately 2% to 6% on average, but will vary for each car. And the error is NOT programed in it is part of an electronic circuit that uses resistors to make the error. Any tiny variance in the values of the resistors changes the percentage of variance. It "can not" be change via programing or any other type of adjustment, except to change the resistors on the circuit board.

Also of note is that the errors listed above for tire sizes also change by manufacturer as each manufacturer and model of a specific tire size has slightly different diameters as shown below.

205/45-17
Dunlop SP SPort 9000 DSST - 24.2
Dunlop Direzza DZ101 - 24.3
Dunlop SP Sport 01 DSST - 24.5 - OEM Tire
Continental ContiSportContact 2 - 24.3

215/40 -17
Dunlop Direzza Sport Z1 Star Spec 23.8
Kumo Ecxta MX - 23.9
Yokohama ADVAN Neova AD08 - 23.7
Yokohama Prada SPec 2 - 23.9
Yokohama S.Drive - 23.8

215/45 -17
Bridgestone Potenzza RE-11 - 25.4
Yokohama Advan Neova AD09 - 25.2
Continental ContiSportContact 2 - 25.3

So by looking at all this data above and knowing that there is no way to adjust the error. your best bet is to stay with a tire that is approximately 24.5" diameter +/- .5" if possible.