[Varse]

My new Cooper was ordered with 15s and then has had 16s fitted at the dealer before delivery.

The dealer is not sure whether this poses a problem with the speedo and mileometer readings due to the car being built based on 15s, as is there any difference in rolling diameter from the 15s to 16s?

Can anyone shed any light on this?

[MGB Boy]

175/65R15
Sidewall Height: 4.5"
Radius: 12.0"
Diameter: 24.0"
Circumference: 75.3"
turns per mile: 842.0

195/55R16
Sidewall Height: 4.2"
Radius: 12.2"
Diameter: 24.4"
Circumference: 76.8"
turns per mile: 825.0

If the speedo is calibrated for the 175 tires, speedo reading with 195 tire will be 2.0% slow, will read 60mph when you are traveling 61.2mph.


205/45R17
Sidewall Height: 3.6"
Radius: 12.1"
Diameter: 24.3"
Circumference: 76.2"
turns per mile: 831.0

If the speedo is calibrated for the 175 tires, speedo reading with 205 tire will be 1.3% slow, will read 60mph when you are traveling 60.8mph. For the record, I plan on getting 17s but putting on 15" rims and tires in the winter, which is why I know this!

-Ricardo

[Varse]

Cheers mate, top info. I'll get mine recalibrated.

[MINIAC]

Here's what I asked MINI USA:

Quote:

I was wondering ... since there are different overall diameters for the three MINI wheel/tire packages ... 23.96" for 175/65-15, 24.26" for 205/45-17 and 24.44" for 195/55-16 ... what overall diameter is used to calibrate the speedometer on a MINI? or is it calibrated for the specific wheel/tire package used?

They replied:

Quote:

No calibration is necessary for each specific tire/wheel package. The difference in diameters among the three tire packages is not large enough to significantly affect the speedometer display. The development department assured me that the accuracy of the speedometer with any of the three packages remains within the legal tolerance.

[BruceK]

FYI - There's an interesting article in the latest issue of Car and Driver concerning speedometer accuracy.

The article asked the question why European cars typically have speedometers that do not reflect actual vehicle speed speed as accurately as speedometers in cars made in North America or Asia. The article specifically mentioned BMW and Porsche speedometers (which I guess would include MINI). Of course having a European car with a less accurate speedometer is counter-intuitive when you consider the high speed autobahn travel possible in parts of Europe.

The upshot: A certain level of inaccuracy is designed in on purpose by the manufacturer to meet EU regulations. Mostly this has to do with the various tire sizes that the owner may fit to the car.

A real interesting tidbit: Trip Computers do not have to meet this EU regulation, so they tend to read more accurately. Meaning if your speedometer says you are going 80 mph and you maintain that speed and reset your trip computer to determine your real average speed, the Trip Computer will show the more accurate true vehicle speed.