1st Gen R53 Cooper S Most liked posts in thread: Speedometer shows 4 mph fast

I set the cruise at 6 above posted. People still pass me and I quietly thank them for sacrificing themselves as LEO bait.

 

NCS expert software will allow you to adjust the displayed speed, or you can put taller tires on the car.

This speed error is due to a goofy European Union law.

 

the on board computer has no way of knowing what tires you're running, what inflation you have and what temperature it is outside. Basically, it may read different and higher from what is displayed on the dash, but it will only match your actual speed if you're lucky and all parameters are perfectly the same as the calibration of that car when it was designed.

GPS is the only way of knowing fairly well what speed you're moving at, and even with GPS you have to make sure the GPS is a) a high quality unit mounted with good reception and b) the sample size is high. There is lag between readings so it won't respond to speed changes very quickly and may be off by a lot until it gets the next proper straight line measurement with good reception and constant speed of the car between those intervals

It is all too complicated to lose much sleep over. Just go a little slower than the worst yahoos out there and everything is fine.

 

It's normal. See Speedometer Scandal! - Feature - Car and Driver

 

I agree, it is normal.


I've got a 2002 MINI, so my car doesn't have the unnecessary digital speed readout in the tach -- I rely on the analog readout on the speedometer itself. One thing I found that partially mitigates the under reporting of true speed is upgrading from 205/45-17 tires to 215/45-17 tires. The slight increase in overall diameter of the 215 tires reduces the speedometer's fibbing, showing closer to true GPS speed.

I would assume it is the same for those of you poor MINI owners that look at digital speed readouts.

 

Only 4MPH? You're pretty lucky. When I was at the Dragon I had Helix re-code the speedo (among other things)on my R56. Much better.

 

I changed to 225/45R17's and mine is now dead on, 75 is 75. it was off by 4.

 

If you have an OBD reader, it will show the correct speed. It's only the speed displayed on the dash that is bumped up by the software. This happens on all other BMW models as well.

 

All cars are like that

 

With my 215/45-18 it's right on. With the 205/45-17 it's off by 2mph.

Nice article Nathan. I remember having a speedo that ticked and waved back and forth over 5mph.

 

That just allows you to imagine that the GPS is an electronic version of the Smiths Chronometric speedometer that really top-end* British motorcycles had in the 1960s/70s. This counted the revs over a short period and then displayed the calculated average speed. So it moved in steps but, providing it was correctly calibrated, was supremely accurate. And, for double Joe Cool points, some Chronometrics had zero at the top right, not bottom left.



https://youtu.be/6dknqvZBfpk for a demonstration of the 'Chronometric two-step'.

* 'top-end' being a relative term, meaning something like 'will reach over 120mph before big ends break free of crankcase'.