[toononame]

I picked up a set of winter wheels (641M + PA4 235/35 R19). The revs/mile is a bit different vs stock, but I'm surprised how off the speed is. At 30 it's about 3mph off. 60 about 5mph off. 80 is 7mph off. This is based on gps from my phone as well as in my dash cam.

Is this expected? Do people have the speedo calibrated when they swap? Or maybe my speedo was always wrong and I just noticed?

[bentom2]

Originally Posted by toononame

I picked up a set of winter wheels (641M + PA4 235/35 R19). The revs/mile is a bit different vs stock, but I'm surprised how off the speed is. At 30 it's about 3mph off. 60 about 5mph off. 80 is 7mph off. This is based on gps from my phone as well as in my dash cam.

Is this expected? Do people have the speedo calibrated when they swap? Or maybe my speedo was always wrong and I just noticed?

Some of that may be just that it was a bit off and you never noticed, but a fair bit of it will be from the tires.

A 235/35 r19 is a smaller diameter than both of the OE tire sizes. Take a look at the tires on tire rack, or use a website such as willtheyfit.com to see the difference. The “35” in the tire size is a ratio of sidewall height to tire width, and not a fixed measurement.

[detroitm2]

The difference between OEM 245/35r19 and PA4's (235/35R19) is only 1.2%. My guess is something else is off.

[jkoral]

Originally Posted by toononame

Or maybe my speedo was always wrong and I just noticed?

(Ignoring the change in tire size...)

BMW's TSB (620296 from May 1996 - "Speedometer - Maximum Permissible Error") states 2.4 mph + 10% as a maximum (to be considered within spec). Not +/-, only plus.

It can never read lower than the actual speed, otherwise it's a violation of European regulation, ECE-R 39, which states indicated speed must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour.

In your case, that means 60 could read 68 mph and 80 could read 90 mph and BMW would consider that within spec since the max error is 68.4 and 90.4 respectively.

[HazChem]

Same with me. On my 763M 245/265, I set my speed at 76 but Waze will show 74. On my 641M 235/235, I set my speed at 76 and Waze shows 72. It’s annoying but somewhat expected...

[toononame]

Originally Posted by bentom2

Some of that may be just that it was a bit off and you never noticed, but a fair bit of it will be from the tires.

A 235/35 r19 is a smaller diameter than both of the OE tire sizes. Take a look at the tires on tire rack, or use a website such as willtheyfit.com to see the difference. The “35” in the tire size is a ratio of sidewall height to tire width, and not a fixed measurement.

Yeah I checked out the revs/mile at least but that's only a difference of 1.2% in the front and 3.5% in the rear, so I was surprised at the huge difference in actual speed.

Originally Posted by jkoral

(Ignoring the change in tire size...)

BMW's TSB (620296 from May 1996 - "Speedometer - Maximum Permissible Error") states 2.4 mph + 10% as a maximum (to be considered within spec). Not +/-, only plus.

It can never read lower than the actual speed, otherwise it's a violation of European regulation, ECE-R 39, which states indicated speed must never be lower than the true speed or higher by more than one-tenth of true speed plus four kilometers per hour.

In your case, that means 60 could read 68 mph and 80 could read 90 mph and BMW would consider that within spec since the max error is 68.4 and 90.4 respectively.

Well that's certainly interesting and good to know. It's definitely reading above at least. I was thinking of talking to the dealer but maybe they'll just tell me it's within spec.

Originally Posted by HazChem

Same with me. On my 763M 245/265, I set my speed at 76 but Waze will show 74. On my 641M 235/235, I set my speed at 76 and Waze shows 72. It’s annoying but somewhat expected...

Interesting! So I'm not alone. I'll have to check my summers the next time I put them on.