[StiffUpperLens]

So I recently bought new wheels for my e90. The car had 225/55R16's on it and I upgraded to 225/45R17's and now speedo is reading about 10% too fast?

Reading the specs for the car, the new wheels are within spec..

Am I missing something?

[dpaul]

Yes - if you had 225/55-16 then the appropriate (same rolling diameter) upgrade would have been 225/50-17 (which at least in the US is the stock 320i tire size).

Since you have decreased the tire diameter, a given linear movement requires more tire revolutions, which is read out as a higher speed.

[Camery]

I recently changed my wheels from 16" to 18" and wondered the same thing.
However, using this website I found out it does not make that much of a difference (for me anyway).

Just fill in your old and new size

https://tiresize.com/comparison/

[dpaul]

16" to 18" by itself doesn't provide enough information to tell whether there will be a change in rolling diameter.

If you choose the proper aspect ration and width to complement the change in wheel diameter, there would be no difference in rolling diameter. The OP changed rolling diameter by 2.7%, which would make an observable difference at the speedometer.

[StiffUpperLens]

Originally Posted by dpaul

Yes - if you had 225/55-16 then the appropriate (same rolling diameter) upgrade would have been 225/50-17 (which at least in the US is the stock 320i tire size).

Since you have decreased the tire diameter, a given linear movement requires more tire revolutions, which is read out as a higher speed.

OK that makes sense. I was thrown because the placard listed the tyres as correct. In fact the old wheel/tyre combo was not listed so I wonder whether a previous owner had calibrated the speed to non spec tyres.. although why would they?

Strange thing though - I plugged in diagnostics using a generic OBD app called OBD Fusion - and the live speed it throws is exactly correct - but the speedo is running 7% over. Even after coding the car with Carly to force it to display accurate speed, instead of calibrated.

Any thoughts on that?

[Jurciks]

So you want to tell me that if I swap my winter tyres (225/55 R16) to summer tyres (F 225/40, R 255/35 R18) i have to calibrate the speedometer?

[dpaul]

Originally Posted by Jurciks

So you want to tell me that if I swap my winter tyres (225/55 R16) to summer tyres (F 225/40, R 255/35 R18) i have to calibrate the speedometer?

You don't have to do anything. However, your rolling diameter change is -2.3%. What you do with this information is , of course, entirely up to you.

I personally would not bother with the speedometer.

FWIW, in the US, the E90 speedometer is purposefully set to read high. There are quite a few threads on this. I've read that in Germany, manufacturers are subject to TUV rules and they leave a safety margin so those rules are never violated.

[StiffUpperLens]

I'm still confused, how is that the car's OBD is reporting exactly the correct speed, confirmed by GPS, but the speedo is reading high by 7%. And it's only since i changed these wheels..

Both digital speed and analogue speed has been set to 'accurate' via coding.

Digital speed on the dash matches OBD reported speed - both confirmed by GPS (on phone).

Analogue speedo reads high.. much higher than before..

Would the OBD speed be taking its measurements from the car's on board GPS (iDrive) by any chance?

I feel like I'm about to get a nervous tick over this....

[Jurciks]

Previously i have used a navigation program like Waze, which shows route and speed you are driving, and it showed exactly the same as my speedo.

[StiffUpperLens]

That used to be the case for me, until these new wheels were fitted. Now only the OBD reported speed matches the gps.. which is why I'm confused.

[dpaul]

OP- if the change in speedo reading exactly correlated with installing new wheels, I think you have at least part of your answer. To be sure, the speedo change may be an unlucky coincidence. But I'd start with the assumption, for purposes of troubleshooting, that it is not a coincidence.

I cannot speak to your vehicle, which I assume to be an F-series as I only have experience with E-series. But I believe that BMW has coded speedometers to read about 5% high for many years - true for E30, E46 and E90 vehicles. It is German law that speedometers can't show speeds less than the actual speed and they can't show more than 110% of actual speed plus 4 km/h and BMW has taken a very conservative approach to this issue.

If you want the analog speedo to read accurately, code the KOMBI BC_V_KORREKTUR to nicht_aktiv in NCS expert. If you want the digital speedo to read correctly, code BC_DIGITAL_V_KORREKTUR to nicht_aktiv. Again, this would be for E series; F's may be different.

I do not know why you might read a correct value from ODB port but it is possible that the KOMBI reports an unfiltered value there. Maybe someone with more direct experience and knowledge can chime in.

EDIT: Sorry, missed the part where you report that the KOMBI has been 'coded' to report correct values. I suppose the first thing I'd do is check those setting with NCSExpert

[hassmaschine]

changing *wheel* size has absolutely no affect on speedometer accuracy. Not unless you're going for the "donk" look.

changing *tire diameter* however, does, which you've done by running the wrong tire size for that wheel.

And the stock BMW speedometer is off by 5% anyway, due to German laws about speedometer accuracy (it can read higher than actual, but never lower).

[dpaul]

Yes, Chris.

OP - it is possible to calibrate the speedometer to any arbitrary scale but it requires NETTODAT coding that is a little more complex to perform than the typical sort. I can't bring it up the values to code off the top of my head but I know it's out there in some thread somewhere.

[hassmaschine]

Ninja post!

[StiffUpperLens]

Thanks all, I appreciate your input. I have to accept you're right dpaul, it must be the wrong tyre. Yet its there on the placard.

The wheels were second hand and the tyres came with them. Of all the wheels of that size i looked at, the tyres were exactly the same specs and the specs are clearly marked on the placard - 225/45R17.

However, they are type 194 wheels which according to what I've since seen online, are meant to be staggered. I didn't know that when i bought them, all of mine are equal size.

https://www.bmwoemwheels.com/en/rim-model/bmw/bmw-wheel-style-194

The old wheels that had 225/55R16 tyres fitted where the 156 type.

https://www.bmwoemwheels.com/en/rim-...heel-style-156

They're otherwise similar specs, the new ones being slightly wider, with a larger diametre, but with the same offset.

Would running the same size all round with these wheels make a difference - i can't see how they would but I'm a noob so I don't pay much regard to my own thoughts on this

I'll head into BMW next week and discuss because it really does confuse me that the OBD reports the correct speed but the speedo is way off, and i appreciate the deliberate overreading but it's way, way off. Even after coding.

I've come from Alfa Romeo's and for 17 inch wheels, they run 225/45R17 tyres - maybe that's where my confusion lies, i see it as a pretty standard size.

[dpaul]

No, running the same width wheel on all four wheels would not make an appreciable difference.

Also, I'm not saying you have the "wrong" tire, I was only trying to make two points:

1) in the US, 320i tires are 225/50-17 for base models or 225/45-18 for sport models, which have the same diameter. If you opt for staggered wheels, they give you 225/45/18 front and 255/40/18 rear, again all the same diameter. I don't know what they give you in Australia.

2) Changing from 225/55-16 (which has the same diameter as 225/50-17) to 225/45-17 will change the way the speedometer reads because the diameter of the tires are different.