caza

C43 2019 19”

Hello I finally changed my run flats for some good Michelin PS4 I went for 255 40 19 for the back wheels instead of 255 35 19 for some more confort and also it was the only model available.

According to tires calculators I’m now 5% faster then what I see on the speedometer.

I want to know how can I adjust that? And is it really needed, I mean, with all the tech packed into the car there’s isn’t a way that the car adjust by itself?

My front tires stayed the same size how does it affect the equation? We need a speedometer expert here.

Also, my winter wheels are 18” so that means I need to recalibrate during winter?

superswiss

The speedometer can only be calibrated by a dealership, but with AWD you should really not increase the diameter of the tires on only one axle. This is not good for the center differential. The rear of your car now wants to go 5% faster than the front, which is putting constant strain on the AWD system.

caza

That’s concerning but I did my research about that before buying and peoples are divided some say it’s fine since the car already comes with a staggered setup and can allow 5% difference. It’s also 70% of the torque on the rear wheels.

superswiss

The staggered setup is only in the width, not the diameter. The diameters have to be as close as possible on both axles. All AWD systems have a certain tolerance even when it comes to tire wear to maintain close enough diameters. Audi for example allows up to 4/32" in tire tread depth difference, which is about a 1% difference in tire diameters. Anything greater than that can cause the AWD system to fail prematurely. I don't actually know what the tolerance is of 4Matic, but I doubt it's as high as 5%. This is about rotational speed, and not torque split. Doesn't matter how much torque is sent to each axle. It's about the speed difference between the gears on both sides of the differential. If you have constant speed differences between the front and rear axle, then the differential has to constantly be active, and that can cause it to overheat and get damaged.

caza

Thanks, looks like I will have to search for the tires again it’s a real hassle everything is out of stock except unknown brands

gabemcclintock

Just ordered PS4S in 235/40 front and 265/35 rears for my 2019 C43 Sedan.

Only 0.4% difference in diameter between front and back (26.4 vs 26.3)

https://tiresize.com/comparison/

caza

I went back to the 255 35 19 for the rear but with a different brand but to be honest I drove 2 days with the wrong tires and even if the calculations were giving me 5% higher speed I tested and my speed was normal.

stockbmw

Maybe the speed sensor is located in the transmission instead of rear differential. While rear tires were skipping along, the speed of the car was matching with the fronts instead.

superswiss

There are wheel speed sensors in each wheel and the car's speed is the average of them. BTW, ESP/TC can also get confused if you have mismatching diameters.

stockbmw

Your speedometer is definitely not the avg of individual wheel speed from abs readings. It must have speed sensor like most car on the road in the transmission.

How do I know? There are few speed traps in my route, car say 50kmh but radar say 45kmh. However, my other car is right on the dot, car says 50, radar say 50.

I am even thinking of testing if larger diameter tires will reduce this gap.

superswiss

Yes, it's the average of the wheel speed sensors. Cars of old had sensors in the transmission. Speedometers showing higher speed than actual is normal, because by law they are never allowed to show less than actual. They can be as much as 10% off.