Wyteboii86

I want to run 305 30 20 tires. Is there a way to calibrate the Speedo in a 2012 cls550 or does it calibrate itself by gps?

Wyteboii86

??

Tbbt CLS

No, it doesn't self-adjust. You just need to know the difference on your speedometer. Do you have 4Matic?

Wyteboii86

Yes it's 4matic. So there's no way to tune the ecm to the tire size? I see people running 22 and 24 inch rims.

Tbbt CLS

With 4Matic, the front and rear tires need to have the same (or very close) circumference. That means you'd need to change the front tires too. You could damage your AWD system if you don't.

Tbbt CLS

Running non-standard tire sizes will make your speedometer read wrong. You just need to be aware of that fact and drive accordingly.

Wyteboii86

That's crazy. With any other make you can correct the Speedo in the tune.

Tbbt CLS

Check with your dealer, maybe with their computer system they could, but it will be expensive.

rick hesek

Not a big difference.
The 305/30 R20 will result in 5.2% increase, from factory 285/35 R18's, in vehicle speed vs speedo reading.
So, at 40 mph speedo reading, your actually going 42 mph.
120 mph = 126.2 mph.

285/35 R18 are 25.85" in Dia, 780.1 revs/mile.
305/30 R20 are 27.2" Dia,, 741.3 revs/mile

Wyteboii86

yeah I did the math and realized it isn't much difference. I ended up going with 295/25/21 and it's about the same.

rick hesek

Good.
Also important to consider the effects on the 4Matic dynamics when changing wheel/tire size.
The factory, front tire to rear tire, ratio is important.
What brand tires did you end up going with? 295/25 R21 are unusual size.

Your thought about GPS being able to determine vehicle engine and speed is also good. But, GPS isn't that smart.
It sees the vehicle location and movements. Not what is in the vehicle.

Wyteboii86

295/25, 255/30 front. So I'm still well within the 3% margin