steinjr

Previous owner of 2015 S550 put 18'' wheels all around and 245/45/R18 tires that are too small. The speedometer is about 4 mph faster than what my cell phone gps readings. This is also effecting the odometer.

Instead of 245/50/R18 all around as Mercedes recommends, I want to do mixed tires so the rears are wider like from the factory.

Are the speedometer/odometer readings taken from the front or rear tires?

Utopia Texas

Magnetic pickup from driveshaft ?

steinjr

So the rear tire rotation determines the revolutions per mile?

Utopia Texas

Yes, I believe so on rear driven cars.
New cars have the option in the software to change settings for different tire heights.
https://tiresize.com/speedometer-calibration/

steinjr

Put 265/45/R18 on the rear today. They are only 5.35 revs per mile different than the stock 275/35/R20s and should only be .5mph slower at 65mph.
The gps on my phone shows about 4mph slower at 65mph. Does it take the computer some time to recalculate the revs per mile or should it instant?

Mitch Alsup

Should change instantaneously.

steinjr

This is what I thought. Really freaking out about my odometer making the car look it has been driven more miles that it really has.
Commute about 2 hours a day and 1900ish mile year additional could effect resale value and warranty will run out sooner.

Utopia Texas

The computer in the car does not know that you have changed tires size. It just reads the number of revolutions of the drive shaft and calculates by the settings chosen when the car was built. Someone has to go into the computer settings and choose a new ratio so that the speedometer/odometer matches the tire size. You have to have the Mercedes Star Diagnostic System to do this. This is how it was done on my SL, the S May be different.....

steinjr

Is this something Mercedes will do, or do I need to find an independent tuner?

Utopia Texas

Use the telephone and call a few places and see what they say....

pdeacon

I went through a tire upgrade with an 2002 C230 K hatchback that I have in the fleet. It is a fun little car with 20K original miles. I wanted to upsize the tire size by going to a plus 1 setup and the use of rear spacers. I kept the same wheel size (17"). Tire rack has a tire sizing chart that gives you the outside diameter on various tire sizes and rim sizes. It appears you went from 18" rims to 20's so that is going to complicate the tire size calculation. But it can be done using the chart. You want to come as close as possible to the outside dimension of the original tire. In my case I came to within .1 mm. This had virtually no effect on my speedometer or odometer. My point is; you can probably solve your problem with a different tire size rather than going through all kinds of computer mods. If you have a rear drive car it will probably involve just two tires. Hope this helps.

steinjr

I went ahead and added the 265/45/R18 on the front as well, and the speedometer and odometer are now correct. No idea how, but all is good now.

Mitch Alsup

Conversely, you could put some huge ghetto wheels on it and preserve the warranty and milage......