johnand

I bought a set of 21" AMG wheels for my 2014 ML350BT and also purchased GL size tires, 295/40-R21 for a little more sidewall to fill out the wheelwells a little better. I think the 30" overall diameter looks better than the 29". Anyway, I have a STAR diagnostic system and have yet to find the location where I can select an alternate tire size, or tire offset to correct speedometer. Anyone have a idea what module that is located in, and if that is even possible with a clone Xentry system, or do I need Vidiamo or similar?

Diabolis

Seeing as all wheel and tire sizes offered for any particular vehicle model have roughly the same overall diameter / circumference and the speedo (cluster) part numbers are specific to said vehicle model, I highly doubt that it's simply a matter of calibration. AFAIK there are no provisions to perform anything of the sort using any standard "Mercedes approved" tool like DAS Xentry. Possibly with Vediamo, but I highly doubt it seeing as pretty much every signal the cluster receives is already binary encoded data.

rapidoxidation

You'd think that a few lines of code written into the IC would allow quick and easy calibration of the speedo for different wheel sizes on these computerized cars. I'm going to look for that this weekend. I've just put some bigger tires on my 20" wheels, and the speedometer reads about 4% low (50 indicated is actually 52mph GPS).

Diabolis

If by bigger tires you mean wider tires, then you're supposed to decrease the profile (sidewall height) to end up with an overall tire diameter that is similar to the factory offerings. If by bigger you mean you've put on taller tires on purpose, then there is no fix. The reason is simple - any such "fix" would also necessarily affect the odometer reading, which AFAIK is illegal in every single country in the world. Tampering with the speedo = tampering with the odometer reading so I would tread carefully.

rapidoxidation

265/50/20. They're wider and taller by about an inch. The 19" rims that were on there had stock size tires so the speedo read too high for a given speed (think 50 indicated, 48 actual). "Proper" size tires, in my mind, allow perfect accuracy on the speedometer. That's chasing rainbows, in my experience. As far as "tampering with the speedo" goes, one does that with each and every tire change that has different diameters. Wouldn't it be reasonable to simply allow the given tire size to be coordinated with the speedo computer to allow accuracy? There are plenty of motorcycle "speedo healers" that intercept the incoming signal from the wheel sensor, adjust it according to pulses per mile, and feed the accurized signal to the speedometer. So far, they're still in business. A far more graceful solution would be a few lines of code accessible via scanner that allows the change of calibration according to tire size.

Potrice

Following this thread. I'm using stock tire size with the stock 19 inch wheels and my speedo is off by a wee bit (going 100 km/h on my speedo but in reality I'm going 96-97 km/h).

Diabolis

A 265/50-20 tire is too high - the proper 20" size in order to maintain the same overall tire diameter is 265/45-20.

As for speedo readings, while every single one in every singe car l've ever driven reads 2-3 km/h higher than actual on purpose, the odometer is perfectly calibrated - in other words, the speedo "scale" is slightly off (even on digital ones), but the pulse / rotation rate is spot-on. As for "speedo healers", their use is no less illegal that it is to simply disconnect the speedo cable or swap the speedo / odo with one from a junk yard with a lower reading. It produces an incorrect odometer reading, which then becomes fraud if you try to sell the vehicle and represent the mileage as accurate.

rapidoxidation

You're kind of making my argument for a built - in calibration function for me. Changing the tire size in the IC computer would allow for a more accurate odometer reading when someone chooses to (legally, or no tire installer would do it) change tire sizes on their ride. In my experience you are correct in that while speedometers read optimistic the odometer is generally accurate for stock tires. There's no reason one ought not be able to keep that built in accuracy while using different sizes by letting the computer know what's on the car.

My OEM Mercedes stock wheels and tires read differently than my OEM Mercedes stock 20's. Which one is correct? Which one is fraudulent? Wouldn't it be nice to keep the odometer accurate when I go from winter to summer wheels? Are you beginning to see my point?

By the way, I just connected my computer to the car. I found no path to changing calibration via tire size, but if someone knows of one let me know and I'll be happy to try it out. There's a little box on the DMV title that can be checked when selling that reads "odometer reading may not be accurate".

Edited to add: Here's a speedo healer setup for Mercedes, made in Germany:
https://superkilometerfilter.com/
Not cheap, but neither is a speeding ticket.

Edited again to add: I like your sig line.

Diabolis

I do see your point, but you're splitting hairs here. All other things being equal (which they are not), the speed difference between the OEM 19s (255/50-19) and the OEM 20s (265/45-20) is about 1.2% which is a normal variation / tolerance. Even tires in the same size from different manufacturers could easily vary by that amount. A tire that has a full 10/32" thread vs. a worn tire that's down to the wear bars at 2/32"s (the difference between these being a 1/4" in the radius, or 12.7mm in diameter) has more variation that the two OEM sizes. Running higher or lower tire pressures will amount to more than that, which in turn depends on whether you're just setting off in the morning when the tires are cold or after an hour of driving in the middle of a warm day, or whether you're in the car by yourself or have three passengers.

Anyway - I completely understand your point that the vast majority of speedos are inherently inaccurate (except on a Tesla where it computes the speed based on a number of factors, chief among them being derived from the GPS signal), but you also have to understand that if you are given the option to adjust the speedo, half of the population will have it set to read 50% lower so that their car mileage when they decide to sell the car will appear to be half of what it actually is. And, the moment you tick that "odometer reading may not be accurate" box or there is an odometer discrepancy on your Carfax, you've automatically lost 25% to 33% of the car's resale value if not more (or the outright sale). That's the only point I am trying to make.