speedo19

Does anyone know if you can change the tire diameter in the dash unit to calculate gas mileage, speed, distance etc? I know how to do it in the Nav but not on the dash, they do not seen to communicate with each other?
I had a 2000 (w208) 320 that got much better mileage that this 2004 (w209) 320 and the only difference I can find is the rear end ratio has been changed. The 2004 car is 350 rpm higher at 70 mph. I want to change the tire size to compensate for this. The 235/45/18 will give me a dia that gets the rpm within 25 of the 2000 320. Is there a way to change the dia the speedo uses to calc mileage and distance etc? My dealer says they do not know how......
Thanks,

Thanks,

Green E-300 DT

There is something really wrong with your calculations here my friend.

Going to that slightly larger diameter wheel/tire combination will not give
you anywheres nearly that big of a difference in rpms at any speed.

Do you know what the rear-end ratio is of each car?

If you do, I will try to explain my figures to you IF you are interested?

speedo19

Sure, I would like to know what you find!
This is what I have found out but am not sure about the car specs.
The diff ratio on the W208 was 3.07, correct me if that is not right?
The diff ration on the W209 is 3.27, correct me if that is not right?
The top gear ratio is unchanged? at .83
So, using the normal formula for rpm:
(MPH X Diff ratio (3.27) X gear ratio (.83) x constant (336) ) /
tire diameter
according to this:
At 70 Mph with a 225/50/16oem (measured 25" dia) the 208 turns 2397rpm
(I am assuming the factory used this size for the tire dia calc in the system?)
The 209 would turn 2553 at 70mph with the same tires...correct?
With the 225/45/17s (dia measured 24.5") it came with the 209 turned 2606rpm at 70mph.
So if I install 235/45/18s which have a dia of 26.3" then the 209 would be
turning 2427rpm at 70mph correct?
So the "official" rpm difference on the 209 between the 225/45/17s and
the 235/45/18s is 179rpm?
So I did stretch the numbers to get someones attention (it worked) but that seems to get me within 30 rpm of the oem 225/50/16 tire?
So far the only thing is to remember to go 5 mph slower than the limit as it is reading approx 5 mph less.
Now I have already done this and found the rears fit fine (even though the car is lowered with a PSS9 kit) the fronts are touching on the inner plastic panel) when turning more than 90 deg. If you find my "assumptions" of ratios correct above, what happens if I go to a smaller dia tire on only the front. Does the system take an average speed from all 4 wheel speed sensors? if so what is the wspd/difference tollerance in the system and will this cause the abs to activate early (on the rear) as it will see the fronts turning slightly faster than the rears? I am thinking of a 25 inch dia front.(1 inch smaller than the rears)
Anyway, we still don't know the answer to my original question, can you change the tire dia that the system uses to calc fuel mileage etc. ...is that possible? And which wheel or combo of does the speed come from?
Thanks and let me know if any of my info on the two chassis is incorrect..
Bob

Green E-300 DT

You are on the right track sort of. But I think some of your assumptions are not correct.

First off, I think both cars have the same rear-end ratios. I could be
wrong here, but both are listed on the MBZ site as having 3.07s.
MBZ has made mistakes in incorrectly listing the proper rear-end ratios on their site.
As an example, the '98 '99 E-300 DTs, listed as having 3.46s, actually have 3.07s.
You do know how to ascertain exactly what your vehicles' have don't you?

Very simple to do, but you must crawl under and rub the crude off the case. It's that simple.
It's embossed on the lower right hand corner.

According to memory and the TireRack the older car came stock with 215/55-16 stock.
The later car has 225/55-16 stock. Not that much differences in revs per mile, right?

Sounds like someone has already been changing tire sizes?

I tried 235/60-16s on the rear only on my diesel for a while to lower the revs when out on the highway.
Of course, that size is hugh. 26.9 or 772 revs per mile. My diesel loved 'em BTW.

It worked fine, but you do not want to do that IF you like your cruise control.
The CC will no longer work as the computer senses the differences
in rotations between the different size tire and the rears.

You do not know that your speedometer/odometer readings come from your right front wheel?

According to the charts available on the TireRack, these are the tire revs for these sizes:

215/55-16 814 3.07 X .83 X 814 = 2074 At 70 = 2420

225/55-16 810 3.07 X .83 X 810 = 2063 At 70 = 2408

225/45-17 829 3.07 X .83 X 829 = 2112 At 70 = 2464

235/45-18 793 3.07 X .83 X 793 = 2021 At 70 = 2357

So your correct overall rpms for these various tire sizes depends on what your rear-end ratios is.

With the 3.07s, you'll see anything from 2357 to 2464 at 70 mph depending on your tire size.
Really not that much difference is there? Barely over 100 revs difference.

Sounds to me that you are barking up the wrong tree.
Unless you want to go to the trouble of changing your rear-end ratio, you
will not see enough difference to make it worth your time and expense.

It is indeed possible to have someone change your computer to correct your
speedometer and/or odometer, but I am sure they will charge a bundle.
Better to calibrate your speedometer/odometer and simply remember
how far it is off. Renntech is expensive don't you know?

You do know how to ascertain yourself how far off your speedo/odo is?

BTW, one cannot turn the front wheel anywheres near 90 degrees.

Let me know if I skipped over anything.

Questions/comments . . .

speedo19

Well the diff case is stamped with 3.27, so the info I got was correct.
The tire size listed in the Manual is 205/55/16 (it came with 225/45/17s)basically the same dia. if 24.9". Problem is new 225/45/17s measure only 24.5".
Never use cruise control so no problem there.
I hope you are correct about the speed readings coming from the right front wheel. The WIS info sort of says it comes from different wheels and combinations thereof depending on the condition (wheel speed differences). If you are correct then putting a smaller tire (normal dia for the car) on the front my speed should be correct and the lower revs on the rear should (I will see) help the mileage a bit.

About the revs:
So the 2000 CLK with a 3.07 and 24.5 dia 225/45/17s turns 2446 revs mile @70 and the 2004 CLK with a 3.27 and 26.3 dia 235/45/18s should turn 2427 @70. (from the 2606 it turned with the 225/45/17s so 179rpm lower) If that is all correct then we mangaged to get the revs close to what they were on the 2000 clk it seems.

I use a radar gun to check my speed.

I was refering to turning the steering wheel, not the road wheel...obviously.

No cost involved, got the 18s for the same cost I sold the 17s on ebay and I needed new tires anyway.

Anyway, thanks for the help and I will see what happens.

speedo19

Well, for anyone who cares after all that it seems to have worked. My gas mileage with the 2000 CLK320 Driving a 2.5 hours trip on the freeway doing 65 to 70 mph I got over 29 MPG many times.

Now with the 2004 CLK320 I was only getting a bit over 27 MPG until this tire change.

Yesterday did a 3 hour trip doing 65 to 70 and got 29.5 MPG! a 2 MPG difference..

The final setup is fronts 225/40-17 and the rear 235/45-18s. I just need to get a accurate calibration of my distance traveled to make sure all the new results are correct. But the front tire size is real close to the original dia.