I just bought a 97 E420 it has 275/40R18 on the rear and 235/40R18 on the front. Anyone one have an idea on how this will effect anything??

 

If they're all the same diameter, everything should be okay. If not, you may have problems with ECU-controlled stability and traction controls, cruise control, and maybe ABS. I don't know if M-B's ECU programming allows for different tires diameters, but it's possible it does and that the tires sizes have been adjusted accordingly. I know that lesser car manufacturers allow this. Such adjustments would correct for any faults.

The only way I can think of to test this is to go dangerously/illegally fast with the cruise control engaged. Go around 90 mph or 100 mph or so. Maybe a little faster. If the cruise control doesn't automatically disengage, you should be okay.

The best and easiest way to check the tires diameter is by using a rolling measurement. This is a two-person job--for the convenience factor. On your driveway, make a mark on a tire at the very bottom and a make a mark directly below it on the driveway. Roll (drive) the car until the mark on the tire makes one revolution and the mark is at the bottom again and make a mark directly below it on the driveway. Measure the distance between the two marks on the driveway. Do this for each tire.

 

The tire manufacturers list rpm (round per mile) for each tire. Even with the same diameter, the tires can have different rpm.
So unless you want to listen to Lugnut and roll each wheel for a mile, better check that with manufacturer.

 

Well, you can listen to that retarded Polack if you want, but I wouldn't. He's a real simpleton.

The reason you do a rolling measurement is because the tire diameters are inconsistent. You're not actually measuring the tire's diameter. If you wanted to do that, you'd raise the car up and measure the tire. By doing a rolling measurement, you're getting the actual diameter, if you will. It accounts for the flat spot on the bottom caused by the weight of the car and the air pressure in the tire, for example.

I think the dumb *** is talking about true tire diameters. Of course, you can't use that figure. It doesn't apply when you mount the tire on a car. There's no way a tire manufacturer can predict what car you're going to mount their tire on, how much the car will weigh, and how much air pressure you run in the tires. So they have no idea what the actual diameter will be. The "round-per-mile," if there is such a thing, is a pre-mounted measurement and is useless in real life. Since it's one number, it can't be correct for all combinations of cars, how much cargo each car has, and how air pressure each tire has.

 

Yes the tires are different sizes the rear tires are about an inch taller, my question is more what will go wrong with the car, and what will not work properly.



Thanks

 

You are not saying how did you get that measurement and you don't even say what tires you have.
You need to help us help u.
For example, I was able to compare Michelins PilotĀ® XGTĀ® Z4
275/40R18 dia 26.7 rpm 780
245/40R18......25.4 ....... 814
So even when the sidewall indicates the same size, the actual diameters are quite different.
Nothing wrong with such a difference mechanically (unless tire touches the fender) but as Lugnut wrote, it very likely will screw traction control programs.
On the other hand it depends lot on the car. I remember putting spare on our ML, which is way smaller and on 70 miles drive had traction light coming on only 2 times and as I recall CC worked just fine at 60 MPH.

 

BTW take a note, that although the tire diameters in the sample are different by 5.11 %, the actual rpm change only by 4.3%. So tire diameter alone is not very liable indicator.

 

The tire's diameter is reliable if you measure it while it's on the car by rolling it. It's the ONLY reliable measurement. Why are you having a problem comphrehending this simple fact? If you use that "rpm" crap number then it isn't reliable at all because it's not meant to be used as a real-world measurement. A car's weight and the tire's air pressure nullify the usefulness of your "rpm."

 

Have you been reading the replies?

 

Sounds, like whenever Lugnut is checking his belt, or pants size he is rolling on the living room floor

Maybe for making your life easier, you should custom-order square tires for you vehicle Lugnut?