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2000 Mercedes-Benz E320 4matic
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Discussion Starter #1
Ok so the car in question (2000 E320 4matic) originally had 16" wheels but the previous owner put 18" AMG wheels on it. I have always read that if you run bigger rims on a car that it is not good to use the 16" spare on the wheels that have the differential. They say the wheels will turn at different speeds and the smaller will seem like it's slipping because it's moving faster, which causes the fluids to heat up and thicken more. Supposedly this can happen pretty quickly and do major damage to the differential. Quickly meaning that if you are driving 50mph for an hour it will get very hot. They say to drive it slower and then rest it for 2 hours every hour or so. So if you drive a rear wheel drive car, they also say you should replace the spare on one of the front non-differential wheels. So if the flat was on the rear to begin with you are moving a good front tire to that rear tire and putting the smaller spare on the front wheels.

My issue is that we have a 4matic so there is a differential on both ends. If my wife gets a flat tire and uses that 16" wheel ... will it cause damage? I have an extra tire so I would just put it on a cheap 18" rim ... but I'm assuming the spare wheel hole won't hold an 18" tire on the E320. I am also assuming that the rim size doesn't matter as much as the overall tire diameter/circumference with rim. So I went to a page that shows you how much your speedometer is off when you change wheel sizes to do the math.

And I put in the stock tire info (215-55-16) and the new rims/tires (235-40-18). I was surprised to see that the overall dimensions are not very different actually.


So I thought I would post what I learned in case there is anyone else wondering if running bigger rims will mess up their car. My new wheels are only .1" bigger diameter and .4" circumference than the original wheels. Here is a link to that website so you can check your shoes out too! Notice that when you jump to a 20" rim you have to be careful with the tire size you choose so that it matches up closely to the stock size.

Tire size calculator

:D
 

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1997 E320, 1997 S320
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2,946 Posts
you are safe to drive with your 16' as a spare, take a drive to mexico and come back with the 16' spare and you will be fine. 215/55/16 is pretty much same as 235/40/18 in your case.
i like the below site better because it allows you to compare 3 different sizes to the one you have. rule of thumb is to stay within 3% difference overall diameter. Tire Size Calculator - tire & wheel plus sizing
 

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E320/E250 Bluetec Ford F350 6.7l
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36,725 Posts
Those calculators don't take actual tire into account. To be fully accurate you need to go to tire manufacturer site and read the real RPM, no the theoretical.
Looks like PO did a good job of matching correct tire size to bigger rims. They should have RPM identical when possible.
Our traction systems have some tolerance. We figure out about 3%. I drove on donut spare on ML and didn't have any problem with ESP or traction. Worse come to worse, you can turn those off.
 

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2000 Mercedes-Benz E320 4matic
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44 Posts
Discussion Starter #4
you are safe to drive with your 16' as a spare, take a drive to mexico and come back with the 16' spare and you will be fine. 215/55/16 is pretty much same as 235/40/18 in your case.
i like the below site better because it allows you to compare 3 different sizes to the one you have. rule of thumb is to stay within 3% difference overall diameter. Tire Size Calculator - tire & wheel plus sizing
Thats a good one too. I like that it shows the percentage.
 

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You are fine. In fact, I believe the AMG 18s are OEM optionals for your car. Note how close the overall diameters are? That's all you really need to consider in this case. If it were aftermarket, you'd also consider the wheel width, but in your case you can drive to wherever with no problems at all.
 
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