hasaini

Hi All,

I just bought a C300 sport sedan (RWD) and it has factory installed 17" rims on it.

They are staggered. 245-40R17 (back) & 225-45R17(front)

I am planning to get 18" all around with 225-40ZR18 size wheels.

I was told by the dealer that putting different size wheels can mess up the electronics of the car.

So my question is - Is the above 18" rim with that size tire be right for the C300?

mr inkredibul

your dealer is full of is...especially since you have different size wheels are there now. The correct size is going to be 225/40/18 for the front and 255/35/18 for the rear.

hasaini

Thanks mr inkredibul,

So I did the math and the setting of those new 18" wheel will make the tire size bigger. I.e the height. Currently I am on 17" and the dealer says that I have to maintain the same tire+rim config i.e. the circumference. I am kind of torn between this. I hope I don't mess the car up in turn voiding my warranty.

Any suggestions?

BenTrovato

Does the dealer know they sell the c300 with an 18" tire option as well?

mr inkredibul

If you ordered the 18" wheels as an option FROM THE FACTORY as many including me did, the tire size you would have been given is..

Front: 225/40/18
Rear: 255/35/18

It's even in your owner's manual. Trust me this is the right size.

hasaini

Hi All,

Would this spec of rims fit -

mr inkredibul

No I think that offset is too low.

hasaini

Hi All,

Can someone suggest a good 18" AMG style aftermarket rims that would fit the c300? I am looking to spend around $1,500.

Sportstick

Lots to choose from here:

http://www.powerwheelspro.com/shop/index.php?cPath=23

I got the 8" wide wheel with 44 offset and it is perfect. You have to click on some models to see all the offset choices.

Why spend so much?

hasaini

Hi All,

Looks like I am running most of the rims in 35mm offset size. Is that too low or not a good fit?

Sportstick

Too low, not a good fit, and wheels will stick out strangely from wheel openings.

hasaini

Hi Sportstick,

Then what is a good offset size? 45mm?

Sportstick

With the staggered set-up, the 7" rim in front has an offset of 47mm
The 7.5" rim in the back is 58mm.

When I converted to 8" rims with ET44 all around, I realized that 13mm=1", but as I understand it, half the increase of the width goes inward from the hub, half outward, so in front, .5" wider at the wheel opening side = ~6.5mm further out. 6.5mm+44mm= 50.5 versus 47 spec, so 3mm is indistinguishable to my eye.

In back, .25" wider than 7.5" spec wheel = ~3mm. 3mm+44mm=47mm, so vs. 58, 11mm further out than spec...less than an inch.

Here's what it looks like:
https://mbworld.org/forums/c-class-w...g-17-reps.html

Note especially the photos from directly behind and in front and how the wheels/tires are flush with the body.

As far as car electronics going from 17" to 18", get the Overall Diameter (O.D.) spec for your original tire, and for the tire you want to replace it with. If they are (essentially) the same, you are fine.

And, if any other forum members think I got the offset calculation wrong, PLEASE COMMENT!! I am working from my best understanding.

hasaini

Hello Once again :-) -

So would these fit -

mr inkredibul

Yeah those seem right but for me its so easy to make a mistake because its actually 11 mm equal half an inch since 2.2 cm equal one inch. Its really just easier to use google "wheel offset calculator" and use one of those. You can enter in current and new wheel size and it will tell you all the changes in the wheel dimensions in relation to your vehicle.

vpdfan

You may want to check out this site to get an idea of different tire sizes.

http://www.miata.net/garage/tirecalc.html

For example, I put in your current rear size 17" , 245/40/17 vs. the 18" factory option 255/35/18. With this change, your speedo will be 1.3% too slow (mostly insignificant). This means if you put the 18" on, at 60 mph, you will really be doing 60.8 mph.

If you have a good dealer, thay can update your ECU for the 18" option to make this slight change go away (hopefully for no charge)

mr inkredibul

That's not a big difference you don't need a reflash, again the sizes are stated in the book, and I don't think the dealer does a reflash either lol. MANY people switch their rims ALL day between sizes for winter and summer. You guys are making this WAY more complicated than it needs to be.

Again for 18" the correct size is 225/40/18 front and 255/35/18 rear.

vpdfan

Lot's of arm-chair automotive engineers on this board!