Mercedes-Benz Forum banner

1 - 13 of 13 Posts

·
Registered
SLK 350 AMG
Joined
·
128 Posts
Discussion Starter #1
I've read several posts about people upgrading their wheels to 18's or 19's, but haven't noticed anyone replacing their tires at the same time - except for a few who opted for wider treads in order to put more rubber down on the road.

However, if you increase the rim size of the drive wheels but keep the same tire height, doesn't that mean your speedometer is going to indicate a lower road speed than the true speed that the stock setup gives you? (That could be real tough if you were pulled over doing 80 when your speedo showed you were doing only 75!) I always assumed you should maintain the same tire/wheel ratio when you increase the rim diameter on the "powered wheels".

If anyone is planning on upgrading their rims and wants to know which tire size SHOULD be fitted on the new rims in order to achieve the correct tire/wheel ratio, there's a handy-dandy calculator (and other good tire stuff) at this web site: http://www.chris-longhurst.com/carbibles/index.html?menu.html&tyre_bible.html
 

·
Registered
SLK350 (apr 15)
Joined
·
673 Posts
um ... if you change your wheel diameter, isn't it the case that you MUST change tire sizes?

I'm not sure I understand your question in that context.
 

·
Registered
SLK350
Joined
·
834 Posts
I think what Essellkay350 was trying to say is that if you upgrade your stock 17's (with 17x7.5 wrapped in 225/45/17 front and 17x8.5 wrapped in 245/40/17 rear) to something like 18x8.5F/18x9.5R or 18x7.5F/18x8.5R, nobody's really talking about tire sizes to match.

Naturally you'll want to keep overall tire diameter as close to stock diameter as possible, and that means reducing the ratio (middle number) on wider tires with bigger rims. I believe optimally for the 18x8.5F/18x9.5R configuration you're looking at 245/35/18F and 275/30/18R. Correct me if I'm wrong. I've heard that anything wider than 265's for the rear will rub.
 

·
Registered
2005 SLK55
Joined
·
664 Posts
Essellkay350 - 5/5/2005 9:19 PM

I've read several posts about people upgrading their wheels to 18's or 19's, but haven't noticed anyone replacing their tires at the same time - except for a few who opted for wider treads in order to put more rubber down on the road.
18" tires wont fit on a 19" rim. They had to change the tires.
Cheers!
 

·
Registered
06' SLK350
Joined
·
2,315 Posts
Most tire shop has the OEM tires/wheels specs so whatever you get, you will most likely get the right tire sizes. It's hard to make a mistake.
 

·
Registered
'06 SLK 350 Mars Red - '07 ML350 Black (Sport Package)
Joined
·
104 Posts
If you buy your wheel and tire package from a legit dealer they will take care of the tire size depending on your new wheels and OEM specs. There are some limitations on absolutely matching the OEM height but the difference will be to small to notice.
 

·
Registered
Volvo 940GLE, GMC Suburban, Infiniti I30, Toyota Prius, VW GTI, MB SLK350, Lexus IS250
Joined
·
1,063 Posts
bcbenz88 said:
Here's a tire calculator to show you how close different tire sizes are to your stock tire height.
If you do need to run a tire/wheel with larger circumferance, you can estimate the speed increase this way (decreases also):

Circumferenace=PI * 2R (2*radius or total diameter)

The total diameter is this:

Dia. of wheel + 2* side wall (sectional wide * profile)

So 245/40/17 has total wheel height of 627.8mm

Since PI is a constant (3.1415), the increase in speedo error is directly proportional to the increase in total diameter. For instance:

245/40/17, standard SLK rear tire diameter
245/35/18, 0% increase
245/30/19, 0% increase
255/30/19, 1% increase
265/30/19, 2% increase
245/35/19, 4% increase

These are calculated sizes without regard to fitment. Assuming you want to mount that 245/35/19. That will give you a 4% error. That is 2MPH faster at 50MPH.

The other way to check is with a GPS unit after the tires/wheel had been mounted.

I have been using a 4% oversize tire on my Suburban without taking it to have the speedo adjusted. But I keep in mind that the car is running 2-3 MPH faster at highway speeds.
 

·
Registered
Volvo 940GLE, GMC Suburban, Infiniti I30, Toyota Prius, VW GTI, MB SLK350, Lexus IS250
Joined
·
1,063 Posts
One word of caution about using larger wheel size is the the larger rim places the wheel's rotational moment further out. Similar to placing the same weight further out on a ruler held in your hand. This higher rotational moment is a torgue robber and stopping distance robber as it takes more torgue to get the wheels rolling and get the wheels stop rolling.

The factory wheel sizes are selected for balanced side wall flexure, torsional moment, total weight. and ride comfort charateristics. AM vendors will sell us what we want but seldom a better solution on the car.

That is why even F1 cars don't have the kind of low profile tires people are putting on their cars.
 

·
Premium Member
2007 SLK 280
Joined
·
66 Posts
I'd like to replace the standard 16" on my 2007 280 with 17" at least. But I've been concerned about the way a size change would change the speedometer and all the trip/mileage and other built-in calculators that are driven by speed. Since 17's are standard on the 350 (and the '09 SLK 300), shouldn't there be an adjustment to the on-board computer to accomodate the change in wheel size? Or is the "plus-1/2/3" wheel/tire guide enough to offset the change?
 

·
Registered
Joined
·
103 Posts
Or is the "plus-1/2/3" wheel/tire guide enough to offset the change?
By definition, yes. Plus 1/2/3, etc. means that increased wheel diameter + lower profile tires = the same (or close enough with only very negligible differences) overall diameter means you are plus-sizing.

If you just put on larger diameter rims and you don't offset that by reducing the tire sidewall height appropriately, then you didn't "plus size"; all you did was put on larger rims and tires, period.

FWIW, I've always used this calculator for my plus-sizing:
Tire size calculator

Havn't tried the other ones linked to in this thread; they may be fine.

I'll throw a little wrench into the works though. I don't know if MB does this or not, but I come from the VW world where many models since about 2000 have had optimistic speedometers that have read higher than the actual speed. Some VW drivers have confirmed this with their own portable GPS units, finding differences of as much as 5mph (e.g.: 70mph indicated at 65mph actual via the GPS). Also they found that it wasn't necessarily consistently off at all speeds (like at an actual 35mph the speedo read maybe 36 where at an actual 65mph it read 70mph on the speedo). If you have a GPS and can run this calculation against your stock tires now, you can see if this is the case with your SLK too. If so, you can either plus-size and keep the same settings or you can vary a little from the plus-size to force the speedometer to more accurately reflect the actual speed.

Rumor has it that VW sort of intentionally made mildly optimistic speedometer readings to ensure that there was room for some error because if the speedo reads too low, the German TÜV could fine VW.
 
1 - 13 of 13 Posts
Top