Wheel Size Acceleration Question - PeachParts Mercedes-Benz Forum

Tump · #1 07-17-2003, 06:45 PM

Hi all --

I've changed my O.E. wheels and tires to wider AMG wheels and tires and am now wondering how the wheel and tire size difference may affect acceleration and speedometer accuracy.

The specfic changes made are as follows:

--- O.E. 7.5" J x17" AMG Monoblock Aero II's, 17 H2 ET 35 in the front and 8.5"J x 17 H2 ET 30 in the rear with Continental Sport Contact 225 45 ZR 17 (front) and 245 40 ZR 17 (rear) and (2 bump spring pads all around)

were changed to

--- AMG A203 400 1302 17x8.5 ET 34, Style III, Composite 5-Spoke, 2-piece alloy rims with Dunlop Sport FM901 Tires, 235/45ZR17 at all four corners, using 4 bump (17 mm) spring pads in the rear and 5 bump (21 mm) spring pads in front.

The thicker pads were installed to solve the fairly frequent rubbing at the front left wheel that I experienced when going over large bumps and dips in the road. Changing from a 2 inch overall drop to a 1.5 inch overall drop and having Benz perform a 4 wheel alignment to Europoean C43 spec's resulting in -1 deg. 30' and 31' camber in front and -2 deg. 31' and 47' camber at the rear, eliminated the rubbing problem.

Handling has improved dramatically, but because the rear tires are now about 1 inch taller than O.E. would that affect speedometer accuracy much? (if so, by how much?) and would it hamper acceleration times? --- checking with the Police installed public radar speed display and my seat of the pants meter, acceleration and speedometer accuracy seem to be about the same. But, I may not remember accurately how the car felt before the modifications.

I do apologize for my lengthy post but would greatly appreciate any help, knowledge or experience offered.

Thanks

#2 07-18-2003, 01:51 PM

Marc, per my calculations the new tire is about a half inch taller than the old ones. Any time you increase the overall diameter of a tire from stock it will change your final drive ratio. In your case with installing larger diameter tires it would be like installing a lower (numerically) rear-end gears which will make your engine turn slower at any given speed.

Having said all that junk, I doubt you will see a decrease in acceleration and at most your speedometer might read a mile or two slower than your actual speed. After all, what's a half inch between friends and tires

need2speed · #3 07-18-2003, 04:23 PM

Hi There: Your old front setup of 225/45-17 fronts would revolve 808 times per mile while your new 235/45-17 tires only revolve 796 times per mile. This is because the new tires have 1.1 inches greater circumference.

Your old rear tires were even smaller, at 2 in. less circumference than the 235/45-17 size. This would produce 816 rev/mile. No problems with your ASR/ABS using the old setup?

Regarding your question on acceleration, I think your camber probably affects this more than the change of tire sizes. Increased negative camber does have a negative effect on straight line braking and acceleration, however you've probably found out that it sure helps around the corners! :p

Tump · #4 07-18-2003, 06:35 PM

JetForeman & Need2Speed --

Thanks for your responses. They have calmed my worries about "improving" my car w/ unauthorized modifications. Handling has improved and I'm glad to know that acceleration has not suffered much if at all. It's always better in my book for the speedometer to be a little optomistic.

Need2Speed -- I didn't understand your question re: ASR/ABS.
Since changing my wheels, tires, springs and camber, ESP and ASR do not intrude nearly as often as they did before. I've not noticed any change in my ABS performance.

However, one thing that I have noticed since lowering the car that I don't like or understand is: at high speeds, while accelerating or braking very hard the car feels smooth as glass, but when the car is coasting or I apply the brakes very lightly I notice a low level vibration in the steering wheel and driver's seat. This vibration seems to be directly related to the wheels and tires. I just had the wheels meticulously balanced by my ind. shop (charged me $100) so I doubt that balance is the culprit. Could it be that since changing the alignment/camber and having driven the car way out of alignment for 7 days before having Benz realign it that the tires have developed a funny wear pattern that will take a few hundred miles to even out and eliminate the vibration?

Thanks

need2speed · #5 07-18-2003, 07:37 PM

ABS/ASR operate by detecting the relative wheel spin between the tires. Some people with staggered wheel sizes, as you had before, have experienced problems with the ABS/ASR computer if the difference between front and rear sizes was too great. If you had no problems than obviously the 1 inch difference made no difference.

As far as the vibration, with my car I've found that a "finish" balance on the front wheels is required whenever I change tires or rims. This is done by balancing the tires by spinning them on the car. Mercedes dealers have this equipment. If you remove the rims for any reason, you will need to mark or "index" the bolt hole position so you don't lose the balance.