Acceleration/Speedo question - PeachParts Mercedes-Benz Forum

Tump · #1 07-17-2003, 07:45 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

gsxr · #2 07-23-2003, 04:06 PM

The difference should be very small. The best way to check is to borrow a friend's GPS and verify the speedo on the freeway. I bought a Garmin GPS III and tested all my cars. My one 87 was 4% low (read 96 at actual 100mph), my other 87 was dead on up to 100, and my 84 is about 2-3% low. Once you know how far it's off you can adjust your driving accordingly.

On a side note, a 235 tire is kinda narrow for an 8.5 wheel. If you just used the stock 245/40 size all around, or at least in the rear, you'd have NO problems at all. Next time you buy tires, consider that as well...

Tump · #3 07-24-2003, 01:57 PM

Dear GSXR:

Thanks for your reply. I want to keep all tires the same so that I can rotate them. I would use 245/40's at all four corners but the O.E. spec for my car specify 45 series tires in the front and 40 series tires in the rear. Why does Benz specify a shorter tire on the rear axle than the front? They do this on all Benz's with the staggered wheel set up.

If I put 40 series tires up front would that adversely affect the increased shock absorbtion needed there because of the heavy V8?

I do have the MB GPS -- how do I use it to measure actual speed. I don't see a function on it that measures distance although I know it does measure distance to calculate ETA -- I of course can measure time.

gsxr · #4 07-24-2003, 05:48 PM

Actually Marc, the 40/45 number is the ratio of the section width. Stock was 225/45, which is a 101.25mm tall sidewall, rear is 245/40 which is 98mm sidewall. While that is shorter, it's almost identical (the 3% difference isn't visible). Your new setup, 235/45, is a 105.75 sidewall.

A 40 series tire up front won't make a huge difference in shock absorbtion. I'd be very surprised if you could tell the difference, given the same brand/model tire. I also like having the same size all around to maximize tire life, btw.

I don't know if the MB GPS shows current speed or not. My handheld units will show current speed, top speed, average speed, and lots of other things. Check in the MB manual & see what is says!

Tump · #5 07-28-2003, 09:22 AM

GSXR:

Thanks so much for your information -- basically because there is only a 7.75mm difference in height between the 45 and 40 series tires, I will be sure to run 245/40's at all four corners next time I buy tires. 245/40's are 10mm wider than 235/45, right? Do you think you could post the various formula's used for calculating tire size, circumfrence, etc. from the posted tire size. Then I wouldn't have to ask the same question over & over.

Unfortuneately my MB GPS does not display mph at all -- I read the manual. But, checking my speed against a public service radar display erected by the local police at the side of the road, I seem to be going about 1 to 2 mph slower than the speed displayed on the speedo. I'll probably get a G-Tech eventually and know for sure.

gsxr · #6 07-28-2003, 11:39 AM

Actually, that's 7.75 on the sidewall, or 15.5mm total height (I think). Here's a nifty Java application for tire size computations, fun to play with:

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

Note that you can take five tires that are all the same size (say, 245/45/17) but from different manufactureres (Dunlop, Pirelli, etc) and each can have different measurements! So always check the mfr's spec sheets for the real numbers, and make sure the "measuring rim" is equal (or almost the same) width.

About the GPS, that's a bummer! The radar displays are handy to test at lower speeds. For higher speeds you'll need a nice hand held GPS. I got my Garmin GPS III for something like $125 on eBay, IIRC. Also, the G-Tech Pro doesn't tell you speeds, only 0-60 and 1/4-mile times. If you want a performance computer, the G-Tech is a POS, IMO. Check out the Race Tech AP-22, those are MUCH nicer and not much more money either. I tossed my G-Tech and love my new AP-22...

HTH,