w202 : will larger wheels affect speed sensor

[thevtechater]

i got 18 inches rims on my 1994 202, will it affect the performance of my speed sensors?

[ScottinSoCal]

Originally Posted by thevtechater

i got 18 inches rims on my 1994 202, will it affect the performance of my speed sensors?

Yes. Your speedometer is calibrated for a certain number of revolutions per mile and that controls what speed is displayed. Going with a larger tire means a lower number of revolutions per mile, and a lower speed displayed. For example my 107 came stock with 14 inch wheels and 205/70 tires. By going with a 16 inch wheel and 235/60 tire I'd change my revolutions per mile from the stock 797 to a lower 744. When my speedometer showed me going 70 mph I'd actually be going 75 mph.

[mbdoc]

Maintaining tire circumference is important on ANY car OR truck with traction systems.

If the ABS system thinks the rear wheels are going faster than the front ones it will try to apply the brakes.

[lkchris]

Originally Posted by M.B.DOC

Maintaining tire circumference is important on ANY car OR truck with traction systems.

If the ABS system thinks the rear wheels are going faster than the front ones it will try to apply the brakes.

Make that ESP of course.

There are two different things here.

First of all, the term "sensor" is almost never used with respect to speedometer.

But, indeed, changes in rolling diameter of the wheel/tire combination WILL affect speedo accuracy.

Secondly "sensor" is in fact most often used with regards to ABS/ESP (braking/traction control) systems and changes in rolling diameter created by different tire sizes DO NOT affect performance of these systems ... AS LONG AS rolling diameter is same front-to-back and (obviously) side-to-side. Doesn't have to be same as original, just still same for every wheel--you can stagger the sizes but you ought to do it right.

[msethk]

Originally Posted by lkchris

Make that ESP of course.

There are two different things here.

First of all, the term "sensor" is almost never used with respect to speedometer.

But, indeed, changes in rolling diameter of the wheel/tire combination WILL affect speedo accuracy.

Secondly "sensor" is in fact most often used with regards to ABS/ESP (braking/traction control) systems and changes in rolling diameter created by different tire sizes DO NOT affect performance of these systems ... AS LONG AS rolling diameter is same front-to-back and (obviously) side-to-side. Doesn't have to be same as original, just still same for every wheel--you can stagger the sizes but you ought to do it right.

but does it matter in a diesel??? would you do it to a diesel??

[need2speed]

It seems that only a few staggered front/rear combinations result in the "perfect" match, such as 245/45-17 fronts and 275/40-17 rears for my E500. I've heard from others that the ABS and ASR systems will accomodate a slight difference, which seems to be confirmed by several factory AMG staggered set-ups that have up to 2% variation.

Now I wonder if the computers for these vehicles are specially tuned or is this slight variation allowed for all MB autos?

A tip for those with staggered and directional tires/rims and lowered suspensions that may be experiencing inside wear due to excessive camber. While you can't rotate, you can "flip" the tire by dismounting from the rim and installing on the other side, so that what was the inside shoulder is now on the outside of the opposite side.

[Dee8go]

Thanks for posting this. This is something I never gave a thought to. My 300TD has 17" wheels rather than the original 14" ones. Would it also be off, or do older models detect speed differently? Mine is an '84.

[hitwtom]

It depends on what size tires you have. Check here: http://www.miata.net/garage/tirecalc.html You can see what size tire will give you the same number of revolutions as the original

[Will_w202]

In sum, if you maintain the same overal diameter of the wheel/tire combo, you will be fine. You're gonna be looking at 235/35/18s or 235/40/18s. Check tirerack.com for their "plus-size" calculator.

Either way, if you have the original Sachs shocks, those things are going to bounce all the way to the top of your wheel wells when you hit a pothole, and if you go with Bilstein HDs, etc, they're going to break your teeth. Lower it with springs, and they're almost guaranteed to rub.

I have 17s with 45-series rubber, HDs, and original springs. I would be afraid of anything lower-profile. I already eat through alignments every 10k, and with 18s, those delicate steering and suspension bits are going to be beaten

[obwielnls]

The circumference of the tire is what will make your speedo read incorrectly.. As long as all four tires are the same, your abs/traction control should work fine.

[G-Benz]

Originally Posted by obwielnls

The circumference of the tire is what will make your speedo read incorrectly.. As long as all four tires are the same, your abs/traction control should work fine.

To add to that, when purchasing larger-than-OEM diameter wheels, you can maintain the OEM tire circumference simply by obtaining tires with a lower profile. The link posted by hitwtom gives you that info.

One caution though...an 18" rim typically takes a tire with an aggressive unidirectional sport tread pattern...that means you cannot rotate your tires in the usual manner as ordinary tires...if your rims are staggered (i.e., wider in the rear than front), then forget about rotating at all.

Sport tires also wear faster...don't expect 50,000 miles out of them...I'm usually going through tires every 18 months on my daily driver!

Also, low profile means increased risk for curb rash! Be careful negotiating turns on intersections!!

Here is a pic of my W124 setup. Although I went from 15" to 17", the tire profile I had to settle for is downright scary!!

[Will_w202]

"The circumference of the tire is what will make your speedo read incorrectly.. As long as all four tires are the same, your abs/traction control should work fine."

"To add to that, when purchasing larger-than-OEM diameter wheels, you can maintain the OEM tire circumference simply by obtaining tires with a lower profile. The link posted by hitwtom gives you that info."

Isn't that what I said 4 posts ago? lol

Question abundantly answered. Next thread plz

[Dee8go]

So, it's TIRE size and not WHEEL size that matters? I don't know where the sensors are located. Would they be on the rims or the wheels?

[jlomon]

Originally Posted by Dee8go

So, it's TIRE size and not WHEEL size that matters? I don't know where the sensors are located. Would they be on the rims or the wheels?

Neither. It is on the wheel hub. The ABS sensor is used, if I recall correctly. If you look on the back side of the wheel hub you'll see what basically looks like a toothed gear with a sensor reading against it. That's it.

[Johnhef]

Originally Posted by Dee8go

So, it's TIRE size and not WHEEL size that matters? I don't know where the sensors are located. Would they be on the rims or the wheels?

IIRC, no US-spec W123 had ABS. No ABS = no wheel speed sensors. Euro models however did have it as an option.

Tom, I believe I did compare the speedo on that car to GPS and it was correct up to at least 80 MPH. To compare, hold one of the original wheels next to one on the car, they're almost the exact same overall size.