Cruise Control

[cwell4373]

I have a 1999 Mercedes SL500 and my cruise will not stay set over 75 miles per hour. Under 75 miles per hour it works fine. Any ideas will be greatly appreciated

[JamesDean]

This might have to do with tire/wheel sizing.

The older cruise control systems only have 1 speed sensor. The newer systems have multiple sensors (usually one at each wheel) and when it starts to see differences in the speeds reported by the front/rear wheels it will disengage.

So when replacing your tires/wheels try to get them in all the same size or close.

A guy from Dubai emailed me with a similar problem. His would not set above a certain speed. He bought new tires for the front wheels. These new tires were larger then the old ones and it put the difference at like 1.5 - 2" in size...making his cruise control see different speeds and thus shutting down.

Here is an except from our email conversation (I omitted his real name of course):

Originally Posted by JamesDean

Generally speaking I do not believe changing the width of the tire will effect the cruise. The change in height is what will effect things. That and the overall tire height, which is a combination of the width/height numbers.

Let take a look here:
The front tires spec is 255/60/17 giving you a tire height of 29.05 inches
The rear tires spec is 275/55/17 giving you a tire height of 28.91 inches.

The difference is 0.14 inches with the front being taller. So ideally your new tires should maintain that same difference or as close as possible.

Your new front tires will be 265-65-17 giving you a tire height of 30.56 inches
Your current rear tires are 275-65-17 giving you a tire height of 31.07 inches

The difference here is 0.51 with the rear being taller.

Right now, you are running spec fronts and larger than spec rears, correct?

255-60-17 front at 29.05 inches
275-65-17 rear at 31.07 inches.

The difference here is 2.02 inches. This is significantly larger than the others

I think going to the 265-65-17 fronts would help being the difference down to a more acceptable level. However I am not an expert on the newer type cruise systems so I really cannot say with any certainty.

Best of Luck! I hope this helps!

and here is the results based on my advice:

Originally Posted by Dubai Guy

Dear Kris,

THANK YOU SO MUCH!!!
IT WORKED, IT WORKED, IT WORKED!!
YIPEEEEEEEEEEEEE!

I just followed what you had adviced. I went for the new front tires - 265-65-17
So the setup now is:

new front tires will be 265-65-17 giving you a tire height of 30.56 inches
current rear tires are 275-65-17 giving you a tire height of 31.07 inches
The difference here is 0.51 with the rear being taller.

This new difference of 0.51 is much smaller compared to the old 2.02 inches and so the Cruise Control Works Like a Charm!!

Kris, I tell you man, you are a genious!
I really Thank You for the great help you have rendered to me