Should I change my tire size to get my cruise working again?

[davis449]

First of all, yes I did a search and found a lot of useful info. I am currently running 235/45/17 in front and 255/40/17 in the rear on my 99 C 280 Sport with AMG stlye III's staggered (switched from the stock 16's). My cruise will not set and hold above 74 MPH (and on when terrain changes it doesn't hold that very long), which in reality (checked with GPS) is about 77 MPH. This is annoying as the speed limit is 80 MPH on I-10 in West Texas where I travel a lot. I did some reading in a previous thread from about 2 years ago and saw that overall diameter is probably the culprit. Right now the difference in overall diameter between the tires I have is .30. It doesn't seem like that much Is that enough to make cruise stop working? I've noticed that the difference between a 225/45/17 and the 255/40/17 is .06, which seems MUCH closer. Should I go ahead and purchase 2 new 225/45/17's for the front and try to get my cruise working again?

This whole thing puzzles me because the difference between a 225/45/17 and a 245/40/17 (stock C 36/C 43 setup) is .25 which isn't that far off from my current setup. I guess the real question I'm asking is: Is it the new rim size I threw on or the difference in tires size that's screwing with my cruise and ASR (to a much lesser degree)?

[Denlasoul]

I would say its the tires. I run 225/40/18 and 255/35/18 and my cruise works just fine.

So you are running 235/45/17 and 255/45/17? The overall diameter between the front and rear wheels are off and most likely the reason why your cruise is off. You can go 255/40/17 with the rear tires (I havent calculated it, but i bet it's close to the fronts).

[ryhi]

yup its the tires. mine didnt work when i was running 245/40/18 all around.

[davis449]

Originally posted by Denlasoul
I would say its the tires. I run 225/40/18 and 255/35/18 and my cruise works just fine.

So you are running 235/45/17 and 255/45/17? The overall diameter between the front and rear wheels are off and most likely the reason why your cruise is off. You can go 255/40/17 with the rear tires (I havent calculated it, but i bet it's close to the fronts).

The rears are 255/40/17.

[Denlasoul]

Originally posted by davis449
The rears are 255/40/17.
Then it must be the fronts, change them to 225/45/17.

[Proven Guilty]

i would agree that it's probably related to the aspect ratio. do you have an ABS warning light on?

[davis449]

Originally posted by Proven Guilty
i would agree that it's probably related to the aspect ratio. do you have an ABS warning light on?

No ABS light, just ASR occasionally and the fact that my cruise doesn't work above 74 MPH.


Then it must be the fronts, change them to 225/45/17.

I'm assuming this as well. I'm gonna wait a month till my next long trip, by then I will have the $400 I need for the matching Michelin Pilot Sport A/S for the fronts.

Thanks for all your input guys.

[jnolte]

Originally posted by Denlasoul
Then it must be the fronts, change them to 225/45/17.


If the rears are 255/40/17

then the rears need to be 225/45/17 in order for the cruise control to work

[davis449]

Originally posted by jnolte
If the rears are 255/40/17

then the rears need to be 225/45/17 in order for the cruise control to work

I don't want to go smaller in the rear if I can avoid it. I'd rather do without cruise than sacrifice that. Plus, my rims are wider in the back, so I don't know if 225/45/17's would work in the rear.

[allirroz]

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

IMHO, I would change to the proper aspect ratio. Why have a malfunctioning ASR/ABS , Speedo, Cruise control just to attain the look you prefer and basically mess up the electronics of the car...just my 2 cents

[rman]

^^^^^^^^^^
that's my favorite tire size calculator! it's in my IE favorites

[davis449]

Originally posted by allirroz
http://www.miata.net/garage/tirecalcold.html

IMHO, I would change to the proper aspect ratio. Why have a malfunctioning ASR/ABS , Speedo, Cruise control just to attain the look you prefer and basically mess up the electronics of the car...just my 2 cents

My ABS isn't malfunctioning, and for that matter neither is my ASR. The only thing that's screwing up is the cruise. I think if I just throw 225's on the front it will solve the problem. I used that tire calculator post above and the only difference between the two is a circumference of 78.5 for the front and 78.6 for the rear yeilding a .1 difference. Everything else is EXACTLY the same so this configuration shouldn't throw anything off.

[allirroz]

yes 225/45/17 should solve all issues if your rears have the 40 series, because your initial post says 45 series
but then under your mod specs it says 40.

[davis449]

Originally posted by allirroz
yes 225/45/17 should solve all issues if your rears have the 40 series, because your initial post says 45 series
but then under your mod specs it says 40.

*fixed

Yes the rear's are 40 series.