GAU8mech

Originally had AMG 18" staggered wheels. I just put on 19" replicas staggered as well. I wasn't sure what tire sizes to use so I worked off of the stock sizes in the door jamb. 225/45/17 front and 245/45/17 in the rear (iirc). I went up to 235/35/19 front and 255/30/19 rear. According to all the tire size calculators I've used, there is little difference circumferentially. Before the swap, cruise control worked fine. Now if I hit the switch it acts like it's seeing the cruise by slightly accelerating but then it just lets go. No warnings on the dash, nothing. What am I missing?

Plutoe

Well I know you won't believe me, but you FU when you put those large tires on the car-----you said----"there is little difference circumferentially"--do us all a favor and take a tape measure and measure the C or take the formula=3.14xR(squared)---now is there a difference between the cars C specs and the pimp tires!!

I bet there is and that's the problem---sell the pimp tires or forget CC!!

Chadzu

I'm sure you mean 2xPIxR

With a Tcu tune you can have the final drive ratio adjusted so everything works.

GAU8mech

Challenge accepted.

First and foremost, I'm trying to understand how you logic these as being pimped tires. I worked at Porsche for a while. Porsche, BMW, Audi, and even Mercedes Benz cars came through my bay with similar sized tires as stock sized. Many of these newer cars are coming with 19 inch and 20 inch wheels with low profile, extra load tires on them just like these. But whatever.

Now I measured the tires that were on the car and came up with 80.5 inches on the front and 82 inches on the rear for the circumference. The new tires are 80.1 inches on the front and 79.1 inches on the rear. Before I changed them, there was an inch and a half difference front-to-rear. Now there is only one inch difference front-to-rear, so technically they are more accurate now front-to-rear than they were before. So I doubt this is the problem. If anything, I could go up to a 35 series on the rear and be that much closer to equal front-to-rear. So I guess I can ask again if there is something I should be looking for as far as a blown fuse or something else that could be preventing the cruise control from activating and remaining on?

sick430

Can also be a speed sensor on one of the wheels

GAU8mech

Wouldn't that generate a code?

GAU8mech

Maybe I should add this little tidbit. The past few days I've had the BAS and ABS malfunction light come on. Says see workshop. If I cycle the key off and back on it goes away. It just occurred to me that this could be related.

MarcusF

The US spec OE tires (Michelin or Continental) were 225/45/17 & 245/40/17. Front to rear, the difference in revs per mile was less than 1%. Even with factory tires, the cruise could disengage at high speeds while in a curve. When the revs are off, ABS and brake assist go wacko. The 19" tires described here have a difference of 1.86%+.