Our cruise control works perfect, until the car goes above 55. At 56mph and up it tries to activate for a split second, but nothing. as soon as I am below 55 it works perfect.

 

This problem is common when the tires are different circumferences.

Are the tires the same size?

Are you driving with a flat tire?

Have you checked the air pressure?

 

Bingo... Amazingly enough, I just said that to my wife. We just bought it last weekend. I did notice the rear tires are bigger than the fronts (215 vs. 205). Will this effect acceleration as well?

 

.................but I have staggered tires on a 2004 Kompressor & mine works perfectly, even at 80mph!
225 45 R17 F & 245 40 R17 R denotes they cannot have the same circumference; 5mm difference, or is it too smalll to make a difference?

 

215 vs. 205 is small width difference, that shouldn't make any big rpm (rounds per mile) difference. What year and model is it?
Don't pay the attention to Lugnut. He is running for the "forum clown" title.

 

It's when there's a difference in the tire sizes from side-to-side, not front-to-back.

Don't expect any good info from a dumb Polack. That would be that Kajtek1 jackass.

 

car model please.

 

Here are two guys who had the exact same problem as you're having. There are more, but why belabor the point? The cause of the problem turned out to be different tire circumferences in both cases, so it's worth a closer check of your tires.

Case #1
Case #2

There's also an amplifier and actuator that can fail and cause the cruise control to stop working all together. This isn't speed dependent, however. There's also a switch on the brake pedal that can get dirty and cause some flakiness, but that's usually not speed dependent either.

Have you checked the air pressure of each tire? Is one tire worn more than the others?

Measure the rolling distance of each tire. One might be unusually worn and difficult to tell visually. It's a long shot, but the tire manufacturer could have mislabeled the tire size on one.

 

it is a 99 c280. the rear tires are bigger than the fronts. Fronts: Michelin Energy 205/60/15. Rear: Goodyear Integrity? 205/65/15

 

it makes the difference indeed.