Have a 2021 CX9 GT. When in cruise control going through hilly areas the cruise control will speed up as you go over top of hill by 5kmh, and take a long time to slow back down. Have been back to Mazda but they cant fault here in Kalgoorlie because we have no large hills. Does anybody else have this issue. Its very annoying as at 5 kms over there's a good chance of a speeding ticket.

 

Just curious, how are you able to determine the 5 kmh difference? But you are correct, there is a small disparity in how the speed is measured. Probably a calibration issue. I've noticed this on my CX-9 as well, where my radar detector - which shows speed using GPS - is about 3 mph higher than what the speedometer shows. Fox example, speedometer shows 65 mph, radar says 68.

It's not your cruise control. This may improve as your tires wear out over time but I'm not sure by how much.

In the meantime, since you're aware of the 5 kmh difference, just set cruise control to 5 kmh lower than the speed you're aiming for just to play it safe. I really doubt this is something a Mazda dealership can fix.

 

So I set the cruise control to 110 kph and away we go at 110 kph. no problems we come to a large hill and up the hill we go doing 110 kph. Bear in mind that the car is now doing its thing to get over the hill at 110 kph. When we get to the top of the hill, the car now starts to go faster and the speed gets to 115 kph and takes about 2 to 3 kms to slow back down to 110 kph. This is not right as the car is not slowing its self down for awhile. So my thinking is a programming or software issue.

 

Yeah I missed the part you said about slowing down. It definitely shouldn't take up to 3 kms to do that. If you can provide a dealership with a video of that behavior, it would help make a better case. Best of luck in resolving that issue. 👍

 

I dont think its an issue. It uses Adaptive cruise control, not a regular one. I've noticed, i live in the US, it'll go 1-2 miles above but not more, because its not easy to adjust for hills.