Cruise Control

[GAXIBM]

Comming out of Greenville SC towards Atlanta on I85 there are a series of low rolling hills. With the cruise set at the 65 speed limit and watching the tac, the transmission is shifting down to 5th going up the hills after the speed on the nav unit only drops one or two miles per hour. What is unusal is that going down the other side the transmission shifts down to 5th and sometimes to 4th, I guess to provide engine bracking, again with only one or two miles per hour increase. The rig is so smooth you might not notice it unless you watch the tac. I moved the transmission from D to M and it stopped the shifting. I think the cruise/transmission is two tight and wonder if it can be adjusted other than going to M. I would rather the car freewheel or stay in 6th rather than provide engine braking which in the rolling hills is just a waste of gas.

 

[hsb]

I think engine braking is generally more efficient than freewheeling.
Fuel injection is stopped during engine braking but not when the engine is freewheeling.

 

[smithsm1984]

Yep. If it's "Freewheeling" it's actually at idle, which means its dumping a bit of gas in to keep the engine going. That's not needed when in gear.

I first noticed this a while back in some modern cars with a manual - they were displaying higher estimated MPG if you leave them in gear downhill than if you just coast in neutral. But it makes sense when you think about it.

 

[hooked]

I am no auto expert and I don't play one on tv, but I have read that keeping it in gear saves more gas than coasting in neutral.. And not just for hills. If you're coming to a stop light, you go as long as you can in the higher gear before downshifting to prevent stalling.

 

[GAXIBM]

OK, I used the wrong word I should not have said freewheeling. When the car downshifts to keep the speed from rising slightly, to me it would use less gas if the speed would be allowed to increase, say 5mph in top gear, so the car could coast down to the set speed on the next hill instead of having to engage power sooner without the 5mph coast down energy. I have never seen a car shift down to limit speed in cruise. It also does not shift down and up if driven without cruse on the same hills and same speed without cruise? I like the way cruise works in M better than D and wonder if this can be adjusted.

 

[kampfire]

Yes you do use less gas when the engine is gear and not coasting at idle. However I can coast a long ways not in gear which in turn gives me better gas mileage. I do this on my 98 GTI.