Cruise control might be better on flat terrain - Fuel Economy, Hypermiling, EcoModding News and Forum

abogart · 04-02-2011, 10:01 AM

So on my 170 mile trip across Michigan yesterday, I did not use the cruise control once, entirely manual throttle control for the whole trip using the MAP, RPM, MPG, and AVG gauges on the Scangauge. The western half of the state is quite hilly, so I practiced DWL on the hills, shooting for around 7.2 MAP (psi?) on level road and inclines. Neutral coasting (engine idling) down the steeper hills gave a nice boost in avg mpg.

I did find, however, that on the flatter eastern side of the state, instantaneous mpg seemed to suffer, I would try to hold about 7.0 to 7.2 on the MAP which varied the speed between 52 and 60 mph, but inst. mpg seemed to top out at about 33.7 in that load range, when I clearly remember getting 35-40 on flat terrain before. Possibly due to headwind? I experimented with rolling the window up and turning the lights on an off, but neither one had a noticeable impact on mpg whatsoever.

This trip averaged 33.1 mpg, where I was able to average 35.6 mpg before in similar conditions. The only real difference was in not using the cruise control for most of the trip. I think when I make the return trip I am going to use cruise on the flat terrain, but practice DWL and neutral coasting in the hilly areas.

ATaylorRacing · 04-03-2011, 12:39 AM

I have noticed this too....every time you hit a hill your car automatically uses more throttle to maintain the same speed.

Kodak · 04-03-2011, 01:34 PM

You're probably right. Sometimes my speed will vary a little on flat roads. Sometimes I'll aim for maybe 45mph, but find myself at 50 for a short while.

It can be tough to finesse the pedal just right, but it can be done. And wind can definitely affect your ideal throttle position.

ShadeTreeMech · 04-03-2011, 01:51 PM

On flat roads, the cruise control is amazing at maintaining a consistent speed, and therefore you get a good MPG. On the hills, it is a different matter entirely.

When drving a long distance, I do still use the cruise control, but when the hill is steep enough that the car is going to downshift, I put my foot on the accelerator where the cruise is at, cancel the cruise control, then give as much throttle as I can without a downshift. Some hills are steep enough where I let the torque convertor unlock but I still try to keep it in fourth.

Interestingly, when driving my minivan (weighs in @ 4K lbs empty) I found on the steeper hills, turning off the overdrive completely and holding a steady speed is more fuel efficient than allowing the car to slow down in fourth, then having to accelerate to maintain above 40. Below that, I find it impossible to get good gas mileage with an auto.

BTW a steep hill is one with more than a 10% grade

abogart · 04-03-2011, 10:56 PM

I have found that on flat terrain, holding a set engine load (with reference to the MAP reading) works for a while, until I get down around 52 mph or so, then it takes the same amount of load to hold the vehicle at that speed. The TC locks at a very low RPM on my car in 4th gear (around 1450 at 55 mph), anything below that speed and the engine just doesn't seem to have to torque to accelerate back up to cruising speed efficiently. Adding just a little throttle brings the inst. MPG well below where it should be, and the engine lugs at this speed as well. It seems that 55 mph is just about the minimum that the engine produces efficient torque with the TC locked.

On the other end of the spectrum, anything above 55 seems to be inefficient due to drag and resistance, as it takes a higher engine load to maintain speed here as well.

So it seems that if I want to cruise at a constant speed on level terrain, cruise control set at 55 is the way to go. I don't really think very much efficiency is lost by varying engine load up and down at a set speed. We know that engines are more efficient at high load in the lowest rpm that produces the most effective torque, so accelerating up hills in 4th gear with the TC locked should not produce any loss of efficiency. Rather, the loss is in the engine braking action due to pumping losses as the car coasts down the hill in-gear, or even adds fuel to maintain speed when none is needed. I have found that backing off the throttle and letting the TC unlock on light descending inclines gives higher numbers than trying to maintain a specific load and keep the TC locked.

Pulse & glide with the engine idling and trans. in N seems to work quite well on both hilly terrain and level terrain, especially between 60 mph and 52 mph. So my new strategy will be to practice P&G EIC on hills and when practical wherever else, but to set the cruise at 55 in traffic or when not wanting to P&G. We'll see if this does anything to my avg mpg over a few tanks.

jtbo · 04-11-2011, 01:02 PM

On relative flat area I got almost 28MPG with cruise in 1986 Pontiac full size STW with winter tires, I was pretty impressed, but I guess it does a good job when conditions don't change too much.

It had that 307 and TH200-R4 box, desert gearing they said, so it had really low rpm at 50MPH where car is limited by law here.