A hypermiling question

[Rogar]

A few years ago I got into some driving habits that I picked up from some hypermiling tips. For example, I rarely drive over 65 on the freeway and usually keep my speed closer to 60 mph. I try top coast up to stops lights and signs, shut off the engine sometimes in long traffic lights, and have been known to do just a little drafting on semi trucks.

I have been in the habit of applying sort of constant pressure on the accelerator up hill which generally results in slower speeds and then lifting off the accelerator going downhill but sometimes going a little faster to gather momentum. I've never had a vehicle with cruise control but as of last month now have a car with it. So what I'm wondering is whether I would get better mileage using cruise control or trying to keep the accelerator in the somewhat constant position that I did with my old vehicle?

[Gardenarian]

i was wondering the same thing - does cruise control really work on hilly terrain? The answer is, NO.
From Edmunds.com:

“Using cruise control can improve your gas mileage by helping you maintain a steady speed, but only if you are driving on mostly flat roads. If you are driving in hilly terrain, using cruise control typically causes your vehicle to speed up faster (to maintain the preset speed) than it would if you were operating the accelerator yourself. Before you push that cruise control button, think about the terrain ahead.”

[bae]

I think the answer will also be very specific to your particular vehicle's engine/trasmission/wheel size/tire size, your speed, and a few other odds-and-ends.

Some of my cars have a display that claims to show instantaneous fuel consumption rate, which is handy for experimenting. Also they show trip fuel consumption/mpg, so you can reset it and take a reading over your course of interest.

[SteveinMN]

Those car computers that show instantaneous mileage are notoriously inaccurate compared to measuring fuel input and calculating miles driven. I wouldn't count on them, but, then, if I'm going to the trouble to calculate it, I don't want to waste my time using suspect numbers.

Rogar, what you're doing is the way to do it -- steady pedal pressure, not steady speed.

[redfox]

I love coasting on hills (easy in Seattle) and seeing how much speed I can generate going downhill make it up the next hill. We now drive a Yaris 2 door hatch 5 speed, and it is fun to drive. I miss the mileage readout that my Insight had. I didn't realize these strategies were called hypermiling... They just seem logical!

I'd ask you to consider not drafting on trucks. I suspect it lowers their mileage, and can create air pollution from poorer combustion. Isn't it also unsafe?

[tetrimbath]

Cruise control dramatically improved my Jeep's fuel economy in and around Seattle - but - only when it can operate for more than a quarter mile, and only when it didn't result in drastic downshifts and excessive revs. As for the hills, I go up slow and keep the revs as constant as possible. Now, I have a truck (inherited) and find it is even more important to use cruise control; but I also avoid Seattle streets unless I'm making deliveries.

[Rogar]

Thanks. I suspect then that maybe cruise control is good for flat terrain and using the accelerator is best for hill country, though sometimes they are in close proximity. I know that as steady as I might try to keep pressure on the accelerator, there will be some human error that cruise control should overcome.

Also, when I was doing a little research, one source said it is more efficient to leave the car in gear on the downhill. I had often shifted into neutral on long down hills, but their theory was that the engine runs faster when idling in neutral than when it's in gear. I may have to think about that one, as it would seem to depend on how steep the hill is. I also had to remind myself of when to shut off the engine. They are saying anymore than 10 or 15 seconds at a stop and it saves gas to shut off the engine. I can see wearing out a starter at that rate, but will pay closer attention to that one.

I don't know that drafting off a semi is that great an idea and could be dangerous, depending. If I'm going about the same speed as a semi and keep a safe distance I may still follow for a few miles rather than pass.

As far as it being common sense, I think standard "common" sense is driving the speed limit or 5 mph or more over. On freeways around here the speed limit is either 65 or 75 mph. I'm usually not in that much of a hurry and can drive 5 mph or 10 mph under the speed limit without being a traffic hazard, though in heavy traffic it's probably safer to go with the flow.

[Tanglefoot]

I agree. On flat terrain, cruise control can provide a bit better mileage but on hilly terrain, the tactic you describe is called "driving with load" and it offers better mileage than the cruise control. The most fuel efficient way to drive hilly terrain is similar to a roller coaster--allow speed to bleed off at the apex of the hills and then let gravity accelerate the car on the downhill side.

I'm a fan of the long-distance draft on the highway. While a close-draft provides the greatest fuel economy benefit, I believe even tracking behind a high-profile vehicle at a safe distance (in view of at least one of the vehicle's side mirrors) allows you to drive in a less-turbulent air stream, similar to following a long distance behind a planing boat in the smoother surface wake.

I don't follow the 75 mph limits on freeways either. I try to find a slower "blocking" vehicle in the right lane (bonus if it's a high-profile vehicle that offers a long-draft) and track it at around 55-65 mph, playing with the following distance to mesh with traffic entering and exiting the freeway.

Regarding coasting in neutral, most fuel-injected vehicles exhibit a fuel cut-off when the engine is spinning faster than idle-speed with the throttle closed, such as while coasting in gear, using no fuel at all. While coasting out-of-gear, some fuel is needed to keep the engine at idle speed. You do have to consider the resistance of the engine in an in-gear coast and the fuel cost of getting back up to speed.

I believe that on flat terrain, when a long, out-of-gear coast is possible, the better ability to maintain speed in neutral may show a benefit. On a slight downgrade or if there is an anticipated stop or slow-down ahead, it probably helps to make use of the fuel cut-off by coasting in-gear.

[jp1]

I'd ask you to consider not drafting on trucks. I suspect it lowers their mileage, and can create air pollution from poorer combustion. Isn't it also unsafe?

Actually they just discussed this on cartalk a couple of weeks ago. Supposedly both vehicles benefit. The basic idea is that the two main forces of drag on a moving vehicle are the air that the vehicle has to push out of it's way in front of it and the backdraft of air swirling around behind it. When a vehicle drafts another they essentially function aerodynamically as one long vehicle. The front vehicle benefits from not having the backdraft behind it and the back vehicle benefits from not having to break through the air in front of it.

http://community.cartalk.com/discussion/2296315/drafting-front-vehicle-does-benefit

That said, you have to get pretty close to the vehicle in front of you for much effect and for obvious reasons I don't think any of us want to be driving down the road "nascar" close to the vehicle we're following.