Driving back home the other day (a good 3 hours) I got to wondering whether all the cars going over the speed limit (the those who were constantly passing me) are saving gas?
I don’t know how fuel efficiency works, but I’ve heard most cars top out at 55 mph. I’m thinking, however, that if you’re going faster than that, you’re covering more ground, and thus using less fuel. Does it balance out? Which is more efficient?
I am not going to use logic as mks57 used, but rather hard experimental evidence.
I recently bought a 05 Outback with a manual transmission that has the feature of reporting instant and average mpg.
Car/drive specifications: The car was advertised as having 25 city and 28 highway IIRC. I drive exactly 70 miles/day (35 each way) and take one of two routes. One route is all highway with an average speed of 85-90 mph (speed limit 75) while the other is half highway, half country roads with speeds of ~55 mph (more traffic and stop and go but much shorter as far as milage is concerned).
I, being somewhat of a nerd and bored from my 40 minute commute, have spent many days trying to maximize the fuel efficiency by varying my driving speeds and styles. I have found that speed does not really matter; what matters is acceleration. When I drive using my normal style (somewhat a leadfoot) I average 26.7 mpg for highway driving. Being as careful as possible (and we are talking pretting anal here, I am a scientist and I do work in a lab for a living) I can increase this to a maximum of 28.5 mpg (the subaru computer has a resolution of 0.3 mpg). To get this value it does not seem to matter how fast I go, but I do need to go 0 to 60 (85) in as long a time as possible, just barely touching the gas pedal (actually I accelerate as quickly as possible without letting the instant milage drop below ~30-35 mpg). This also means losing speed on hills (I drop to ~65 on my drive) and gaining speed going down (~90 mph). On the other route I find it almost impossible to get above 27.6 mpg due to all the stopping and starting.
No, pushing harder on the pedal allows more air into the engine.
That being said, theoretically, the most efficient speed for a car in terms of fuel economy is most likely to occur with the transmission in top gear and the engine speed getting up close to the flat part of the torque curve. This assumes a car with a decently low coefficient of drag. YMMV
sewalk both you and Reeder are correct.
When you press down on the gas pedal, you are opening the throttle to allow more air into the engine. BUT and this is where reeder got it right, you have to inject more fuel to maintain a correct air fuel ratio. So pressing down the gas pedal does put more fuel into the engine, and does allow more air into the engine.
The question in the OP is pretty much a no brainer. Slower and smoother saves gas. Example, I drove to to San Luis Obispo using cruise control at 75 MPH. Got 20-21 in the SUV I was driving. Not too bad considering it has the areodynamics of a barn. When I went to leave SLO I had a flat and had to mount the spacesaver spare. The spare has a restricted speed. I drove home with the cruise on at 55MPH I got 29 MPG. Also the return trip the entire SUV was full of my daughter’s stuff being moved back from college, going up it was empty.
While I’m sure it varies somewhat from car to car, I think I recall reading that maximum fuel economy usually appears in the 30-50 mph range. Go faster and you’re fighting significantly increasing wind resistance, go slower and you’re in an inefficient engine speed range.
Yay, I’ve wondered about the OP! and also about how to strategize the gas pedal going over hills, which Eyer8 has answered.
So in gratitude I dig up the San Jose Mercury News, Gary Richards’ Roadshow column, today’s “Ways to burn less gasoline and spend fewer dollars”. Summary:
Fast starts & stops can lower mileage by 33% on the highway, 5% around town.
Air conditioning, by 20%.
Proper tire inflation, 3.3% gain
Dump load: 100 pounds reduces fuel efficiency by 2%.
Check-up: air filters, radiator thermostats and sticking brake calipers by 4%
Faulty oxygen sensor repair can give 40%
Incorrect motor oil (10W-30 vs 5W-30), 2%
He doesn’t give his references and I don’t doubt some of these could be argued over, as some must depend on the circumstances. FWIW.
Not to mention, if you went from all bad on this list to all good, you’d gain 109.3%, and if you went from all good to all bad, you’d lose 109.3% of your original mileage… Of course the wording is, “can [gain or lower] mileage by”
I’m going on limited empirical evidence, since I can’t stand driving too slow; but according to my economy meter (going from recall) I get lower mileage at 30 mph than I do at 55 mph in my rather draggy Jeep Cherokee (4.0 litre six).
FWIW, it seems that 55 mph is the most efficient speed for the Jeep. I can get 25 mpg on average at that speed. I can get up to 22 mpg at 65 mph. When I reach 70 mph, mileage drops to 20-21 mpg. Faster than that, and the mileage falls off faster.
A factor to consider is the terrain. Driving uphill on the freeway yields mileage numbers (using the ‘instant’ mileage meter) in the mid-teens, while going downhill results can result in ‘99 mpg’. A feature of the Jeep’s engine seems to be that it ‘shuts off’ (or does something similar) when it is not under load. Computers do much to increase mileage!
I also noticed that my Yakima roof rack cost me 2-4 mpg, even when the accessories (kayak carriers, padding that replaced the kayak carriers, or Load Warrior basket) were not installed. The Cherokee is already draggy enough, and adding drag hurts.
There’s obviously going to be a certain point at which you’ll get the best gas mileage. At near 0 speeds, you’re getting practically no work done for the fuel you’re using. Then there’s an upper limit where the engine starts straining and you have to use a lot more fuel just to get a little more speed. The peak efficiency will be somewhere between and will depend greatly upon the engine and aerodynamics.
On the tangential topic of driving style, I remember an article about maximizing fuel economy that discussed “constant throttle” vs. “constant speed.” Constant throttle was most effective and was used by drivers in economy runs (constest comparing fuel mileage from different brands of gasoline) on closed courses. Of course, in traffic it would mean getting rear-ended when going uphill and rear-ending someone when going downhill, so for normal driving constant speed (to the degree possible) is recommended. While constant speed involves some acceleration and deceleration, especially on hills, being smooth and gentle about it works pretty well. Vigorous acceleration and vigorous braking are wasteful.
Back to the topic at hand, this site says that most cars do best in the 40-60 range, this one says 45-50, and here it says around 40.