This was a few years ago. I was driving home on I40, headed east just across the border into the Texas panhandle. It was 3AM on a Sunday, and I was having trouble finding a gas station that was open.
After the last little town that had one (closed) station, I decided to push on one more exit, but I was still worried I’d be out of gas by then (let’s say 25mi or so).
So, I tried to find the most fuel-sparing driving strategy. At 45mph, I gave it 85% throttle or so until 70mph, then turned it off, put it in neutral and allowed it to coast down to 45 again, then roll-started it in 5th and repeated. Thankfully, I made it on vapors to a station that took credit cards automatically.
So, is this the optimum method? I know that specific fuel consumption of gasoline engines is best near wide-open throttle, but I didn’t floor it because I’ve heard the computer enrichens the mixture then to keep temps down. Also, the roll start was to avoid the “auto-prime” that the computer would give upon engine start.
Incidentally, for those who almost certainly will bring up the safety angle, it was Sunday, 3AM, west Texas panhandle…I had the road to myself.
Since air resistance increases with the square of the speed, and rolling resistance is also non-lonear, you wasted a lot of fuel. Years ago we had the Mobil Gas Economy run, where drivers go extraordinary mileage from regular cars. The average speed was normally just under 50 mph. You have to look at the total resistance to motion, and the most effective performance of the car in the highest gear.
Air resistance gets serious at 60 mph, so you try to stay below that. you need to get the car in the highest gear (OD?) so that your rpm are minimal.
I had the same experience as you recently when one of my regular gas station had closed for renovations; I made it to the next one on the freeway 50 miles further “on vapors”, by driving STEADY at 55 mph with the A/C off, radio off, fan off, and window closed tightly.
Forget about “specific fuel consumption being best at W.O.T.”; that only works if you have enough load on the engine to justify that level of powere, and as pointed, out, anything over 60 mph rapidly increases fuel consumption per mile.
I suspect the most efficient would be driving as slowly as practical in the highest gear (not slowly enough to lug the engine of downshift an automatic). This will minimize the wind resistance, rolling friction, and internal engine friction while still using the highest gear ratio. I would guess this would involve a steady 40 mph or so in most cars.
I agree that “specific fuel consumption of gasoline engines is best near wide-open throttle,” but that does not mean you will get the maximum mileage under those conditions. The goal is to use the minimum power and let the engine management system worry about using the minimum fuel.
I doubt it. The actual numbers will vary for a number of factors. Back in the days of the economy runs, special built cars would run with extra small engines and they would accelerate to about 70 mph then shut down and cost to about 30. Doing this with that very small engine they could ssqueeze out about 70 mph. Later they were braking 100mpg still using similar techniques. However your car is not made the same and does not have the aerodynamics those three foot tall cars had.
I suspect the best you could have done is to cruse at 45 mph all the way.
I can remember my father and big brother laughing at me when they were talking about almost running out of gas and I asked them why they just did not go as fast as they could so they could get further before the gas ran out. I guess that is why I ended up as a math major rather than an engineer.
Joseph, there were several classes of cars in the Economy Run. The ones I was referring to were totally stock, such as the Nash Rambler, Studebaker, and other lightweight, barebones 6 cyl. cars with tall gearing and manual transmissions.
Agree with everyone else that above 55 MPH, wind resistance is the biggest sapper of fuel and energy. Anything you can do to reduce it will help.
In an EMERGENCY ONLY, I’d say your HYPOTHETICAL best bet is to find a big rig to draft about 75-100 feet back at slower speeds. Mythbusters did this a while ago and their results were something like 35% increase in fuel economy drafting a big rig at I think 75 or 50 feet… I’m not endorsing this, as that is a good way to get yourself smashed into a trailer.
That can be difficult to verify without experimental information. I might opt for a combination of ‘shut off while coasting’ and ‘accelerate to a limit(30-35mph?)’ and repeat if there’s no minimum speed requirements and/or minimum average speed requirements. If the car has a carburetor then you might need to install a fuelcut system to prevent gasoline from being sucked into the engine until the pistons come to halt after turn off the ignition. As for ‘drafting’, it would work at certain speed range but no ideas whether ‘rolling starts’ is a good idea or not.
Either way, you’d better avoid doing that. I-40, 3AM, west Texas, sounds pretty tempting for highway robberies. Good luck.
My only problem with your ‘burn-coast’ method was the 85% throttle. I agree with Mr. Meehan that dropping speed to 45 and running gentle on the throttle in the highest gear available would use less gas overall. Accelerating to 70, even in modern cars, still gets you into the range of air resistance that works against your efforts to save fuel. Keeping the RPMs as low as possible and the wind resistance down is what saves fuel. That is the reason the 55 mph speed limit was set, and overdrive transmissions are in virtually every car and truck made today.