Question for Scotty...RPMs/MPH/Gear = fuel usage

[Sopher]

I got to thinking on the way to work today...
In 3rd gear, at 45mph i was at around 4500 rpm.
In 4th gear, at 65mph i was at around 6500 rpm.
etc.

Does one use MORE gas (Less efficient) when in a lower gear to go just as fast as if I were in a higher gear? Does gearing/RPM/MPH have any relationship to fuel usage?

Granted the RT runs better at 5000+rpm, I just find myself in 4th gear on most highway runs. 5th gear seems to start to bog the engine.

[Prense01]

I got to thinking on the way to work today...
In 3rd gear, at 45mph i was at around 4500 rpm.
In 4th gear, at 65mph i was at around 6500 rpm.
etc.

Does one use MORE gas (Less efficient) when in a lower gear to go just as fast as if I were in a higher gear? Does gearing/RPM/MPH have any relationship to fuel usage?

Granted the RT runs better at 5000+rpm, I just find myself in 4th gear on most highway runs. 5th gear seems to start to bog the engine.

Think of it like gears on a bicycle. It is about the use of energy to maintain the current rate of speed or accelerate from a given speed to a given speed. Fuel is the combustion source for the engine to produce energy.

For example...if you are in a low gear trying to reach and/or maintain 15 mph on a bicycle, there is minimal resistance, but you are pedaling your a$$ off! If you are trying to accelerate to 15 mph from 5 mph in a high gear, resistance is high. In both cases, more energy is being used than necessary.

Below 5k rpm, the bike seems like it is lugging some. Lugging would = less fuel efficiency and possibly damage your engine over time. Nice flat road without a strong headwind, you could probably cruise below 5k rpm and be a little more fuel efficient at the same time. The closer you can stay to your target cruising speed, the better.

Not sure if anything I said makes sense or not, and my name is not Scotty. Take it for what it's worth.

Chris :spyder:

[MouthPiece]

6500 RPM doing 65 in 4th gear? Seems a bit high to me. Seems to me that mine is usually around 5500 RPM in 4th at that speed. I'll have to check and get back with ya.

"The other Chris"

[NancysToy]

There are a lot of factors that enter into fuel mileage. If an engine was equally efficient across the entire rpm range, and there were no other factors, the lower the rpm...the higher the mileage would be. Afraid it is not that simple. For one, engine efficiency is not a linear function.

Aerodynamics plays a role, too. The more drag, the lower the mileage. More weight equals more drag, so the mileage goes down as you add cargo or passengers. More tires equal more drag, so three wheels will do worse than two. The more "slippery" the better, so a sleek car with the same weight would probably beat the Spyder in the mileage department. Wind platys a role too, of course. The worst offender is speed. As the speed increases, the drag increases exponentially. I'm not saying that at twice the speed you get one fourth the mileage...the equation is more complex than that, but that is the general idea, nonetheless.

To be honest, the worst offender in the mileage department is almost always the driver. Remember that old adage about driving like there was a raw egg between your foot and the gas pedal? The more briskly you accelerate, the worse the mileage...period!

All you can do is find what works best for you. I doubt there is a huge difference in fuel mileage between two different gears (if they are appropriate ones) going down the road at a given speed. If you get one mile per gallon worse on a tank of gas, it is only a couple of tenths of a gallon difference. For $0.50 worth of gas per tank, I don't fret about mileage. I just use the gear that will get me through the traffic or up the hills.

[Sopher]

...

All you can do is find what works best for you. I doubt there is a huge difference in fuel mileage between two different gears (if they are appropriate ones) going down the road at a given speed. If you get one mile per gallon worse on a tank of gas, it is only a couple of tenths of a gallon difference. For $0.50 worth of gas per tank, I don't fret about mileage. I just use the gear that will get me through the traffic or up the hills.


Thanks Scotty. I'm not worried at all it was just a thought while I was driving. DWT. Driving While Thinking.

[NancysToy]

Thanks Scotty. I'm not worried at all it was just a thought while I was driving. DWT. Driving While Thinking.
That's always dangerous... :roflblack:

[Prense01]

Fairly certain that weight has nothing to do with aerodynamic drag.

Chris :spyder:

[Lamonster]

Fairly certain that weight has nothing to do with aerodynamic drag.

Chris :spyder:
After doing a little Google search on that I think you're right. Here's some good reading on this.
http://www.a2wt.com/Wind%20Tunnel%20Testing.htm
http://www.hybridcarblog.com/2008/08/volts-aerodynamic-design-means-80-cent.html

[NancysToy]

Fairly certain that weight has nothing to do with aerodynamic drag.

Chris :spyder:
No, but it does affect mileage in much the same way, as does road friction. In theory, a heavy vehicle will have the same top speed as a light one, but accelerating to speed will take more horsepower with a heavier vehicle. Most folks don't have the patience or the clear distance to accelerate very slowly until the desired speed is reached, so weight becomes a factor. It also becomes a factor when trying to maintain speed uphill.

[Prense01]

No, but it does affect mileage in much the same way, as does road friction. In theory, a heavy vehicle will have the same top speed as a light one, but accelerating to speed will take more horsepower with a heavier vehicle. Most folks don't have the patience or the clear distance to accelerate very slowly until the desired speed is reached, so weight becomes a factor. It also becomes a factor when trying to maintain speed uphill.

Not arguing that one bit... Just that weight is not a factor when calculating aerodynamic drag.

Chris :spyder:

[NancysToy]

Not arguing that one bit... Just that weight is not a factor when calculating aerodynamic drag.

Chris :spyder:
Yes, I shouldn't have tried to simplify my explanation that much. Serves me right for trying a shortcut.