It is easy to say that fuel usage increases with speed, and that wind drag is the major increasing factor.
But just how great is the effect?
Is it the same for all vehicles?
Does driving slower always save fuel?
Lets look at some hard data: page 29 in Chapter 4 of Transportation Energy Data Book: Edition 30 - 2011, published by the The Center for Transportation Analysis at the Oak Ridge National Laboratory gives some figures for a variety of vehicles sold in the USA. Most of the vehicles reach their best economy at either 50 or 55 mph, with some of the physically larger cars better at lower speeds than that, and just one (the Toyota Celica - a reasonably aerodynamic sports coupe) being most efficient at 60 mph.
If that link is unavailable, then try here, which is a summary of the various data from the same source.
Just pick out the data list you think is the most relevant for you needs.
They all point to pretty much the same conclusions, whichever you pick.
Some important conclusions can be drawn, whatever type of automobile you drive, and whichever data source you choose:
Well, engines work most efficently at a particular engine speed - find out the engine speed for your car that gives maximum torque (the manufacturer usually publishes this).
If you are in top gear, then when the engine is running at the "maximum torque" speed, that is likely to be about the best speed for you to drive for economy.
For my car, maximum torque is generated at 1750 rpm, and in top gear this works out at about 47 mph - so about 50 mph, of just a bit less.
I am lucky enough to have a fuel computer built into my car, so I can confirm that this figure of 45 to 50 mph does indeed return a good gas mileage on a run, and that the gas mileage drops off quite a bit if I venture up to around 70 mph. This speed also ties in with the overall conclusions of the data we looked at earlier.
Note that it doesn't quite work for all automobiles - some have rather low gearing to allow them to tow things or more heavier loads (think pick-up or station wagon), and some have a rather high top gear to try and give maximum economy, even though in that top gear the vehicle is pretty gutless to drive!
My automobile is quite a bit taller than average (it is not very long, and "boxy", and as a result wind drag is more at highway speeds than with a lower (more "normal") vehicle. Consequently, my best gas mileage (as measured by the onboard fuel computer) is achieved at a little less than the "maximum torque" engine speed.
Clearly the gain in engine efficiency is being outweighed by the gain in drag, and consequently, the optimum gas mileage speed for my vehicle is about 40 mph (as measured by the onboard fuel computer).
But just how great is the effect?
Is it the same for all vehicles?
Does driving slower always save fuel?
Lets look at some hard data: page 29 in Chapter 4 of Transportation Energy Data Book: Edition 30 - 2011, published by the The Center for Transportation Analysis at the Oak Ridge National Laboratory gives some figures for a variety of vehicles sold in the USA. Most of the vehicles reach their best economy at either 50 or 55 mph, with some of the physically larger cars better at lower speeds than that, and just one (the Toyota Celica - a reasonably aerodynamic sports coupe) being most efficient at 60 mph.
If that link is unavailable, then try here, which is a summary of the various data from the same source.
Just pick out the data list you think is the most relevant for you needs.
They all point to pretty much the same conclusions, whichever you pick.
Some important conclusions can be drawn, whatever type of automobile you drive, and whichever data source you choose:
- in ALL cases, fuel efficency drops off after 60 mph, and in almost all cases after 50 or 55 mph.
- driving at 75 mph rather than 65 mph results in a drop in fuel economy of between 10 and 20% (depending on vehicle), with the results averaging out at about 15%
- driving at 75 mph rather than 55 mph results in a drop in fuel economy of between 13 and 30% (depending on vehicle), with the results averaging out at about 24%
Well, engines work most efficently at a particular engine speed - find out the engine speed for your car that gives maximum torque (the manufacturer usually publishes this).
If you are in top gear, then when the engine is running at the "maximum torque" speed, that is likely to be about the best speed for you to drive for economy.
For my car, maximum torque is generated at 1750 rpm, and in top gear this works out at about 47 mph - so about 50 mph, of just a bit less.
I am lucky enough to have a fuel computer built into my car, so I can confirm that this figure of 45 to 50 mph does indeed return a good gas mileage on a run, and that the gas mileage drops off quite a bit if I venture up to around 70 mph. This speed also ties in with the overall conclusions of the data we looked at earlier.
Note that it doesn't quite work for all automobiles - some have rather low gearing to allow them to tow things or more heavier loads (think pick-up or station wagon), and some have a rather high top gear to try and give maximum economy, even though in that top gear the vehicle is pretty gutless to drive!
My automobile is quite a bit taller than average (it is not very long, and "boxy", and as a result wind drag is more at highway speeds than with a lower (more "normal") vehicle. Consequently, my best gas mileage (as measured by the onboard fuel computer) is achieved at a little less than the "maximum torque" engine speed.
Clearly the gain in engine efficiency is being outweighed by the gain in drag, and consequently, the optimum gas mileage speed for my vehicle is about 40 mph (as measured by the onboard fuel computer).
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