Letters to the Editor

Myths about mileage

In letters to the editor in the past few days, the writers have claimed that the faster you drive the more fuel you use, or to put it into conventional terms your gas mileage reduces. This is one of those cases where conventional wisdom, though it seems logical, is not necessarily correct.

For example I have on several occasions measured the mpg on my van. At 70 mph it averages about 23 mpg. At 60 it averages about 20 mpg. In typical city driving, it averages about 19 mpg. Overall efficiency depends on how the engine and transmission are matched up. If the engine has excess power, (such as on the van) the thermal efficiency of the engine is low at low cruising speeds because it is throttled back. Throttling an Otto cycle engine reduces the operating effective pressure ratio. Thus, best efficiency is obtained when the engine can be operated at reduced engine RPM but at high power levels. A diesel cycle is less affected by throttling because it continues to operate at the same pressure ratio but just injects less fuel.

If your car is underpowered, reducing cruising speed might improve the mpg. To find out try cruising at several speeds and compute the mpg.

Dan Rundell


Posted in: economy

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