Variables accounted for in GAS LOGS

Article copyright D. S. Brown

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HIGHWAY MILES is a steady speed with minimal starting or stopping. Ideal highway MPG conditions would be several hours of driving 70 mph in light traffic on a flat dry interstate.

CITY MILES indicates city driving including heavy traffic on a secondary highway with traffic lights and stops. Even an interstate may qualify if it passes through a major city amidst stop and go gridlock.

OVERALL AVERAGE is not similar to the EPA 'combined rating'. This is a real life summary of how many gallons were used over the course of all driving. The ratio of City and Hwy will vary with each individual so it's not a precise apples to apples comparison, just a real life figure for one person. When it is actually possible to know how much of each type of driving was done brackets indicate the ratio of City to Hwy. For example (C 40/ H 60) means 40% of the mileage was in the city, 60% on the highway.

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Body style, engine, carburetor or fuel injection, exhaust, transmission, differential and tires are all big factors in MPG. Items like automatic transmissions and A/C add weight and sap energy from the engine.

Older automatics were nicknamed 'slushboxes' because the method of power transfer occurs initially through a fluid coupling which causes slippage and loss of efficiency. Lock-up convertors have solved this energy loss allowing modern cars with computer controlled automatics to get better MPG than manual versions.

The age of car, how many miles it has been driven, engine rebuilds and so on affect efficiency.

Deviations from stock such as a change to dual exhaust from a factory single exhaust improves MPG (even though it adds weight). A roof rack robs a car of MPG because of wind resistance and weight.

Modern radials may improve MPG due to less rolling resistance, but the diameters available for old school 14 and 15 inch rims may be smaller than the original factory tires, canceling out some of the gains. Higher inflation pressure aids ease tire roll, too. See the story about Tires, also in the GAS LOGS.

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Brand and octane of gas affect MPG, particularly the new 10% ethanol gasoline which contains less energy and hence hurts gas mileage. Oil viscosity plays a role as does the use of synthetics. New oil formulas create less resistance to moving parts inside the engine, increasing MPG.

A car that is always parked in a heated garage will get better MPG than one left outside all the time. Cars using block heaters get a head start on the warm-up process, too.

Driving style is major. In the HIGHWAY MILES section one driver may be travelling with 4 people and 200 pounds luggage at 80 mph with the windows open, while another is running at 55 mph with just a driver and windows rolled up. One may be cruising in hot summer weather while another is slogging through frigid weather on wet roads. As many factors as possible are detailed in the descriptions of driving style to help compare each figure.