Speedometers and Miles Per Gallon

Story copyright D. S. Brown

Measuring MPG requires an accurate odometer and speedometer. Most speedometers don't tell real distance and average speed. 

Car manufacturers usually engineer more accuracy into the odometer than the speedometer because the service intervals are tied to this number. The speedometer frequently gives high readings intentionally, especially for cars made back in the 1960s. Even modern electronic speedometers which are capable of great accuracy usually have a built in 'exaggeration factor'.

Some speedometers are even further out than the usual 5% or so. One of the craziest examples of this happened in the 1950s and 1960s when a high performance variant of a pedestrian sedan received a 140 or 160 mph speedometer. The face of the unit was changed while retaining the 120 MPH passenger car internals. They didn't even bother to recalibrate!

The speedometer of older cars used to run off the front wheel. By the sixties most cars ran the cable off a gear in the transmission. Newer speedometers are advanced electronic units capable of great accuracy. A bit of 'extra' is put in to insure the manufacturer is blameless if someone is nailed for speeding.

In older cars, the speedometer is a mechanical device. A transmission turning at a specific speed will produce different road speeds depending on variations in tire diameters and rear axles. Putting different diameter tires on a car will allow the car to run slower or faster at a set engine RPM. The mileage and speedometer indications remain tied into the factory issued tire and wheel size. The speedometer merely counts wheel revolutions, oblivious to the actual speed or distance covered. 

The same variables occur with optional high or low geared rear axles. Sometimes the car is not fitted with the correct size speedo gear in the transmission when an optional axle ratio is installed. The genuine speed will differ from indicated speed in the same ratio that the axle differs from the standard issue one accounted for by the number of teeth on the transmission speedo gear.

Speedometer problems crop up when outfitting a classic car with modern radials. Current tires have lower profile and hence smaller diameter for the same width as a comparable old bias ply tire. This will cause the speedometer to indicate 65 mph when you are really only going 61 mph. Your odometer says that you travelled 100 miles when you only covered 95 miles.

One way around this is to fit a wider tire, so that the lower profile radial has the same sidewall height as the original bias ply, creating approximately the same diameter as the original tire. Caution is needed here because the extra gripping power and unsprung weight of wider tires creates strain on spindles, bearings and other suspension components.

If you want to know exact, true MPG you need to correct the speedometer and odometer. Companies can do this for a fee. A simple and free way to do this is to check the car's readings against highway mileposts. Interstates frequently make this task even easier with 'odometer check signs' sporadically posted in open stretches. Any stretch of regular mile posts will accomplish the same goal. 

The speedometer is frequently calibrated differently than the odometer. You can check the speedometer using the second hand in the car clock, or a stopwatch while driving a steady speed past mileposts. Time how long it takes to travel one mile or the full five miles of the check posts. Easiest is to go a steady indicated 60 MPH which is exactly one mile per minute. If you hold 60 for all five mileposts at the end of five miles your stopwatch should show five minutes exactly. Slow or high readings indicate how far off your speedometer is.

An easier way to do this is to use modern satellite technology. Smart phones now have GPS programs that can tell you your current speed. If you drive a few miles at a steady speed you can compare your speedo reading to the GPS result.