Considering the price of gasoline today, a car’s fuel consumption is an important issue. Along with the simple economics, decaying fuel economy is also a warning that something is going awry. Some drivers keep tabs with each fill-up, others only on long trips, and others never check it. However you may choose to do it, the calculation is simple math.
For the mathematically challenged, there are numerous apps available for the iPhone that will both calculate mileage and keep a record of it. These apps range in price from free to about $6; you can pick the features you want and download the one that fits your requirements. The most basic ones will record fuel consumption and mileage, calculate mpg and chart it. Some will also issue reminders for such routine maintenance requirements as oil changes and air filter replacements. The driver’s only task is to enter the fuel quantity, cost and the odometer reading; the iPhone app takes it from there.
The fuel mileage calculation is simply the distance traveled divided by the number of gallons of fuel consumed, resulting in a miles per gallon (mpg) answer. Beyond this basic operation there are some things to be considered; some will seem so very obvious as to be insulting. However, one lady complained innocently that her car’s fuel consumption was varying wildly. One week it was 40 mpg, the next week 13 mpg! Upon reviewing her records it was clear that she was never filling the tank up, but buying gas in varying quantities. Her calculations were meaningless.
The first recommendation is important, even though it is obvious to most – fill the fuel tank up each time and calculate the mpg from fill-up to fill-up. Failure to follow this procedure, like the aforementioned lady, would give you a false number of gallons and the calculation will be wrong. The calculation will prove to be most accurate if the tank is filled to the same level and on level ground.
The second recommendation is to record the fuel consumption and mileage over several tanks of gas. This will give you a more accurate average mpg. Over the course of a few weeks you will have likely traveled on interstate highways and on city streets. It is understood that city driving consumes more fuel per mile than a run on the interstate. Averaging the fuel consumption over a long period of time yields a more useful number.
Charting your mpg is useful in detecting pending car problems. Modern, computer controlled engines tend to stay in tune longer but problems such as a dirty fuel injector can hurt mpg. Older cars, however, are more sensitive to the state of tune. Worn spark plugs or dirty air filters will hamper fuel economy; charting your fuel consumption will expose a decline in mpg and signal an inspection is warranted.
Fuel mileage is also affected by under-inflated tires, worn or failing automatic transmissions or worn differentials. Anything that hinders driveline efficiency can show up in depressed mpg.
Charting fuel mileage over a long period of time will expose some interesting patterns. A brand new car’s mileage will typically be lower during the break-in period. As wear components “seat-in” the mileage will improve. As long as the vehicle is maintained properly the mileage will settle in to an average value. Weather conditions will play a part as well; fuel enrichment during cold starts and cold weather will reduce fuel mileage to some degree. When the engine wears to a point where it is nearing the time for a rebuild it will signal its condition with declining fuel mileage, among other things.