How to Keep Accurate and Useful Fuel Consumption Records
Disclaimer: Direct from ipd’s Tech Tip archive! This tech tip contains information from previous publications. Products mentioned may not be available or the information may not be accurate due to changes in supply, manufacturing, or part number association. Please contact ipd Customer Support if you have further questions.
There is a great deal of misinformation about the subject of fuel mileage. Most drivers do not understand how to keep records. This article will help you to understand this subject.
Fallacies-
"I got about 30 miles to the gallon (MPG) on one tankful, but wondered why the next tank was 33 MPG, and the tank after that was 27 MPG."
"I put $7.00 worth of gas in and drove all week on half a tank of gas, so I must have gotten about 25 MPG."
"I told the service station attendant to fill it up."
"The sticker on the window of my new car said I would get 37 MPG."
Facts-
Record keeping is not a difficult thing to learn if you understand the basics and stick to a system that works for you.
You have to establish whether or not your odometer is accurate. Most are not. Our '92 240 Volvo 5-speed station wagon registers a 3% error. If you drive 100 miles, 97 are indicated.
You should fill your tank to the same level each time to get accurate results. This takes time and effort; making sure the car is level and shaking the car to "burp" the tank to get it full. Service stations generally aren't very accommodating about topping off your tank. You must keep local and trip mileage segregated. If you lump them all together, you will end up with a jumble of useless information.
You should "top off' (T.O.) the tank at the beginning and ending of trips. This establishes the MPG for the trip. When you return to "local" or short-trip driving, you will still keep records of the fuel used, but don't have to T.O each time. At the start of the next trip, you need to T.O. the tank again.
After a short period of time you should have some useful baseline information on hand. For example, 28.5 MPG on trips and 24.7 MPG on local driving.
Expect seasonal variations. Cold weather requires a longer warm-up time which uses extra fuel to bring up the engine to normal operating temperature.
Heavy A/C use can also effect economy, although the newer "rotary" compressors don't use as much horse-power as the older "piston" compressors did. Make notations such as heavy load, fast-driving or air-condition-ing to eliminate confusion with variations from normal driving.
Round gallons off to the nearest two decimal points. A sample entry for me would be:
Date Odometer Gals.
Date Odometer Gals. June 1, 94 44,936 12.78 T.O. Trip to Portland June 2, 94 45,201 8.66 T.O. Eugene
The computation for mileage would be 45,201 - 44,936 = 265 x 103% (odometer error) = 273 miles divid-ed by 8.66 gallons = 31.5 MPG. My wife and I average 31.5 - 31.7 MPG on trips regardless of who is driving.
For local driving, fuel usage is recorded at fill-ups and computed at the time we go on our next trip to give us our "local" economy.
By now, you should have some useful baseline information on both trip and local MPG. You will learn from experience how fast you can go without penalty for fuel economy. You will soon have information that will show that you may require a tune-up or other maintenance, because changes in mileage often indicate a problem.
On our 92 Volvo, the trip mileage gradually improved from 28 MPG to 31.5 over about 10,000 miles and has stabilized. Incidentally, while my wife and I have similar driving habits, our daughter is a "leadfoot" and consequently gets poor mileage.