TOPIC: How I Calculate Miles per Gallon.

In this post I'm going to explain how I determine miles per gallon used in a vehicle and welcome any discussion this may generate.

Fuel filling basics
For most accurate results, if possible, always fill at the same gas station at the same pump. Fill at the same time of day and same temperature (This isn't always possible and is only optimal). This will remove any variability between pumps and reduce fuel expansion variability. Use the same octane.

Place the nozzle at the same depth during each fill up.

Start by filling the tank. When you do so, listen for the sound of the filling fuel to change, you can tell when it is almost full. Allow the pump to automatically turn off. Don't add any more fuel. If the nozzle is at the same location every time, you can't get more accurate.

Set your odometer to zero.

Empty your tank to some level. The more gallons used the more accurate the reading.

Re-fill the tank. Make note of odometer reading and reset odometer.

Keep a note pad or card in the vehicle to maintain a fill up log. Log in the date, odometer reading at fill up, gallons to fill it up, and your calculated mpg. The miles per gallon is calculated by dividing the odometer reading by the gallons it took to fill the vehicle. I also note anything different such as different driving terrain (city, highway, hills), mods made to engine, A/C on a lot...any variable that may affect the mpg.

When I go to report mpg's over many tanks, I drop out any tanks that had a major variable different from the others. For instance, a tank from a long highway trip will skew the average artificially higher if I don't usually drive the highway, idling for an hour will change it the other direction etc. you probably get the idea. I take the total of all the miles and divide by the total of all the gallons consumed.

Other thoughts...
Some of you have computers for calculating mpg. I still recommend you keep a log to note date, mpg and the variables between tanks.

It takes time and miles to get accurate calculations. I recommend four or five tanks before you can feel confident you have an accurate calculation. Make sure you drive the car the same before and after the modification. Some major gains can be artificially created just by driving differently.

How do you calculate miles per gallon? Any thoughts?

I am guilty of one small deviation on the fill-up with my Hyundai. Although I have been monitoring the mileage since I purchased the vehicle, When was going on regular road trips, whether short haul or long haul, the routine was the same. Making sure the tank was full for the next day by making the last stop before home to be the gas station. So I have a history on this vehicle that shows its consistency. My "bad" has always been rounding up to the nearest dollar, usually meaning a couple of 10th's of extra fuel at times. When your mpg is fairly high to begin with the extra 10th's only show as a 1-2 mpg variation. If your getting 18 and all of a sudden your getting 20... that may be the cause.

I did note to myself on the last fill that I had to stop doing that, so there was a slap on the wrist as a reminder.

If it is a vehicle that you can log the mileage regularly, the mileage should be fairly consistent even when you use only a quarter of a tank. Obviously that is a little tough when it is out of your control so the higher the miles on the tank between fill-ups is the better way to go.

Another item that I have found important with the smaller engines. Make sure your fluid levels are where they need to be. My Hyundai (1.5L), with 126K+ miles on the odometer drops 10mpg on a trip if it is a quart low on oil. That has usually been my reminder to check the oil.

It's a Groove thing...

Karl Fortner
Tacoma, Wa.

Here's how to calculate miles per gallon without a trip computer. This manual technique is as accurate as you can get.