What's the best way to manually calculate the fuel mileage? Thanks.

 

I think the best way is to divide the number of miles travelled by the amount of fuel consumed.

 

As said above, miles driven between tanks divided by the fuel put in. But bear in mind that the calculation can be and probably will be off, it isn't an exact figure. to be exact you would need to be at exately the same grade, exactely the same level in the tank. One can easily be a gallon or two off wnen gettin fuel at different locations or even different pumps not to mention the almost impossible act of hitting exactely the same level. Averaging several tanks will give you a better picture, otherwise you have an approxamation and questionable bragging rights if your number is unusually high

 

Here's some things you can to to optimize accuracy:

1. use a GPS or highway spedometer check station to check the accuracy of your odometer. remember both the odometer and the spedometer use the same input (on older cars they used the same cable) so don't be trying to hold it at 65 mph while looking at your watch. compare (with the marked mileposts) odometer miles traveled down to the nearest "half a 10th", and figure out how far off you are. that will account for tires and everything. Its pretty easy to do it in your head while you're driving: just remember these steps:

(a). find the number of 10ths off you are in 5 miles. you can estimate down to the nearest "half a 10th".

(b). multiply this answer by 2 and you have % error.

for example, if your odo turns over "one and a half" tenths less than 5 miles over that distance, then you know that your odo reads 3% slow, and that you travel 3% further than the odo says.

2. When you fill up, there will be some error in that you can't always fill to the same level. with the fuel expansion hazards are they are, I still try to top off to where I can see the fuel in the filler neck, and try to fill up to the same level every time. Even with this method you'll have some error, so several tanks may be necessary to obtain an accurate average.

3. apply the correction factor you obtained in step one, to determine actual miles traveled. For example, if you traveled 1000 miles and your odo is 3% slow, then you really traveled 1030.

4. Divide actual miles traveled by actual gallons consumed.

 

jiminey crickets, lets not forget temps, barometric pressures, humidty, true wind and the phases of the moon and Venus too.

 

[quote author=1Thom link=board=17;threadid=17651;start=0#msg165263 date=1059497058]
As said above, miles driven between tanks divided by the fuel put in. But bear in mind that the calculation can be and probably will be off, it isn't an exact figure. to be exact you would need to be at exately the same grade, exactely the same level in the tank. One can easily be a gallon or two off wnen gettin fuel at different locations or even different pumps not to mention the almost impossible act of hitting exactely the same level. Averaging several tanks will give you a better picture, otherwise you have an approxamation and questionable bragging rights if your number is unusually high
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I keep a log of the odometer reading at every fill up. I always go to the exact same station and fill up at the exact same pump....and I always fill it up to the top of the neck where it won't hold any more. Doing this I feel like I can get a pretty accurate representation of my fuel mileage. Even when I can't go to my usual spot....just filling it all the way up to the top of the neck every time will still get you pretty close.