ways to calculate MPG with 2 tanks
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Mountain Pass
Joined: Jun 2024
Posts: 114
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ways to calculate MPG with 2 tanks
I think I am having trouble calculating my mpg on my truck. When i get gas, I fill one tank and run it until its almost empty then fill the other tank. I think that because the tanks are different sizes, and because I alternate filling tanks I am getting wrong calculations. Any tips on how I can properly calculate the MPG?
Also, any ideas how to calculate when I dont fill the tank? I did figure out (I think) how to calculate it when I fill it but I dont know if it will work if I dont fill it
And no, I wont just fill both tanks at the same time. I like to only fill one and use one at a time.
Also, any ideas how to calculate when I dont fill the tank? I did figure out (I think) how to calculate it when I fill it but I dont know if it will work if I dont fill it
And no, I wont just fill both tanks at the same time. I like to only fill one and use one at a time.
Last edited by Jsouz50; Dec 13, 2025 at 04:53 PM.
Fleet Owner
Joined: Jul 2006
Posts: 20,322
Likes: 789
From: Mi'kma'ki
If you're only going to use one tank at a time, keep filling the same tank. You need to fill it each time and to account for inaccuracies between fillups, you need to fill the same tank consecutively for at least 10 tanks. There's no good accurate way without filling and without consecutive fillups.
Fleet Mechanic
Joined: Oct 2025
Posts: 1,669
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fill a tank all the way and divide the number of gallons to fill the next time by how many miles.
reverse way is to drive it empty and put x number of gallons. Divide the number of miles you get by that number when it’s empty again. Ex 10 gallons and. 190 miles when it dies would be 19 mpg. You shouldn’t get an end result different between tanks. Small variation for driving habits is expected. The driving conditions need to be as close as possible (mostly highway or stop and go) for you to be able to say it’s different between a tank. 20 gallons difference between tanks won’t change mpg. You’ll just have bigger numbers to divide.
you can’t drive 120 miles on a 16 gallon tank and say it took 18 gallons to fill the other tank in your math.
reverse way is to drive it empty and put x number of gallons. Divide the number of miles you get by that number when it’s empty again. Ex 10 gallons and. 190 miles when it dies would be 19 mpg. You shouldn’t get an end result different between tanks. Small variation for driving habits is expected. The driving conditions need to be as close as possible (mostly highway or stop and go) for you to be able to say it’s different between a tank. 20 gallons difference between tanks won’t change mpg. You’ll just have bigger numbers to divide.
you can’t drive 120 miles on a 16 gallon tank and say it took 18 gallons to fill the other tank in your math.
Hotshot
Joined: Jun 2015
Posts: 12,761
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Exactly. That only works if you take the total gallon from both tank fills by total mileage for both tanks.
Cross-Country
Joined: Mar 2025
Posts: 61
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I think the easiest procedure is as follows (assuming your wheels are within ~5% of stock):
1. Fill up both tanks to full, reset trip odo
2. Switch as normal when the first tank runs low
3. Record gallons when you fill up first tank
4. Drive until second tank runs low, record/reset trip odo and switch
5. Record gallons when you fill up second tank
6. Calculate MPG
7. Repeat from step 2 onwards
I just take pictures of the odometer and pump display at each fill-up. It also helps me track mileage over time.
1. Fill up both tanks to full, reset trip odo
2. Switch as normal when the first tank runs low
3. Record gallons when you fill up first tank
4. Drive until second tank runs low, record/reset trip odo and switch
5. Record gallons when you fill up second tank
6. Calculate MPG
7. Repeat from step 2 onwards
I just take pictures of the odometer and pump display at each fill-up. It also helps me track mileage over time.
Last edited by Lepidopterodactyl; Dec 14, 2025 at 07:12 AM.
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Hotshot
Joined: Jun 2015
Posts: 12,761
Likes: 2,419
or… set trip odometer to zero. Run the first tank down toward empty, switch tanks and drive until the second tank is near empty. Fill up both tanks and divide by the mileage. 
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Cross-Country
Joined: Mar 2025
Posts: 61
Likes: 26
I'm in agreement, but it seems OP always wants a full tank on tap. In my case, it takes me about weeks to get through one tank, and I wouldn't want to leave a tank near empty for that long; but that's just me.
Hotshot
Joined: Jun 2015
Posts: 12,761
Likes: 2,419
Then your regular odometer is also not very accurate since they are driven by the same gears in the speedometer.
Frmr Ford Trans Engr
Joined: Aug 1999
Posts: 24,695
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From: SE Florida
I gave this some more thought, and there IS a way to do it.
Record the number of miles for each tank. Divide that number by the number of gallons it takes to fill the tank that you used to accumulate those miles.
For example, if you run the rear tank and go 240 miles, save that number. Now you're filling the front tank, record how many miles you drive on that tank. Say it's 250 miles. Now when you fill the rear tank again, divide 240 miles by the gallons it took to fill the rear tank. That's how much you used to cover the 240 miles. Next time that you fill the front tank, divide the 250 miles by the number of gallons it takes to fill the front tank.
Last edited by Mark Kovalsky; Dec 15, 2025 at 08:34 AM.
i ain't rite
Joined: Mar 2005
Posts: 65,421
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From: Marlboro Mental Hospital.
i never pay much attention to fuel milage in trucks. if i want a gas sipper, i drive one of the cars.
the 65 falcon with 289 4barrel engine and C-4 green dot trans gets 25 mpg if i keep my foot out of the throttle.
FTE Legend
Joined: Jun 2006
Posts: 31,930
Likes: 1,499
From: Ottawa, Ontario
This isn't rocket science it's miles driven divided by gallons used. It doesn't matter if you use 1 tank, both, or even a partial tank as long as you know how many gallons it took to go that distance. To that end first make sure your odometer is accurate, if it isn't then you're just guessing. And it doesn't matter that the two tanks are different size, that is the beauty of math it will return the same MPG number for both as long as the truck doesn't have the fuel transfer problem.
