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How do you calculate gas mileage

4K views 45 replies 19 participants last post by  Rick_crew 
#1 ·
Ok so at every fill up ive been recording the odometer and gallons ive pumped in. For about 5 fill ups now. My question is since there was fuel still in the truck when i did my first fill up how do i calculate this?

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#7 ·
its miles or km divided by gallons or liter put into fuel tank.

but when you fill up the tank dont put more fuel after the fuel pump stop by itself and when you fill up next too for have the same fuel into the tank for better average
 
#14 ·
A lot of variables can alter true mpg calculations. Running the AC, Speed your driving, Weight you're carrying. And unless you're running stock sized tires, you mpg won't be correct unless you've been tuned and/or had your pcm adjusted for different tire sizes. It's not rocket science, but I have been doing this since the stone age as some would say lol.
 
#17 ·
Dang, you beat me to it!

LIke mentioned above you fill up the best you can. Then drive for however long you want to. Then go back and fill up. It's a little difficult because you can't fill it exactly as it was before. But it's very close. Do your best to guess how much extra to fill after the trigger stops. You can't just pull the trigger and let it click off and quit and make it the same. The flow rate could be varied from one trip to another, or from one pump/station to another. Squeeze in as much as you can but not so much that you splash into the vents. The small amount of difference wont' give you much difference in overall readings. For example, calculate out 200 miles / 17.3 gallons and calculate 200 miles / 17.7 gallons and you'll see it's close.

Driving one day using 4 gallons is the same as driving 5 days and using 17.4 gallons. You get better accuracy with more miles driven though.

Now, as you have mentioned your tires have thrown off your odometer. I purchased Ultragauge for other reasons, but it has a calculation feature to correct for tire size. It is for display purposes, not tuned. For example, I had to input a value of like 1.04 or 1.05 to calculate properly. I used GPS over a 5 mile stretch of road with cruise control. I first set my cruise on a long flat stretch of highway. Stopped at the exit and made a calculation adjustment. Drove back along the same route and observed the changes. It took me 4 times to get it as accurate as I could.

According to Ultragauge I am consistently getting 15.4 to 15.8. I can check it during one tank or leave it go. I've had several tanks/fillups with total miles of almost 1,000 before reseting it and I am still at 15.x. I have tires about 9.6% over stock on my Z85 truck and my mileage as dropped quite a bit, I used to get around 20 on stock tires.
P.S. That light bar (or whatever it is) isn't helping your mileage any either.
 
#18 ·
Yeah, if you want to maximize fuel ecomony, minimize wind resistance and weight. Ditch the roof rack, ditch the push bar, nerf bars, toolbox, etc and make sure your tires are inflated properly.

"How" you drive makes a big difference too. I'm not telling you to granny your truck around, but I've read it's best to get to speed "briskly" when taking off and try to use your brakes as little as possible. Anytime you hit your brakes you're going to waste the momentum you just burned fuel to make.

It's part of the price you pay to drive a lifted 4x4 truck. To get that look, you make a sacrifice.
 
#19 · (Edited)
Of course driving habits are something to take into consideration. It doesn't make much different for me. I mean I don't mash the pedal when the lights turn green but I don't drive like I too old either. No matter what I do (within my own habits) my mileage is always 15.something. I more or less drive around town with a 3 mile (at most) trip down the highway. I get on the highway the same all the time. Sometimes I want to have fun and I'm beyond 70 to enter the highway. Sometimes traffic won't allow that. I drvie the entire stretch (LOL) about 70, maybe more. For my truck, I could drive that route at 60 as well as piss off everybody on the on ramp and the mileage would not be different. Probably just because the distances are too short to persuade the overall numbers.

I don't mean to say habits won't affect anything. But for me in my driving, habits don't change it much for me.
 
#20 ·
The thing to realize is that you cannot measure by when the nozzle clicks off.
You must read the "gallons pumped" display on the pump and record that number, including tenths.

Anyone who assumes the nozzle will click off at the same point from one fillup to another is kidding himself, even if you use the same pump each time.
The nozzle cutoff is designed to prevent spills, and nothing else.
It is not consistent from one fillup to the next, not nearly enough to calculate gallons pumped/used.


The readout on the pump that displays amount pumped though is very accurate.
It has to be, by law.
And your state checks it once per month for accuracy, to ensure no one is getting cheated. All states do this.



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#21 ·
1. Fill up tank with gas
2. Zero trip meter
3. Drive vehicle
4. Fill up tank with gas
5. Record gallons of gas from meter at pump.
6. Record miles traveled from trip meter
7. Divide miles by gallons (ex: 300 miles /20 gallons = 15 miles / gal)
8. Zero trip meter and repeat process.
 
#22 ·
What he said, but also keep a running total of miles driven and gallons pumped.
Do the math on those totals.
Over time, you'll get far more accurate results.
 
#26 ·
You still have to fill the tank back to near full like it was when you started though.

Filling to full full then driving 200 miles and only pumping in a partial tank does no good for numbers displayed at the pump. Filling it until it clicks off then doing ones best to 'guess' where full was before is as accurate as you can get without an electronic device to read usage as it's driven. The Ultragauge and Scangauge will do that for you.
 
#31 ·
I used to use the fill-up method but unless you really keep track of stuff and fill the tank almost every time it doesn't work. Especially if you're like me and need to use your trip odometer for other purposes.

I use a free online website designed for big trucks called MyGauges.com. Uses a weighted average method so if you only partially fill your tanks it takes that into account.
 
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