Noob question: how to calculate mpg manually?

divide the miles driven on each tankfull by the number of gallons you pump. average all tanks by dividing total miles driven by total gallons pumped. example: 500 miles driven divided by 10 gallons = 50 mpg. the more miles you drive and fillups you do, the more accurate your mpg. all the best!

 

It is easy. If you are not doing this already, once you get your car, reset either the Trip A or Trip B to record your mileage until the next fillup. At your next fillup, record your miles driven and the gallons of gas needed to fill the tank. Divide the miles by the gallons to get miles/gallon or mpg. Compare this to the CONS mpg displayed on the MID.

It will take a few tank refills to get a good average due to the variations in fill levels, different gas pumps, temperature, etc. I record mine in an Excel spreadsheet.

My error between displayed mpg and calculated mpg averages 3.3 mpg high displayed, or about 6%.

 

Personal opinion: Do the calculation as described above the first few times you fill up. Figure out the approximate amount your calculated MPG is different than the computer's MPG. After that, just take your computer's MPG and subtract what you determined is the difference between your calculated MPG and the computer's MPG.

Maybe recalculate every 6 months or so. My reasoning is that the numbers won't change much. Do you really care that your MPG is 52.3 vs. 52.4? But you should have an idea of what your approximate MPG should be because in case of large differences, maybe there's some mechanical problem.

I calculate my MPG almost every time so that I can brag to my co-workers. Not much other purpose for me.

Mike

 

You can also figure gallons per mile (GPM) by swapping the calculations - divide the number of gallons by the miles driven. I know this might seem to be a non-issue, but it helps determine how much gas it takes to go a mile. The more gas it takes, the less MPG you get. You try to maximize MPG and minimize GPM.

If you are familiar with Excel, it is very easy to set this up as a spreadsheet that will make the calculations for you. All you have to do then is just plug in the numbers from each fill up.

 

If you happen to be metric:

Reset the trip meter at each fill-up. Follow the same behavior at each fill-up, ie: when the pump first clicks off, stop. At the next fill-up:

1. Record the kilometers travelled, as shown on the trip meter, on the gas receipt, then reset it.

2. Set up a ratio equation as follows:

liters to fill-up over kilometers travelled equals what over 100 (kms)

As an example with 35 liters at fillup and 700km travelled:

35/700 = ?/100

? is your liters/100km number. To solve the above, you "cross multiply":

? = 100*35/700

yielding

? = 5.0 liters/100km

See how much easier metric is...

Per other responders, I'm finding the car's display of mileage optimistic. In the above example the car usually tells me me something like 4.6 liters/100km.

 

I actually wouldnt mind some assistance in this area myself. Sorry, dont mean to hijack your thread, but it showed as an existing topic when I went to start a new one.
I filled 9.47 gallons a few days ago. I needed more gas today. Since the 9.47 fill and now I drove 439 miles. That part is straight forward I guess, but today it only took 8 gals. Am I missing something?

 

You have a Gen 2, which has a flexible bladder. This can complicate manual mpg calculations. There are a few threads about this on PC.

 

Doesn't work for me.
On first tank the Computer MPG was 2 more than the calculated MPG.
On second tank the calculated MPG was 2 more than the computer MPG.
It was at different gas stations though so could be a filling issue.

I calculate it every time too.

 

Again, ZitterZap, you have a Gen 2 which has a bladder which complicates things partly due to varying capacity depending on temperature. Yes, I've observed what you're talking about as well. I track averages from both the MFD and manual calculations in a spreadsheet.

Gen 3 (2010 and beyond) doesn't have the bladder anymore. DumbMike has a Gen 3.

 

The 9.47 gallons represents how much gasoline you had to replace for the previous tankful, ignore that for this tank. So your mileage is 439/8 = 54.9 mpg (not bad!).

The reason for the differing amounts is either you didn't let it get as low this time before filling up, or you didn't fill up as much. The latter could be because of temperature changes affecting the flexible bladder in the gas tank, or the pump clicked off early, etc. What I usually do is average the mileage over the last two or three tanks. Also, if I think it clicked off early, I'll fill to the second click. (You don't want to go beyond that or you could overfill the tank and that can cause expensive problems with the vapor capture equipment). Of course, it could also be because your fuel efficiency improved, for any number of reasons.

And this is a perfectly acceptable reason to continue the thread, it's not really hijacking, especially since it was dormant.

 

Thanks for pointing this out. I forgot about the bladder.

 

Wow that's awesome info. The on screen readout showed an average of around 48 for that tank. Lots of quick trips dropping my kids at school, coffee drive through, etc really killed my MPG average I thought.
I think I'm going to start doing this manually and watch and see what the numbers do

 

So i started putting my info into fueleconomy.gov. I'm a bit lower than I hoped for. I'm averaging around 44MPG for the subsequent two fill ups after my initial 54mpg one. Same route, same amount of driving, same speeds (slower maybe even). Tank still isnt wanting to take more than just over 8 gals.
Oh well, better than I was getting before I got the Prius

 

Too difficult!

Calculate litres per one km, and multiply by 100

 

What? Now that's just crazy talk. Litres are what we buy coke or pepsi in when we are having a bunch of people over. Putting coke or pepsi in our gas tank?? That's nuts.

signed, an ignant american