How to calculate MPG / fuel economy formula
 

As surprising as it may seem to the hardcore fuel economy nuts who frequent Ecomodder, there are people who have never given much thought to the process of "manually" (ie. without benefit of a ScanGauge or other onboard device) figuring out a vehicle's fuel economy.

I had someone ask me how to do it yesterday.

After the initial shock wore off :) I came up with these instructions. Feel free to comment/add suggestions.

1) Next time you get gas, *fill* the tank. You can't calculate mileage if you're in the habit of just adding $10 or $20 worth each time. So fill until the pump clicks off once or twice.

2) Reset your trip odometer - if you have one. If you don't have one, write down your current odometer reading.

3) Drive, drive, drive...

4) Next time you need fuel: again, completely fill the tank until the pump stops. Get a receipt.

5) Write on the receipt the distance from your trip odo (or regular odometer) you went since the previous fill up.

6) Divide the distance driven by the amount of fuel used. That's your fuel economy. If your odometer is in miles, and the receipt shows gallons, the resulting number is your MPG (miles per gallon).

E.G. let's say you went 360 miles and you completely refilled the tank by adding 8.7 gallons (US). 360 miles / 8.7 gallons = 41.4 MPG.

7) Keep doing this each time you get gas. Save your receipts/calculations in your glove box or wherever. After a few fill-ups you'll be able to see what's "normal" for your car (assuming you do the same type of driving for each tank), and you can start using this info as a baseline from which you can try to improve.

Might want to say something about a shortfill not being accurate and once you have the FE bug try to use the same pump for every fill.:turtle:

Good points. For "moderate" fuel economy types.

You will know you've gone hardcore when you proceed to step 8: validating your odometer's accuracy and adding a correction factor in all your MPG calculations because it drives you nuts that it was off by 1.5% :)

And I thought I was OCD. I'm just comparing to myself, basically, so I have skipped this step.

I like to do l/100km, so I just divide the other way, the number of litres by the number of kilometres (on my phone calculator), then shift the decimal point until it makes sense (two places).

The only problem with hypermiling is that it takes longer to get accurate feedback this way!

I've been doing this for a little while now (though the car was off the road for a couple of months, so there's a hole in my figures) and it's quite interesting to see the trends.

I've been using a OO spreadsheet to track distance, fuel usage, cost, time between fillings, km/L etc, as well as the differences between values. Probably too many things, really, but it is interesting.


Here's the spreadsheet. Only seven entries so far, so the averages aren't the most accurate. And the numbers are all metric, and australian dollars. But it might be a handy template for someone else.

The EV bits on the right end are based on figures I've gotten around the intarwebs for comparison - one of these years I'll convert a car to electricity and test it out myself.

The file is .ods for Open Office, I can provide an .xls version if asked. :)

Ever since i signed up here I have been writing the mileage on my receipt and stuffing it into the glove box. One day I will actually pull them all out and enter them into the fuel log... i wanted to ignore it for a while so i could get a good back-trend. Maybe its that time.

i love using excel to do all the work for me.

heres what i do:

lets start at line 5

A5 is where i enter the date

B5 is my calculated gas mileage
-enter " =D5/F5 " without the quotes

C5 is where i enter my trip OD miles for that tank

D5 is my "miles adjusted for tire size" formula (takes info from C5, its also the cell used for B5)

E5 calculates my cost per gallon (to keep from having to enter it myself every time)
-enter " =G5/F5 "

F5 is where i enter my gallons pumped

G5 is where i enter total fill-up cost

H5 calculates "cost per mile" for me.
-enter " =G5/D5 "

I5 is for notes


if you don't have mileage correction, just skip that one, and move your formulas over accordingly.

once you enter the formulas a few times (remember to change the number to whatever line you are on, and letters stay the same), excel recognizes the pattern and fills in for you.

i also did different width borders, put the mpg in bold text, and some other little things. i need to print it out and scan it as a .jpg, so i can post it up as a picture.


once its all set up, and remembers the pattern, keeping up with it is a piece of cake. i just fill-up, enter miles on the receipt, enter info into my computer, and throw the receipt into the recycle bin at work. :thumbup:



btw, the "cost per mile" has been really informative, almost more than mpg. when gas prices hit their summer/fall peak, i was getting my best mpg, and it was costing me .20 a mile to drive. now that the weather has gotten colder, my mpg have fallen. but so have gas prices. now i'm paying .10 a mile

Prior to getting my scanguage (just a couple weeks ago) I had been filling the tank past when the pump clicks (as much as 1 to 1.5 gallons). Now I am using the same pump, same station and only fill until it clicks. I suspect this will more accurately represent my mpg as filling past the click could be off by as much as half gallon every time I fill depending on how much time I spend trying to get it to max fill.

The only reason I filled past the click was to get another 2-3 days of driving in before having to fill up. With only an 8 mile roundtrip commute for work the fill past click saves a stop once or twice a month. But, to keep my sg2 accurate I'm just going to the click now.

Since I got my current car 1 year ago, I also log all my fuel receipts in a spreadsheet (found elsewhere). As with the above MPG formula, consistency is key - always fillup to the same point.

So far I always forgot to write down the Current tank average on my ScanGauge before I reset it, so I can't compare its precision with the spreadsheet. Oh well, next tank, or the one after. :D

I do the same thing :o I'm always too excited to see what the error is, that I forget to write it down :p

Hello everyone, in Puerto Rico, where I live, fuel is sold by liters, instead of gallons, so I have to divide total liters purchased by 3.7854 to convert to gallons. I've been using an Excel spread sheet I prepared about 5 years ago and have been modifying since then. I have used this spread sheet for my Suzuki Swift, Jeep Cherokee and Ford F-650 Tow Truck. I am going to include the link for any of you that might be interested in it.

You simply enter the following data:
Odometer reading, liters purchased, date, Price/Liter and Fill Up Cost

Optionally: Location, Fuel Station and Remarks/Notes:

It will then calculate the followings:

Miles Driven
Galons
MPG
Average MPG
Price/Galon
Fuel Cost
Total Fuel Cost
Total Miles
Miles used so far
Miles Loged
Gallones Consumed
Cost per Miles
Overall MPG
Fuel Cost

It also has to graphs, a MPG graph and a Cost graph


Plus it has a part where it reminds you on the next tune up items like for example:
Oil & Filter
Air Filter
Spark Plugs
Ignition Cables
Coolant Level
Coolant
Brake Fluid Level
Front Brakes Check
Rear Brakes Check
Front Brakes Pads
Rear Brakes Shoes
Tire Pressure
Timing Belt Replace
Ribbed Belt Replace
M/T Fluid Level

This spread sheet is pre-filled with some information of my Suzuki Swift, remember to change it accordingly could be modified and tailored at you're convenience: Any question, feel free to ask, help this spread sheet is useful those that are starting at this, or want to keep a better, more precise and organized fuel log.

Tire size calculator Tire size calculator Axle ratio calculator Axle Ratio calculator

have a hardback field book in my glove box, keep all details on my truck, gas, oil change, etc in it.

I too kept a hard cover book in my console for each vehicle for years.

It was amazing to see the step up in MPG in May and back down in Sept./Oct every year. I asked a "carburetor engineer" that worked for Chrysler and he told me about the different formulations for winter and summer gas.

It also showed that one tank meant nothing but more than a few were harder to do under similar conditions. For 6 months I was burning about 10 gal / day and it gave me a good chance to compare a couple brands A-B-A-B etc for 10 tanks of each and found that the Silverado got 1.1 MPG better on Amoco than Speedway (YMMV).

I also noted that even with Mobil 1 and all highway driving that my mileage would begin to degrade after about 8500 miles even though the oil still looked clean and felt slick.

After I got my Impala I found that the computer's record per tank was within .7 MPG of calculated so I go lazy and quit keeping the paper record.

...been keeping log books in my cars since 1965...in every car since.

...section I = FUEL: mpg fillups
...section II = FLUIDS: oil changes; transmission fluids; coolant change, etc.
...section III = ELECTRICS: spark plugs, battery/alternator, lamps, fuses, etc.
...section IV = SERVICES: recalls, engine or transmission repairs, etc.
...section V = PARTS: what's "equivalent?" ie: GM partnumbers vs. aftermarket parts.

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calculating mpg+++++
 

Nice work. Thank you

phw

I don't know if this is "hardcore," but as there are mechanical discrepancies, as well as tire size discrepancies, and it seems many of us change tire size too (you can't really believe the size on the tire is perfectly accurate - it varies) you might want to check the accuracy of your odometer reading this easy way: Pre GPS days (2004 for me) I would sync the odo (or trip odo) with the milemarkers on the expressway when I had a long trip. Much easier to do these days with GPS. Use that adjustment when calculating mpg.

The odo on my car, with the winter tires, is 100.95% accurate. Or I'm accumulating about one extra mile on the odo per 100 driven. Pretty close, but when you want to see that exact mpg... ;)

This thread is very interesting. I am going to start keeping track of the fuel efficiency in my truck. There are variables to this due to transporting big canvas projects (increased weight/resistance) on some of my trips. Will post some of the stats when compiled. Debbott

I keep track of my fuel mileage and I like to make sure I get pretty close but some of you guys astound me at the attention to detail you give everything...

I guess in order to have that perfect combination, you need attention to detail.

Fuel Economy
 

I am on an island in the Florida Keys. Therefore, short trips reduce mpg. I am currently getting 22 mpg with a 1991 gmc sonoma pickup. When you consider huge, heavy canvas, that isn't bad. Most of my trips are under 5 miles from the shop. You are right. Every detail matters and will affect youhttp://dotable.com/images/smilies/turtle.gifr mileage.Debbott

Been doing this for a while.

17-18 mpg Discovery
34-37 mpg hyundai accent

Problems with my method?
 

This has been my method for years:
1)Fillup on the fast pump setting, clicks off
2)put on lowest setting, clicks off
3) repeat #2
4)fill until fuel visible
-Stop at anytime if fuel gushes out.

Then follow the typical miles/gal. This has worked well for me over the years over the tankful (56-64). My mgpuino is going in over the Christmas holidays that should validate my data.

Problem:
Recently, I have been using shorter trips 180-250 to separate the town and highway portions of my tank. This gave expected results in town (44.8 mpg over 230 miles) due to the car lines (like a drive thru for picking kids up at school) and high speed (75-80 mph) highway (61.5 mpg over 180 miles). Problem is my normal slow speed highway version (50-65 mph, over 180 miles), is giving me numbers that seem far too optimistic (90.4 mpg). Is there something wrong with my fill-up process?

There are settings on the fuel pump?

Oh! These are the pumps that have the three little bumps so you can go slow, medium and fast, right?

I don't think these are allowed in CA anymore, which is why I was confused about you're talking about. We have two speeds: off and on! :p

What? Really? Any idea why that might be?

Huh. When I filled my gasoline car about two weeks ago, the stops were still on the nozzle. I always take advantage of that because I like to check the oil while the fuel is pumping. Must be very recent, though I can't find anything in the news. I'll have to see what I can learn about it.

Sorry, off topic... :)

The "cruise control" is not what's been banned. The pumps used to have notches on the bottom part of the handle. For whatever reason, I can't find any articles about it though.

I do the same thing: let it pump itself, and then check my oil (and clean my windshield for good measure).

In Europe, we only have 1 setting where we can lock the pump lever these days.
If you can lock it at all, as the locking feature is often removed from pumps, so you have to squeeze it during filling.

You can manually squeeze the lever a little bit and get a slower fuel flow though.
Don't the Canadian pumps allow for that ?

A gas cap in the handle works well as a lock if the lock has been removed. I've only had to do this a couple of times in my trucks (40 & 120 gallon tank) when the lock was broken. otherwise I'm not there long enough to worry about it.