Finally had an opportunity to calculate my MPG at the pump after filling up 7.753 gallons and traveling 285.9 miles. On-board computer calculated (and yes I know this is likely a computer estimation) 38.6 MPG, but my actual MPG was 36.9. I'm not really complaining as this is a pretty narrow margin and still great mileage, but I was wondering: has anyone else calculated their MPG the old fashioned way and also noticed a discrepancy? Thanks!

 

I did my first fill up yesterday.

Computer estimation: 36.4
Actual: 37.89

 

This to me is one of the best kept secrets in the entire auto industry. I'm convinced the algorithm that does the calculation must only look back at the last 20 miles. I wish they would just come out and tell us that the mpg is based on the last (xx) miles.

I've only bee in one vehicle ever that was accurate. A 2010 Chevy truck with the diesel. It would be to the tenth but I believe it also kept track of fuel to the tenth on the information center. It's been a while so I may be explaining that a little loosely. What I do know is that it was accurate.

 

Just found this.. Very interesting thing about ethanol and the numbers. I plan to get a full tank of pure gas very soon. I hope to see if the numbers are more accurate, as they should be.

Your Fuel Economy Gauge Is Fibbing

Quoting from the article linked above, hope this is okay to quote.

"Roger Clark, senior manager of GM's energy center, explains that the fuel economy gauge makes a calculation by counting the number and duration of pulses made by the fuel injectors as they squirt gasoline into the combustion chambers of the engine. The onboard computer system divides the distance the car travels by this estimated fuel consumption. Clark says the gauge is "dead nuts accurate" — if you consider all the variables at work during driving, including temperature, driving conditions and driving style. The biggest fluctuation occurs because ethanol, which is blended with gasoline in varying amounts, contains less energy.
"When you fill up, you are paying for a gallon of gas, but the energy in that gas varies significantly," Clark says. This means that while the car's computer assumes the gasoline is providing energy to drive a certain distance, the fuel might have less energy and not propel the car as far.
The 5.5 percent average variation in the vehicles Edmunds tested "seems like a perfectly reasonable range to me," says Paul Williamsen, national manager of the Lexus College, where his responsibilities include service training for Lexus staff, dealers and corporate personnel. "I can't imagine any reason that any automaker would want to make drivers think they can get better fuel economy than they were getting," Williamsen adds."

 

I've been calculating MPG since I was a teenager in the 1950s with various cars and motorcycles. My 2013 FIT Sport MT is the only vehicle I've had that ever estimated it for me.

With about 40 fiillups (many from the same gas pump) it is rare the the calculated figure matches the estimate. Most of the time estimated is about 1.5 MPG higher than calculated. Once the estimated was 4.3 higher. A very few estimated were lower than calculated, but never more than 1 MPG. All fillups involved more than 230 miles traveled.

Dave

 

I think the owners manual says that the average is based on the last tank and miles driven.

 

I like the explanation above that says that due to the ethanol in the gas having lower Btu content, the calculated number is always high. Don't know whether it's true or not, but I like it!

I don't have enough experience with the Fit yet to form an opinion of its accuracy, but my Subaru Forester always calculates an mpg number that's high by 0.5 to 1.5 mpg (over a range of 22 to 28 mpg). I wonder if there's an actual variation in the ethanol content of the gas I buy (almost always buy BP at the same station, same pump)?

Mostly an academic exercise here . . .

 

On my GE, I've averaged 38.3 real world MPG, according to the computer my lifetime average is 39.68.

 

So, not to hijack this thread at all, but what is the best fuel to use to get the best MPG? The manual says 87 octane or higher. Where I live I have choices of 87, 87 with ethanol, and 93. All unleaded of course. Is fuel with ethanol better/worse than fuel without ethanol? Does you receive a MPG increase for fuel with an octane rating over 87? I'm only on my second tank, but I seem to be getting about 34 mpg.

 

87 without ethanol is what I'd run in an ideal world.

 

I'm a fan of 87 and after trying each.. 87 will remain my 'go-to.' Price is my simple answer to this. What I plan to do since downloading the app... Ethanol-free gas stations in the U.S. and Canada

 

I would do 87 no ethanol if I had the option. As far as using a higher octane fuel, I did an experiment with a previous vehicle and ran the same 200 mile trip 4 times (family stuff I was dealing with required a bunch of trips in a row). I took the same route and got fuel at the same station. Most of the route was interstate. For 2 of the trips, I used 87 and for the other 2, I used 91. I did get about a mile/gallon better average (hand calculated) with the 91, but when I figured in the additional cost of the higher grade, it was a wash. I noticed no drive-ability differences between the 2.

 

I just completed my first fill up this morning. My manual calculation was 42 average. I pumped in 9.5 gallons. The computer stated avg was 40 so I beat the computer. My best trip was a 32 mile country road at 51 mpg.

 

I am glad to see this thread. My first fill on one yielded 36.7MPG actual vs 33.xx calculated. I was happy regardless for around town driving in my manual fit.

 

My actual is always less than the calculated. Usually 35-36 actual vs. 36-37.5 calculated mpg.

 

My first tank told me 36.1, but it was actually 33.8. Ethanol was involved. I guess I can't really complain for a lot of city driving.

 

After 3200 miles my actual mpg is 39.63 and the gauge usually shows 41, so it's pretty close. On my fuel log app the last car comes in at 16.76, so I'm a happy camper.

 

Why would energy content effect the gauge? I would think it would be a simple injector volume compared to miles driven calculation by the computer.


Therefore, there should be calculation differences on each fill up based on not getting the exact same fill volume but over many fill ups the gauge mpg and manual mpg should converge.

 

I filled mine this morning and got a calculated 40.2, which the car said I was getting 38.4. I'll be glad when the warm weather gets here, and my mileage gets back into the mid forties

 

My 2010 Fit fuel computer will usually say about 36 but when checking with math gallons/miles is usually 2 mpg less. I only get about 32 to 34. I drive all highway logging 80 miles a day. Speed limits are 60 and 65. This is the 5 speed automatic. I wish it would get more mpg than it does.