Fuel Mileage Accuracy

Newer vehicles can calculate and display both instant and long term fuel mileage. When I calculate my fuel mileage using the traditional method (miles traveled divided by gallons of gas used) and compare it to the display, the traditional method is often 2-3 mpg less. Certainly “filling the tank” can vary between fill-ups, causing some error in the traditional mileage calculation but I would like to think my fill method is relatively consistent. Any thoughts on the accuracy of the electronic fuel mileage compared with the traditional method?

I use the gallons and miles driven, though it is fun seeing 60 mpg while coasting to a stop!

It is quite simple . Those read outs are not done by NASA grade instruments. They all will have a plus or minus factor when compared to actual numbers. Our Volvo in combined driving will read 1 to 3 miles high . On a long highway trip the read out will be very close.

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Years ago I used an electronic flow meter to do science experiments. I found I had to calibrate the flow meter for each experiment using the tried and true method of putting the hose into a bucket and measuring how much liquid accumulated in one minute. The flow meter apparently wasn’t able to compensate for the specific flow patterns inside the tube, that would change depending on what I was doing. The speed of liquid flowing in a tube is not constant across the tube, but tends to be slower near the wall, and faster in the middle.

Then there’s this

I haven’t done a paper calculation in probably 20 years. I’m not sure what the point is. The computer is measuring how much gas is going through the injectors. I’ve found it fairly accurate. But either way what difference does it make? It will use the same amount regardless. It’s just not something I worry about. Kinda like getting a receipt for a purchase. It doesn’t change the bank balance any different if you know or don’t know.

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I never use the MPG display in my Corolla because I find it wildly optimistic. Besides, the clock setting is much more useful.

Mileage displays don’t need to be accurate, just consistent. Any large swing in the displayed MPG can alert you to a problem or show an improvement, regardless of whether the MPG is accurate or not.

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I pay attention to my actual mileage and if it changed I’d notice. I doubt most drivers pay that much attention to the display.

Could make a difference with the Dave Ramsey budget, if there is a dollar out of place a person might be expelled from the club.

The difference between 30 and 33 MPG wouldn’t keep me awake at night but this is the sort of thing that keeps vehicle owners and service departments busy during the warranty period.

Neither of my vehicles have the instant reading. But once I compared the cumaltive with the pen and paper method. The vehicle display was 0.5 MPG higher.

The one on my Mustang consistently reads around 0.4-0.7 MPG high with the higher margin of error tending to happen more often on longer road trips (90% highway) and being more accurate with more in-town driving. You’d think it would be the other way around.

Yes, my experience has been very similar. My current car–a 2011 Outback–is my first car with an mpg readout on the dashboard, so I was initially skeptical about its accuracy. After several pen & paper calculations, I concluded that the dashboard’s readout was consistently .4 mpg higher than reality, so I became comfortable with relying solely on the car’s instrumentation.

+1
In addition to making me more aware of my driving style and how it impacts my gas mileage, I think of the MPG display as a quick & dirty diagnostic tool. Even if the CEL isn’t lit-up, a significant drop in the displayed MPG can alert the driver to a problem that needs to be attended to.

The dash on my 2017 Tucson reads consistently 7% higher than the traditional measurement.
Found that out over several fill ups.
My current tank of gas, ~200 miles in, displays 26.5 mpg at avg 16 mph.
So “real” mpg is about 24.6.
On a road trip I typically get ~33 mpg indicated, adjusts to 30.7 mpg, etc.

Calculating mpg by dividing miles / gallons is the most accurate way. However…a one time fill-up doesn’t give you a true reading. You need to do several fill-ups. The more fill-ups - the more accurate.

Example:

Fill-up 1 - 100 miles and 10 gallons = 10mpg.
Fill-up 2 - 150 miles and 11 gallons = 13mpg

Now you take the totals and calculate again.

250 / 21 = 11.9mpg.

The on-board calculations do estimates based on rpms and injector pulses and duration of pulse. Problem with that method is injectors wear over time.

My Accord EX-L with the 4-cyl and CVT gets a bit over 36 mpg in mostly commuting. The Honda measured mileage is consistently about 0.5 to 1 mph higher than dividing the miles between fillup by gallons added. Once in a while, the difference is slightly negative, and that occurs when I get gas at a station I don’t usually use. I wonder how much of the difference is due to when the gas pump shuts off during a fill.

The greatest difference I’ve noticed is my trip mileage. I get a bit over 40 mpg going to work, and low 30s going home. I can go 60 to 70 on the highway the whole trip in, but rarely get that fast going home.

The system in my Lincolns have been accurate to within point 1 MPG. That was a huge surprise to me the first time I checked it many years ago.
It is kind of fun to coast down from highway speeds with the Current MPG feature enabled and watch it get 99 MPG; which is as high was it will go. Only does it briefly around 45-50 MPH.
It’s a bit painful to watch it under acceleration as MPG tanks even during mild acceleration.