How accurate is the guage cluster mpg meter?

[A_A_G]

Set the cruise at 60 mph (the legal speed limit here), reset the meter and almost got it to read 40 mpg! The best I ever got it to read prior was 33 mpg. I almost got run over by all the idiots doing 15+ over...

06 Altima 2.5 S 4 speed auto
Changed oil, plugs and air filter about 1k miles ago.

 

[Donald]

I have often wondered that also. Unsure how they calculate it. On a boat they have a flow meter, but do not believe they have one in a car. Thus my guess is they get info on what % of say a 10 second sample interval the fuel injectors were on, they average the MPH for the 10 seconds and they have as a fixed number the ounces of fuel the injectors would put out if they were on for a full 10 seconds. If they have that info they can calculate MPG.

 

[OriginalRocket]

Yeah I do that too when I drive my mothers navigator, it will read 19 mpg highway makes me feel better.

Last time I asked her millage she said 8.6 based on full tank and actuall miles driven. The car mpg computer read 11.1

It's a continuous average.

 

[ryansride2017]

Easy way to test that...pay for a full tank of gas and go for a Sunday drive on the interstate. Go 250 miles out and 250 back. Then tell us how accurate it is.

 

[AandPDan]

Only 40? That's nothing. I can easily get 99, the highest it will display on my 4.7 Dakota. Of course I'm going downhill at the time.

I routinely check my mileage and it's usually almost dead on with the computer.

The computer calculates based on the injector pulse width and vehicle speed. I bet it can do it a lot faster than every 10 second.

 

[L_Sludger]

AA_G, your car will easily do that in ideal conditions - namely, a pretty decent tailwind.

 

[tommygunn]

I drove a Prius as a rental, and wow, did I have some fun with that thing, tail gating behind semis down hill, I got some crazy MPG read-outs - it made for a fun and entertaining trip....and let me tell ya, it kept me AWAKE....

 

[Benzadmiral]

My mpg readout is pretty consistent in being ~10% higher, and the gallons used about 10% lower, than I get by the calculation method.

When I gas up, I check the gallons used readout, add 10%, multiply that by the price per gallon (not forgetting that .9 cent), and round up to the next $5.00 increment. That's how much I hand the clerk, and I always get change back.

 

[NateDN10]

The ECU calculates MPG using its commanded fuel injector duty cycle and the flow rate of the injectors. I'm not sure if it compensates based on closed-loop fuel adjustment; that may be a source of error.

 

[raaizin]

My 2002 Maxima was of by 4 MPG when I clocked it on a 1200 mile roadtrip. My 1996 corvette was only off by about 1/2 MPG

 

[MCompact]

The X3 and the Mazdaspeed have digital average mpg readouts and in both cases I have found my calculated mpg to consistently average about 3% lower than the mpg readout's number.

 

[Hokiefyd]

The MPG indicators on our ChryCo minivans weren't all that close. They over-estimated fuel economy by a good bit. The '03 was closer than the '07 was (to actual), but they were both off by at least 5%. The MPG indicator in our MDX is always extremely close to measured.