Significant discrepancy between trip computer mpg and actual mpg calculations
 

All,

I know that there is a big thread on this forum regarding the difference between the dash MPG display and the trip computer MPG display. I've been trusting the trip computer to date.

However, the last couple of tanks, I've been verifying the MPG reported by the trip computer against doing the division myself (trip computer miles divided by gallons reported on the gas pump when I fill up).

This last time, trip computer said 28.8 mpg, but dividing the trip computer miles by the gas pump gallons, I got 27.4mpg.

Has anyone else noticed this - any clever explanations for it?

Thanks,
Wes

Re: Significant discrepancy between trip computer mpg and actual mpg calculations
 

A difference like that for a single tank can be for any # of reasons. I've had a few tanks like that. It's not worth sweating over.

Remember- the computer is calculating using the odometer and what it says the fuel flow has been, which may or may not be super accurate. Even Honda may slip up on a calibration now and again. A sudden temp swing, or who knows what, can affect accuracy.

Meanwhile, you are calculating using the odometer and what you think you actually put in the tank...based on a pump that may or may not be super accurate, and which will vary from fillup to fillup by some amount- depending on whether the tank 'burped' the vapor out or not, etc.

Several sources of error on both sides...it will be a rare instance when they cancel each other out and the final results match. Some might call it accidental. Point is, neither method will be 100% accurate 100% of the time. If you have a fairly consistent offset most of the time, you're doing pretty well.

Re: Significant discrepancy between trip computer mpg and actual mpg calculations
 

Way off on this tank. Trip computer says 419 miles, 31.8 mpg, for a fuel usage of 419/31.8 = 13.2 gal.

Gas tank reading = 14.933 gal, implies actual mileage is 28.1 mpg.

Hmmm....

Also, less than 30 miles left on the range on the trip computer and less than 15 gallons filled??? No way the tank is 17 gal.

Re: Significant discrepancy between trip computer mpg and actual mpg calculations
 

What does the dash read? If the dash is consistently much closer to what you calculate, then the issue is in the navi.

Do you use the same gas station/pump? Try the opposite- if you always use the same station, use different ones. If you use different ones, stick with a single one. Do anything to change or identify the source of error.

I misspoke on the last post. The dash gage uses the odometer as distance input; the navi uses GPS. Navi takes a poxition fix every XX fraction of a second and connects the dots and smooths the bumps from fix to fix. Every fix can be off by up to 10 meters- compounding in some cases, averaging out in others. Any satellite dropouts and it has to guess.

Re: Significant discrepancy between trip computer mpg and actual mpg calculations
 

Toyota Prius computer exagerate MPG

Every tank I gas up, my MPG manual calculation from tank to tank is 5 MPG lower than Prius built-in computer display.

Green Hybrid web master should mandate all users to use manual caculation. As a group, we can see which model has an acurate MPG according to its computer display.

Re: Significant discrepancy between trip computer mpg and actual mpg calculations
 

New Yorker:
Honda Civic used to exaggerate too. That is, until Honda was sued. Now it's pretty even. It's usually an mpg or 2 under, but sometimes it is over. Plus your Prius has a flexible bladder, that can make the capacity of your gas tank change.