MPG Inaccuracy - problem or not?

Are you resetting both Trip A and the Consumption screen?
If not, you may be seeing a cumulative MPG and a reset Mileage.

 

Just go by the MFD display. It is accurate enough. Unless you plan on breaking out the calculator every time or using an Excel spreadsheet you'll never get the numbers right. People on PC have been tracking the numbers long enough to show that the MFD is not far off so that is good enough for me.

Ohh, and don't assume your tank will hold 12gallons. It rarely if ever does.

 

No you shouldn't assume the gas gauge is accurate like that. If I did that I'd be calculating 70 MGP for the first half of the tank and 35 MPG for the second.

Ideally you should record the total distance travelled over about three or more tanks and divide it by the total amount of gas you've put in.

 

Exactly 1/2 on the guess guage does not mean dead nuts accurate half full tank. It could vary by 2 gallons in my experience.

 

I'm one of those who have been tracking the numbers from "Day One" ... AND ... have been doing so for each-and-every fill-up from both perspectives: MFD read-out and Hand-Calc.

After 40 fill-ups, the MFD has been off by more than 10% on individual fill-ups on four occasions; twice HIGH ( +16.3% / +11.7% ) and twice LOW ( -10.4% / -11.7% ), but after 40 tanks, the LIFETIME variance is only 1.7%. Over the life of the car, the MFD suggests mileage of 49.3 mpg, whereas my hand calculations suggest 48.5 mpg.

In short and as F8L noted above ... that 1.7% lifetime variance is good enough for me ....

As an added note: in case anyone wonders why I said above that "the MFD has been off" ... well ... when IT starts paying for the gas I pump into Mithril, I'll take its word for it ....

 

And for that I am grateful my friend.

Mithril would be fine indeed but us tree folk would be happy with a bit of lembas bread to sustain us on our journeys.

 

The OP has a North American Gen II Prius, which means it has a bladder in the fuel tank. Because of this, all bets are off when it comes to manual calculations of fuel economy. Volumetric errors from fill to fill are very large, so forget about calculating mileage on a tank by tank basis. If you total up ten tanks or so, then the error starts to cancel out, and the manual calculation becomes meaningful.

Likewise, because of the bladder you can only safely assume your car carries 10 gallons of usable fuel. On a good day you may get up to 11 gallons, but don't count on it.

Tom

 

My spreadsheet of 36 tanks shows that ~90% of the fills result in calculated mpg difference of 12% or less from the MFD. However, I had three fills that were a mess and they were sequential. In the first the calculated mpg was 18% lower, the next was 53% higher (tank would not take gas), the third was 22% lower. In short, the bladder tank/closed vent system make individual fill variances substantial and sometimes can give you completely off the wall result. But over the long term the difference between calculated and MFD now stands at 0.8% for the car.

 

Situation normal in my experience. If a high & a low tank are in sequence and taken together they are average then you don't have a problem. If you have a series of below average tanks then it is time to investigate.

 

Be careful about overfilling. Two Very Bad things can happen because of it.
1. The vapor collection canister can flood. If it happens too often it will need to be $$$$ replaced.
2. The bladder can pressurize from the overfill...possible bad things:
a) gasoline squirts out the nozzle all over the side of the car and/or you. Fire hazard here!
b) The bladder can burst, requiring $$$$ replacement!

 

Agreed! I've had 2.a happen and the gas just continued to pour out of the filler neck for approx. 10min. It was ugly.

I recommend never leaving the pump going without you standing there to stop the filling process in case the auto shutoff doesn't kick in.