Why is the milage on the screen always different than the real milage?

In short, because of the flexible fuel bladder. Lots of threads on the subject. The search function is a wonderful tool.

 

Besides the variability on fill ups due to back pressure from the bladder, the computer doesn't actually have a flow meter to measure gas use. It estimates gasoline use from the timing pulses sent to the fuel injectors. It also doesn't recognize that the energy stored in the HV battery is also a variable.

I agree that the computer tends to estimate a higher MPG than the Prius actually averages, but only by one or two percent.

JeffD

 

The basic problem, which exists with all cars but which the Prius bladder magnifies, is that manual MPG calculations usually use the miles you *have driven* since the last fillup divided by the number of gallons you *will burn* until the next fillup. Unless you always put in exactly the same amount of fuel at every fillup the MPGs you calculate this way are always a fiction and can only be approximate. You can improve this by keeping records, and dividing the miles driven by the gallons put in at the previous fillup.

Minor complications:
- gas stations have an economic incentive to overstate the gallons shown on the pump, which exaggerates manually calculated MPGs
- some fuel is lost from the tank to evaporation, which exaggerates the MFD MPGs

 

Sometimes its the other way around I filled up last week and had 702 miles it only held 9.44 gallons my MFD was showing 62.8MPG but if you use a calculator it comes up 74.36..So unless you can get gas at the same station and it be the same outside temperature, then you have to go by the Display ..I think in the end it all evens out anyways sometimes its to high sometimes its to low..

 

Sometimes it's high. Sometimes it's low. Sometimes it's close. Sometimes it's not. I don't think the Prius' is worse than any other car. Welcome to the Guess Gauge..

 

There is always some guesswork. Temperature variance (your tank is warmer than the cool fuel pumped from underground) is a problem, so the emptier the better. It's also advisable to use the same station, same pump if possible. Your economy gauge is designed to precisely measure fuel flow and vehicle speed, and does a remarkably good job--the indicator is trustworthy.

 

The mileage on your screen is the real mileage. You might ask the question: "Why is my calculated mileage always different than the real mileage?"

In actuality, neither of these mileage figures is the "real" mileage. They are both approximations.

Tom

 

Bladder: You don't always get the same amount of fuel when the tank is "full".

... Brad