Why do I average the same distance after every pump? Does the MPG display really matter?

If you drive 350miles, then pump in 10 gallons, you are averaging 35mpg. If you drive 350 miles, then pump in 7 gallons, you are averaging 50mpg.

The trip computer is usually accurate within a couple percent. So a 23% error (45mpg to 35mpg) or a 37% error (55mpg to 35mpg) is unheard of.

My first guess would be your math is wrong. Go grab a fillup receipt and post the gallons in and what the trip computer read when you filled up (assuming you reset the trip to 0 after a fillup).

 

Using your worst case numbers of 350miles at 8.3 gallons, that is over 42mpg. Using 7.9 gallons that is 44.3mpg.

The numbers don't add up. Why do you think you are getting 35mpg?

 

If you use fuelly.com, you will not have to rely on memory. All the history of my car can be found by clicking on my signature.

 

Also, I suspect you are not 'topping off' the tank. I usually (if no one is waiting in line behind me) will completely fill my tank (probably an additional gallon = 50+ more miles per tank). Full, as in having visible gas at the cap.

 

highway miles make no difference except the mpg's you are getting while driving them. if you get 5 mpg, you're not going to go very far compared to 75 mpg. i'm not understanding your logic.

 

Be consistent with your fill ups. Always stop filling at the first or second click. This will reduce the number of erroneous tanks.

 

I will add this for completeness, but it may not effect you.
In my most frequent 'commute' the cheap gas stations are 55 miles apart.
This means that almost no matter how well I do, I will drive the same distance per tank. Because running out of gas where gas is 20% higher, if available, does not appeal to me.

 

In Missouri, with the C, if I stop filling when the pump shuts off, I am a gallon and half low. I always put in another 1.5 gallons after it shuts off. I fuel up mostly at night when no one might be waiting in line. By filling the neck up the car holds 10 gallons, therefore I have my ScanGauge set on 10 gallons. I had the Ultra Gauge set at 9.5 gallons and it was not accurate. The ScanGauge is as accurate as my calculator. Now, I think you need a pencil and paper the next time you fill up. Then, use a calculator to figure the mileage. If it is as far off as you think it is, with the onboard computer, then take it back to the dealer. I can always run 450 miles on a tank, if I choose, but do not like running it that low. Well, maybe not "always". Interstate driving with a headwind @ 70 mph, I would not trust the 450 mile thing. My onboard computer is not accurate but it is not that far off, maybe 1.5 mpg liberal, at the most.
Dan

 

These are both horrible pieces of advise.

On a GenIII (lift, V, and C) there is about 1gallon to 1.5gallons extra in the tank if you include the air regions like the neck and top. That means the tank is closer to 11.5gallons, not 10gallons. The 1.5gallons is for fuel expansion. Depending on temperature and elevation the fuel will expand and contract. That air buffer gives it room to do so. By filling up the neck you not only take away that expansion area, you cover the venting system and it will spill into the evaporative control system.

When your car says 0miles to go, you really have about 1-2 gallons left in the system. Treat it as an emergency reserve, don't drive on it. But that is where those extra gallons are that do not get filled. When you are on 0 and the gauge is blinking, you are not running on fumes like other cars.

And for others reading this thread that might have a GenII North American model, overfilling the fuel tank means it could leak into the bladder itself which is a sealed system. The only fix is replacement which is over a thousand dollars usually. GenII in NA does not have a "tank", it has a "bladder" and overfilling can cause serious and irreversible damage.

A GenIII (C,V,Lift) has a conventional tank, so the dangers are conventional. Still stupid to do it.

 

Dan,

Topping off is bad for your car. Search for it on these forums.

ECOman,

The official fuel capacity of the Prius C is 9.5 gals. Most Priuses (Prii) will still have around 1-1.5 gals after the fuel light comes on. So, if one drives it to zero DTE (Distance To Empty), you would only end up putting 8 gals if you stop fueling as Justin (F8L) mentioned above.

If you had 2 bars left, I'm willing to bet you had more than 2 gallons left. 350/7.5 =46.7 mpg which is right on the money. As others mentioned, the mpg indication is optimistic by about 5%-7%. In this case, your mpg and distance for the whole tanks is about right. The ones getting 400 miles + are topping off the tanks which again is bad for the car. Stick with what you are doing. I think that 350 miles per tank is right on.

Also, Jimbo and others suggested, you should sign up on Fuelly.com and log your fillups there. You can even fill in the information from your smart phone after each fillup.

 

2K1 Toaster,

Great minds must think alike .... at about the same time. I guess I type slower than you. Great advice.

 

Oops. Do this at your own risk, should have been the warning label.
Dan

 

I would also suggest filling up based on the gas gauge, not the distance remaining, as I (and others) have found that the distance is not that accurate. The gas gauge might not be either, but I have a feeling it's more accurate that the distance. Perhaps fill whenever you reach two bars remaining (or whatever you're comfortable with--I do two, and usually put in 7-8 gallons).

 

As a stupid driver - I was warned to NEVER TOP OFF - my Scion xB because of the evap system. I retired that car at 350,455 miles with no evap repairs required. I suspect my 4 Taurus/Sables would have come with the same warning if I'd been a member of forums with them.

 

I have 2 xb's and top then off also and never had a problem.

 

I don't take it personal. I always top my vehicles off unless I am in a hurry. I have reworked my entire fuel system on our motorhome, including the evap system. When I get the chance it also gets topped off. I am not saying anyone should do this, but I do. I still own my first car, purchased in 1970. It is a 65 Ford Mustang Fastback, with the tank right on the rear end. It gets topped off also. This is my first hybrid so I may have some things to learn.
Life is good. This is a great forum.
Dan