Computer MPG vs Real

I just placed an order today for a Magnetic Gray. Please tell me how much you love the color.

 

The Prius fuel tank has a bladder in it. This thing stretches and contracts with the amount of fuel in the tank, it's purpose is to allow fuel to be used without introducing air into the tank therefore reducing evaporation for emission control purposes.

It's pretty good in warm or hot weather but it still doesn't allow the tank to hold the same amount of fuel on each fill up. When it gets colder you may notice your fuel capacity shrink a bit, maybe a couple of gallons or so.

Unless you are going to check it over a long period, say several thousand miles it's hard to get good mileage figures the old way. Mostly I just trust the consumption screen figure and check it the old way on long trips.

 

The fuel guage is also pretty non-linear and quirky. There are always tonnes of posts and questions about the bloddy thing. Like Tumbleweed mentioned, the bladder is sensitive to temp changes. Also, different fuel pumps will dispense different amounts of fuel depending on how sensitive their automatic shutoff sensors are. This makes it really difficult to have any consistency when trying to do manual MPG calcs. There's just a tonne of error because you only really know how much fuel you put it, not how much you burned. Then the temps are constant, I've found that the manual calcs are 2-5% below what the MFD says. When there are wild temp swings the numbers can be WAY off. I generally go with the MFD numbers because I think there's a lot more consistency there. The MFD displays MPG calculated by the amout of fuel that goes through the injectors. Congrats on the new car, BTW. Glad to hear that you're enjoying it.

 

What the last two said. PLUS...

Just because your guess gauge said "full" when you took delivery, don't put any stock in that. The gauges will read F even if they're 1-1-1/2 gal. short. Dealers seldom carefully fill them for delivery---or someone has to drive it here or there just before delivery.

The MFD error on the Gen 1 was worse than it is now---10% or a little more in some cases. Now it's about 3-5%. (Overall, the MFD error is ALWAYS in it reading higher than the actual.) I've had 4 Priuses, 2 Gen 1 and 2 Gen 2, totalling 150K mi.

 

My 'calculated' number is usually about 2 MPG lower than my MFD shows, but once it was 2 MPG HIGHER and my last fill up was 3.6 lower. This is my experience from 5 full tanks <g>

I know my tank was completely full when I bought the car - the salesman had me follow him to the closest gas station where they filled it up for me.

 

<div class='quotetop'>QUOTE(Jeannie @ Jul 3 2006, 09:32 PM) [snapback]280772[/snapback]</div>

Or at least it was as full as it was going to get on that particular day at that particular pump.

 

I'm still mystified as to why people assume that gas pump readings are more accurate than the car's display. Sure, Toyota could have arranged to make drivers feel better about their fuel economy by setting the Multi Function Display to overstate MPGs, but it could also be that gas station pumps tend by some strange coincidence to overstate rather than understate the amount of gas pumped (and thus increase the amount we pay them; notice that any overpayment is pure profit to the station owner).

Anyway. sl7vk, just be sure that you buy gas immediately if the last bar on the fuel gauge ever starts to flash. The fuel tank bladder makes it foolish to try to estimate how much gas you have left. This forum is littered with sad tales of people who were certain that they had 50 or 20 or 10 miles worth of gas left in their tanks.

 

I have kept my lifetime MPG on a spreadsheet ever since I got the car (over 2 years), and it shows that the MPG on my MFD is about 1.9% higher than the calculated MPG from my fillups (I "reverse engineered" the MFD MPG to show lifetime figures on my spreadsheet). You can't depend on only one or two fillups, since every pump is different and the bladder will be differend depending on the weather. I think most people who track lifetime mileage for both calculated AND display for the 2004+ models show *about* a 2% discrepancy once they've gone through enough tanks to even things out.

 

I had a 2004 Prius for 22.5k miles (just sold it in April to get a 2006) and I religiously kept a spreadsheet with what the computer said and what I was actually getting. I kept rolling averages as well as total colum averages (about 150 gals) to limit the variation and I found the computer was telling me about 2 mpg higher than I was getting. I asked the dealer and they told me a 1 to 3 mpg variation either up or down is expected.

Also, the bars don't really tell you anything as far as percentage of gas left in the tank.

I always recommend a log book anyway because if something is wrong with the car, the first thing to go down is your gas mileage. In addition, I keep a record of everything that's done or noted on the car (tires rotated, window tinting put on, touch up paint used, scotchguard, wiper blade change, etc). It helps a lot if you forget when the last time you had your wipers or brakes done, and it is a great selling point.

John