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Well, I've been looking for an answer to this question, but never really found one. I know there is at least one out there because I started reading it, but then had to leave so I didn't get to finish it.

Basically, my MFD says that I got 59.0 MPG after going 599 miles, but when I filled up with 11.19 gallons I figured it to be 53.53 MPG. I've always calculated it to be about 5 MPG worse, but I never understood why.

Can I get some help on this?

Mine always calculates to be about 10% worse. No idea why. Maybe it is because I pump my gas at night so I get less gas(gas is more dense when it is cold and less dense when it is warm so you don't get as much). I don't really know.

The temp difference might be the reason.

Ground temp should be pretty stable so the gas would be measured fairly consistently when you're refueling. If it's warm out it would expand in the tank. Assuming the MFD tracks fuel droplets or cc then it's measuring the heat-expanded fuel.

I'm probably thinking too much. I need to leave, drive to work through a snowstorm.

My TCH always shows an average of 1 to 1.5 mpg more than what I calculate after filling up (around 3 percent). I'm not sure why this is, but it seems fairly consistant.

There was an article about California wanting to require fuel pumps to regulate the fuel temperature (http://www.consumerwatchdog.org/energy/pr/?postId=7453) when filling to more accurately measure the amount purchased. As I recall, the claim was the amout dispensed was shy by a small percentage of the amount charged on warm days.

I think the programming has something to do with it too.
Does the manufacturer adjust the fuel volume with fuel rail temperature? Is the injector size computed at 43.5psig test pressure or rail pressure? Does it recompute for reduced rail pressure (high vacuum)? There are a dozen things in the formula that can make big difference between the display and "real world".

I think the programming has something to do with it too.
Does the manufacturer adjust the fuel volume with fuel rail temperature? Is the injector size computed at 43.5psig test pressure or rail pressure? Does it recompute for reduced rail pressure (high vacuum)? There are a dozen things in the formula that can make big difference between the display and "real world".

I agree. I first noticed this with my C5 Corvette (not that I Hypermile it but I do keep track of every gallon that goes in with a spreadsheet!), it consistently gives about 10% too high numbers compared to actual gas consumption. It doesn't take into account all the parameters but it also always reads higher than actual.

Now, I am seeing the same in my Insight, always reading high.
I bet software could compensate correctly but why would the manufacturer want it accurate when it gives the driver a better happier feel when its reading higher... Note, they ALWAYS read on the optimistic side!

At least they now have the speedometers reading right. My first new car was a really underpowered '74 Opel. Its speedometer read a full 10% higher than actual..

Gary

I think the only car I read about that was pessimistic was the HCH II.