No Idea What My Actual Mileage Is

What you do, is you keep a logbook of every fillup, and all mileage.
After 5 or 10 tanks, do the calculation manually.
Your average is what is important, not each individual tank.

 

And no car can be exact in the sense that you described because of such things as tire pressure throwing the numbers off by a little.

It's close enough for government work, but if you want you can do a measured mile at constant speed (i.e. set cruise at 60 and find yourself a measured mile marker on the highway.. time it, and the difference between your time and one minute is your fudge factor) every fillup, or once a month, or twice a year, or whatever makes you feel reasonably confident that you're getting a closer estimate. Then jot that down with the spreadsheet that Rae Vynn recommended, and you can adjust your numbers to give you a closer estimate.

Personally, I don't do it because I can reasonably guess at the difference, and the difference doesn't mean much to me anyway. As long as I'm setting personal-best-tank-avgs and improving lifetime FE, I'm happy

 

In European cars the speedo deliberately reads high, to unconsciously discourage speeding. Maybe this is done a little with US cars also?

But the speedo and the odometer are independent. If your tires are the right size then I'd bet that the odometer is correct.

As for MPGs, the readout on the MFD is accurate enough.

 

I have tracked my fillups ever since I bought my car 3-1/2 years ago. I also "reverse engineered" my display mileage to show lifetime stats (the 2004 completely resets after each tank). The display lifetime mileage is consistently about 2% higher than what I calculate with the gas I put in the car. At 50 mpg, that's only 1 mpg off...

 

Non-military GPS devices are accurate only to 3-10 meters... depending on the model...

 

<div class='quotetop'>QUOTE(Ichabod @ Oct 12 2007, 03:51 PM) [snapback]524814[/snapback]</div>

I believe this to be the case as well here in the U.S., that the speedometers read optimistic by several mph. On the Suzuki Burgman site (a maxi scooter), it is a frequent bone of contention that the speedometers intentionally and consistently read on average 10% optimistic... i.e. 70mph indicated is only 63mph actual. Be happy that you're only off by a couple of mph...

 

my GPS always registers the speed as being on average a mile or two lower than Prius speed. but my GPS also registers in 3D so going up and down hills will dramatically change the readings as well, so trying to figure mileage from GPS is not really practical.

one thing i have noticed is that even at much higher speeds, the difference does not seem to increase. now the speed on the Prius is determined by rotations of the wheel. since a true speed involves the rate traveled past a fixed point, a straight line is assumed. i for one, will say my driving is not straight. albeit, the variance is probably small, but GPS also does straight line calculations and GPS is not even advertised as 100% accurate, so if i were you. i would just take what your Prius says... it may not be perfect, but lets face it, nothing you have access to is either.

 

Speedo and odometer acuracy aren't nessesarily connected. My bike speedo is out by 11% but the odometer is within 1% over a measured distance. I can't be sure of all markets but here vehicle standards require speedos to read from 1% to 10% fast.

 

<div class='quotetop'>QUOTE(patsparks @ Oct 14 2007, 11:00 AM) [snapback]525420[/snapback]</div>

In Australia the law states that speedo errors need to be within PLUS or MINUS 10% actual. This means speedometer readings can be faster or slower than actual.

http://www.aussiemotorists.com/misc/MSACon...ELIX2_2004.html