I'm not talking about airplanes. I'm talking about a very basic concept: if you want to check the accuracy of your car's calculated FE, then reset the calculation when you fill up. Calculate it by hand on the next fill up, then compare the two numbers at that same point in time. You are comparing the same usage over the same time period (and usually, you'll see that your hand calculation is actually a little higher than the car's real-time estimate). Simple as that.

I just re-read your original post. Perhaps there's a language barrier here and I just don't know what you're really trying to say. If so, sorry.

 

I reset and recalculate and compare after every tank. Since the purchase of my car, the FCD MPG has ALWAYS been lower than the hand calc. Sometimes the variance is as low as 0.3 MPG, and sometimes its as much as 1.1 MPG.

The FCD never reports higher mileage/gallon for a tank than the hand calculation.

My last tank showed a difference of o.5 MPG after 568.8 miles of driving.

 

Rob: Your this is silly comment is silly to me, so I responded with a silly airplane comment. See? Now you're silly too! Some people just don't care to figure out this sort of stuff. Others have too much free time on their hands, and enjoy getting down into numbers and details. I've actually got a bunch of RS232 comms captures sitting on a linux box waiting for disassembly to see if I can't figure out the protocol to an old scoreboard I bought. So what if it doesn't float your boat. I like this sort of stuff

rysa (or anyone else for that matter): Have you been keeping track of both methods like I have? I'd love to have a larger data set to work with!

 

Allow me to make this even more interesting. The odometer and speedo on my 06 HCH reads about 2% higher than reality as verified several times using GPS (and even the factory NAV). Interestingly, my ScanGaugeII MPH and dist readouts are consistent with the GPS. The ScanGauge pulls its information from the onboard computer as the FCD probably does too.

This argues the hand calcs, at least in my case, need to be reduced by 2% which makes them awfully close to the FCD numbers. As for the mis-information on your speedo and odometer? well, Honda and other manufacturers have been in trouble for this before. It's common practice and has two advantages: 1) warranty periods end sooner, and 2) you're less likely to get a ticket in those **** states that will snag you for just a few MPH of the speed limit.

At the end of day, I think you should take the numbers that make you happiest with your car. For me its the truest numbers: GPS corrected distance and Gallons refilled at the pump.

-dan