Anyone done a comparison of the estimates of mpg that we get on our display, to a real 'empty tank - to- fill up' calculation? Being a skeptic, I'm wondering how optimistic the figures that are shown on our vaious dashboard displays are. I've only been driving my new TCH for a week now and the mpg average that I seem to be getting (38.2) is encouraging.

 

not very...at best they're ~1-1.5 MPG high

 

I took a trip involving a few hundred miles after filling then filled up again. The listed mileage was within 0.1 mpg of dividing trip odometer by amount of gas added.

 

I disagree that it's not very accurate. 1-1.5 mpg high is very accurate. I've had other cars that weren't within 15%. 1.5/45 is 3%. When you consider temperature differences on fuel and other issues, that's pretty good. Also, I have found that the odometer is off the other way. When you put the numbers together, I calculate the mfd is within 1% of the true mpg.

 

I found the mfd readout on mpg is always 1 to 2 mpg higher than my calculated readings.

I even tried using a carpenter's level at this one gas bay to make sure it was level.
It helped but it was still reading 3/4 to 1 mpg high, although I use the mfd readout for a good mpg driving reference. I like the calculator to figure my true mpg.

I should mention, I fill my tank with the gas nozzle set to auto low. I allow it to auto shut off, then I put up the nozzle. No pumping in any extra gas. This way, I seem to get a more consistent mpg reading from tank to tank.

 

I find my MFD to be about 2.5% high.b

 

I find my display is 1-1.5mpg better than actual trip milage. The displayed odometer reading is 1.5% under mileage posts and/or my GPS and getting higher as the tires wear down at 14,000 miles. So, it's almost a wash.

 

I wish their was a better way to measure than going off the click at the pumps. Seems untrustworthy to me but I have no idea really.

When I do the math its usually a mile or two off mpg.

 

The fuel pumps at many gas stations are not accurate. In most states, they are supposed to be within one percent. But the state won't prosecute if it is even close. And then there is the question of thermal expansion of the fuel itself. Fuel pumps are capable of compensating for the thermal expansion. But, in most states, that compensation is not required, so it is not turned on. The net result is that the fuel measurement at a gas station is often off by 3 or 4 percent. You can guess which way.

 

One of the keys is to keep records consistently over time. You may calculate a little higher MPG on one tank (if you don't fill it quite as full) but the you'll pay a penalty on the next on where you actually put more in. It's a much better representation of the true MPG when you look at long term history and let the overages and underages wash each other out.

My historical mileage calculation (life of the car) compared with a compilation of the MFD readings (weighted for the miles run on each tank) shows the MFD displays about 2% high for me.