Gas Mileage: Whom Do You Trust?

A nearly six mile per gallon discrepancy over a span of 2450 miles is pretty huge. Was the car listing when you filled it the last time? LOL Trust your computer's mpg assessment. It's not affected by the compounding of errors you get from each filling. mark_

Hmmmm. I have measured the digital readout vs. calculated MPG at each fillup for 2 Prii for over 100Kmiles (most on a "Classis 02") and have found that the digital display typically runs about 5% higher on average than actual calculated MPG (ie gallons put into tank divided by miles driven on that tank of gas). This seems to be true as well on my

With that said, the bladder is affected by temperature, altitude, the rate of fuel coming from the pump and who knows what else. On top of that, while the pair of Prii I own are by far and away the absolute best vehicles I have ever owned, I have to say the entire fuel gage / MPG calculation system is inexplicably weak. You would think that a car that is pretty much completely computer controlled would be able to measure and display fuel consumption exactly, but apparently Toyota still uses the time honored tradition of guessing at fuel use by monitoring engine vacuum pressure or something similar, which is not very precise. Oh, well.

When in doubt, use your calculated numbers. Oh, and enjoy the Prius. Did I mention its the best car I've ever owned? ;-)

Davoud wrote:

I have a theory, with absolutely no evidence to back it up (but hey, that's never stopped me before ;-)). My theory is that the Prius calculates average MPG with respect to time rather than distance, which tends to give a higher than expected value.

For exmaple, suppose you drive for 10 miles at 30 mph (20 minutes) with an actual consumption rate of 60 mpg, followed by 10 miles at 60 mph (10 minutes) with an actual rate of 50 mpg. The "normal" way to calculate the average consumption would be to to average over distance, which would give 55 mpg. If my theory is correct then the Prius would calculate an average figure of (20 * 60 + 10 * 50) / 30 = 56.7 mpg.

Paul

The Prius counts fuel injector openings. The fuel injectors must be well-calibrated, otherwise combustion would not be well controlled and the emissions would be much worse.

An alternate explanation for this discrepancy is that the gas pump is overstating the amount of fuel pumped. Considering that this means extra profit for the gas station, I decide to trust the Prius MFD.

It is certainly not consistent. Thus one cannot assume that the amount of fuel added at fillup number N is the same as the amount burned since the previous fillup. Even if we assume that every gas pump is completely accurate, this variability in the Prius gas tank capacity makes a hash of trying to calculate MPG manually, except as an average over many tanks.

And please don't top off. Topping off can saturate the fuel vapor recovery canister and greatly increase emissions of unburned fuel from the tank. Vapor recovery canisters have been around long enough that by now topping off has become an old fart habit.

Thanks for telling us that Richard. This old fart will quit topping-off. :-)

Something else to complicate your calculations. . .

You can't always trust the numbers you read off the gas pump. They can be -- and sometimes are -- rigged to show more gasoline pumped than you actually got. I've even heard in some cases they are even programmed to be accurate up to the first five gallons, so people filling a five-gallon can won't notice, then they start skewing the numbers.

Maybe it's just an urban myth, I've never set out to prove this first-hand. But you know, gasoline scams have a long history (from watered-down gas, to selling regular from the premium pump), so jiggering the pump readings isn't far-fetched.

I have recorded the pump MPG and the readout MPG for the last 35 fillups of my

My fillup end point has not been very consistent. The true calculated MPG has differed from the Prius display value by as much as 6 mpg, but it is generally much closer. The averaging should minimize this variability.

I have moderated my driving and the last 4 tanks have displayed over 55 mpg on the screen. I doubt that the next fillup will achieve that goal.

-Al

OK.

Nope. Gas station owners are making sufficient profit that they aren't willing to risk committing such a crime. They are subject to unannounced visits by state weights-and-measures inspectors.

Davoud

Yep, urban myth, paranoia, or whatever you want to call it. It just isn't done. That may not mean "never," but it means that such a practice is exceedingly rare. Accidental inaccuracy is as likely to be in your favor as against you. The flow meters are designed for high accuracy, and that accuracy is tested by governments and by private citizens who like to make sure that the station owners stay honest. I've tested it at many stations (without meaning to) buy buying as much as 50 gallons of fuel for farm tractors in five-gallon cans.

Davoud

Finally!

Maybe so Dave, but I don't see them jumping to correct an issue when they're told about it either. 5 days went by before weights and measures guy finally got around to check one station. Three pumps on two islands were shut down. He was also reluctant to say how much the pumps were off because anyone with a receipt has a legal right to be compensated. I noticed self service stations are more likely to have pumps that are inaccurate. I wonder why. Could it be that blanket one year testing intervals are either too far apart or that volume should be the determining factor instead of time? There's also an issue with the flow back valves that are meant to keep the lines pressurized. When those valves don't work properly not only do you have to pay for the gas to fill the hose you lose out on the gas in the hose after the fill. The hose may contain as much as a coffee can full of gas. I know because I used to siphon gas when I was a kid. Instead of siphoning out it siphons back into their tank and the next customer has to pay to fill the hose again. Yeah, it pays for them to be woefully ignorant.

The inspectors allow a certain amount of uncertainty. If by some funny coincidence the gas pump always overstates the amount dispensed yet remains within the allowed accuracy...

You may not have heard of a dodge that was common some years ago. The inspectors always collected a certain quantity of gas into their carefully calibrated container. The computer-controlled pump was set to accurately display that certain quantity, but to overstate the amount pumped for any other quantity.

Back in the late '50s I had a 125CC BSA Bantam. In those days the gas stations all closed around 10 p.m. I could get a tank of gas by draining the hoses at a couple of stations. :-)

I use the odometer/tank to track my overall MPG and the online display to handle specific segments or learning how to best drive my Prius. With just 10 months and a little over 14,000 miles, I'm running 51.4 MPG in an NHW11, 03 Prius. Given the EPA rating of 52/45 (City/Highway), I'm OK with the performance.

I find the MFD useful for specific, short term measurements but when it comes to what I plan by budget or trips, I use my Odometer/tank measured, MPG vs MPH chart:

Davoud:

richard schumacher:

Yeah, I've heard that old wive's tale and, I think, all the others.

Davoud

In what way did you moderate your driving?

I had a quarter (25 cent piece) with a hole drilled in it and a first string guitar string attached to it. I cleaned out the soda machines by dropping it down the coin slot and pulling it back up over and over. I saw it done in a movie used on a pay phone. But what goes around comes around. When my stolen van was recovered the police said ever since the movie Terminator came out the ignitions on the columns were being smashed just like in the movie. A simple pull on the exposed metal bar started the engine. They say movies and TV don't influence kids. Yeah, right.

Most of the difference has been moving from cruising the left lane to following the pack in the rightmost lane. ;-)

Al