That's why performing experiments and receiving real time feedback is helpful while trying to reduce the influence of variables (e.g. going both directions in a route to try to cancel out effects of wind and elevation change).

One guy who unfortunately no longer posts at Priuschat did plenty of experiments and posts his results. Examples: http://priuschat.com/forums/gen-iii-...mph-chart.html and http://priuschat.com/forums/prius-hy...tml#post954643.

For cars w/o any sort of instantaneous MPG indicator, http://www.scangauge.com/ can help. Even if it hasn't been fully and properly calibrated, it can help tell you whether things have gotten better/worse.

The above guy has even run his car out of gas intentionally over 3 dozen times (http://priuschat.com/forums/generati...eriment-2.html). I can't find the post where he talks about why but it was for gas experiments like http://priuschat.com/forums/gen-ii-p...tml#post398610. He had to rid his car of any previous fuel by running it out in order to accurately make any determination another fuel.

 

Idk what you guys are complaining about i average mixed driving always above 20... Ive only drove a few hundred miles since ive had it.. I reset the gas mileage on the hwy cruising 75-80 for about 15-20 miles no cruise control and i was getting 29 mpg.. Not sure how acurate the gauge is but i went from a lifted truck to this so im LOVING the gas mileage

 

I was capable of attaining 34 in mine. Usually though due to 'fun' I would achieve 26 city/highway.

Instantanious mpg readers in cars are not 100% accurate. they are good though, and close. but not as close and accurate as miles over gallons (upon fill up). Also driving both ways on the free way to try and eliminate variables or differences is a good idea, however if you really want to be ****, road conditions may be different on either side. freshly paved or not, one is cement, the other not. bumps in the road, debris. additional cars or trucks in front causing vortexes. constantly changing wind speed/direction, temperature changes which may effect road condition, tires, pressure, intake, combustion etc etc etc. the list is endless (and quite ridiculous).
I think an easier way to test petrol/gas would be in a lab. com-busting it under control and determining the variable ignition points, amount of energy stored and released per brand. Which ever produces a better result should translate to a better fuel. then test it in the car.

to many variables all the time at any given time.

fill her up, put her in gear and GO. Just be safe.

 

omg people like this dont belong on the road geez next ull be driving in reverse to save miles too jus go get a nissan leaf n call it a day

 

I really wanted to clarify this as many of the commenters do not seem to comprehend. I in no way am bashing at the MPG reading; by all means, I find my MPG pretty good compared to my previous cars, averaging around 20 MPH. This was just a curiosity that had occurred to me since this was...pretty much how I drove my whole life; accelerating, gliding, and accelerating. I guess you can say curiosity killed the cat.

 

owning a BP, it does not matter how fast you put the gas in the tank. (yes, sometimes the nozzle will click off as the air over pressurizes the nozzle with the fuel as well, but that is just too much psi for the nozzle to handle clicking it off) When the fuel is used there is space. and what takes up that space? "air"...so when filling up at the pumps as fuel goes in air is being pushed out through the same hole. The nozzle itself has that black rubber piece that is sort of spring loaded. Well that is designed to press up and capture the "air" as it is pushed out from the entering fuel now taking up that space. VAPOR so to say is what the EPA calls it. (lol)...anyway the hoses attached to the nozzle's are designed to suck out and transfer the "vapor" or "air" back to the underground tanks. So when the Gas tanker truck comes to refill the tanks he now takes on the "vapor" as he drops the fuel.

I usually complain to the "b1tches" that come from the epa who inspect all my nozzles and what not trying to give me fines.

I tell them that most vehicle when taking off the gas cap release the pressure out into the atmosphere even b4 i use the damn nozzles, so wtf is the purpose...ya know? Such a waste of money and BS specially here in NJ. Everyone (government) is trying to make a buck at the small biz mans expense.

Anyway there you have it...BUT if it makes your cousin feel better just let it be.