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Fuel Mileage Results and Grades of Fuel

Old 12-04-2005, 04:44 AM
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Question Fuel Mileage Results and Grades of Fuel

Well seeing both the Highlander Hybrid and RX400H state in their manuals that better performance is achived by using Premium fuel it made me wonder if the EPA MPG test was done using Premium fuel Vs Regular, which most of us are using. Because there seems to be such a big difference in what we all are showing in the real world Vs the EPA test something doesn't add up. I know the EPA test are NOT real world, but I normally get pretty close to what they state in all my other cars & trucks, why not the HH.

Has anyone on the board tried the different grades of gas and done a comparision on MPG difference's when using higher octane fuel grades?
Old 12-04-2005, 09:45 AM
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Default Re: Fuel Mileage Results and Grades of Fuel

EPA FE testing is performed using the grade of gasoline recommended for use by the mfg.

If the HH.RX400 state that 91 or better is to be used, then it was tested with 91.

Tests are done with non-oxygenated summer fuel. If you are in a region with RFG, you will lose 2-4% due to the MTBE or ethanol oxygenate. Winter fuel gives you another 2% penalty or so. At 30mpg, that's a net loss of up to 2mpg in the lab...often more on the road.

Toss in the well-documents shortcomings of the EPA tests with respect to hybrids and much of your shortfall is pretty well explained.

Peruse the forums here and try out the techniques offered up- you'll be amazed how quickly you can exceed EPA numbers, even on 87 octane!
Old 12-04-2005, 10:51 AM
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Default Re: Fuel Mileage Results and Grades of Fuel

Oops! I voted in the poll before reading what the poll was for. So, since I don't drive one of those SUVs, subtract one vote for regular fuel.
Old 12-04-2005, 11:16 AM
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Default Re: Fuel Mileage Results and Grades of Fuel

Octane rating does not correspond to energy contained per gallon. In fact, lower octane fuel has MORE energy per gallon than the more refined high octane fuel. This is due to the longer, more complex mollecular chains in less refined fuel. Unless an engine NEEDS the ignition related characteristics of high octane fuel, there will be absolutely no power or efficiency advantage to be gained by burning high test. Just more $$$ for the marketing guys at Oil Co., Inc.

This high power-myth about high octane gas sure dies hard!
Old 12-04-2005, 05:14 PM
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Default Re: Fuel Mileage Results and Grades of Fuel

True. Summer fuel and non-oxygenated fuel, however, do have more energy than oxygenated winter-blend RFG, for matching octane levels.

Higher octane fuel has lower energy content not so much because of the refining level or length of the HC chains, but dominantly because of the additives used to boost octane- MTBE, ETBE, or ethanol. All reduce the energy content of a gallon (and oxygenate it). Yes, there may be more actual isooctane/less heptane in premium, but much of the octane gain is from additives. TEPb was used for the same reason (in addition to lubrication)- higher octane without higher refining cost (and subsequent yield reduction). Lower-octane fuel actually has shorter chains on average- dominated by heptane (7 x Carbon). 100% octane (8 x Carbon) is defined as 100 octane fuel. They're piped off the cracking tower at different points- same amount of refining, just a different boiling point.

But if an engine is designed on premium, then chances (in my experience of owning 3 cars that stated "91 or above") are good that MPG will suffer with lower octane. Not because of energy, but because of combustion factors- earlier ignition and whatnot the engine contols can't adjust for completely. One would think that any modern engine computer could handle any octane by now, but, being an engineer, I know 1st hand how easy it is to paint oneself into a corner, design-wise. Sometimes it is a whole lot easier and cheaper to get outself of the corner by putting limits or conditions on the end user. Ideal, no. Good for getting the product out the door, yes. Life is not an ideal system.

Last edited by gonavy; 12-04-2005 at 05:20 PM.
Old 12-04-2005, 05:37 PM
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Default Re: Fuel Mileage Results and Grades of Fuel

Hi All:

___GoNavy, you have been doing your homework! Nice job …

___The Acura MDX (EPA rated at 17/23 on its recommended 91 Octane) was first driven beyond 30 mpg over a tank using Regular Unleaded. Is anyone else starting to get tired of the Premium vs. Regular threads even if the car/truck/SUV asks for Premium? It is the driver, not the gasoline unless said gasoline was bad in the first place. If you are pushing your hybrid into the realm where Premium is needed to reduce knock/advance pull back, you sure as hell aren’t receiving anything in the fuel economy department and the question would be complete waste of everybody’s time.

___As for all this Premium/Regular stuff, if you want every last bleeding ounce of performance out of your car/truck/hybrid, your FE is going to suck. If you want all the FE your hybrid/non-hybrid can muster, drive it for FE. Filling it for FE is non-sense unless you have a choice between non-E10 and E10 at the same station for the same price.

___Good Luck

___Wayne R. Gerdes
Old 12-09-2005, 09:54 AM
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Default Re: Fuel Mileage Results and Grades of Fuel

hello people. 1st post. i hope to post more but probably won't have much time until after the holidays. plus i'm a regular @ vwvortex since my other car is a dub 1.8T. recent owner of hihy 4wd.
i tested a tank of 87 oct last month. i averaged 24.48 mpg in 60/40 city/hwy driving. i drove easier than usual for the test but nothing ridiculous. i'm sure i could get >25 mpg if i really try.

re: reg vs premium
i'm thinking the performance increase (hp) is strictly from the ecu adapting to the higher octance and advancing the timing? or is there something else going on?
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