OK I know the naysayers will come out of the woodwork but this is what I have experienced. I have taken several trips now into the eastern sierras. The trip consists of a slow gradual increase in altitude from sea level to aprox 8-9k feet. The one way leg is aprox a 7 hour drive at 70mph. I have tested premium and regular Shell gasoline 5 times now. I seem to avg. aprox 8-10mpg better going uphill with premium. The downhill is less with aprox 3-4mpg. I have also noted that with a side or head wind the premium gets aprox 8-10mpg better as well. My question is this. Does premium have more energy per gallon? Any other ideas?
I have no idea what could cause this. From the physics of it, there's no way you should be getting better mileage with higher octane gas. I wonder if any of it is a psychological effect that changes your driving style; a way to test this would be to have someone else fill up your car without telling you what the octane rating is. I would think that high altitudes allow even lower octane gas; isn't regular gas rated "85" at higher altitudes? Overall, if you're averaging 8-10 MPG better with the premium gas, who cares what the reason is? You're saving money over just buying 87 octane gas.
higher octane fuel requires more push to burn.. or less air. 87 being the easiest to ignite and 101 racing octane being the hardest to ignite.. which is still rather easy compared to diesel. some will argue that the prius will alter it's injection so the higher injection makes no sense... but that doesn't really make sense.. does it? that would mean instant MPG increase which isn't always the case. i fugured you were going up hills.. using the compression more, instead of the seamless shifting. lower octane means the car does more "shifting". while a higher octane should make more of a boost letting the car turn the engine over with a tiny bit more torque.. while polluting more too.. (as many will chime in)
You use lower octane fuel at altitude. The Prius is designed to burn 87 octane at low elevations, and 85 octane at altitude. Something else is responsible for your mileage boost, whether it's ethanol in the fuel, placebo effect, or some other factor. It's not the octane. Tom
how can someone get 3-4 mpg less downhill, when all you have to do is take your foot off the gas pedal and you can get 99 mpg? going downhill from mount hamilton, i had 99 miles per gallon for 30 minutes straight according to the fuel consumption screen on north bound 680 in fremont, there is a 6 % dip and if i take the foot off the gas at 65, my car gets 99 mpg for about a mile and a half and after that its going 85 mph... one of these days im going to get a ticket on that section of road, but officer i was just charging my batteries! another thing that is rarely mentioned about octane levels, is that its the MININUUM octane rating that is guaranteed, so the gas you buy today might actually be higher then 87 and it might not..
To answer your question, higher octane fuel has no more energy than lower octane fuel. High octane gas burns slower and is safer to use in high compression engines. When engine manufacturers started making higher and higher compression engines, they found that fast-burning fuel would explode (i.e., detonate) instead of going through an orderly combustion cycle. To prevent detonation, they added octane, which merely makes the fuel burn slower. To this day octane is boosted by use of an additive. At higher altitudes, you can get away with lower octane fuel because the air has less oxygen and there is less tendancy for detonation to occur.
Interesting. I live along the mountains in central Pennsylvania, mostly highway driving and get 50.8 MPG. While our mountains are a lot smaller than the Rockys, I'm constantly going back and forth from a base elevation of 1,000 to 2,000 feet. I'm going to try a tank of premium and see if there is any difference.
Every regional area is going to have a slightly different mix of gasoline with and without oxygenation additives. I have seen Shell ethanol regular and I can believe it loses something. But let's discuss what happens in Huntsville: You'll notice the premium fuel, at higher power levels, seemed to increase the fuel efficiency. Certainly running at 70 mph will increase the engine power range with some improvement over a similar specific energy density fuel. To really answer your question, we need to get samples of both fuels and measure the specific fuel energy and that takes instrumentation. Do you have a scanner or other tools to use with your Prius? Bob Wilson
NO. The stuff that's added to retard detonation has less energy than the gasoline to which it is added.
The higher octane in my tests didn't help. In my experience the exact opposite is what occurres with ethanol. We get 10% ethanol in the winter months, and although it may cut down on pollution, it robs mpg. Never mind, I'm putting a new topic...
It's quite possible that the Prius is able to increase ignition timing with higher octane fuel and eke a bit more efficiency out of the engine. It should be simple to test this if you can monitor engine speed and ignition timing using a ScanGauge or similar. Find a steady slope. Engage cruise control at your normal speed. Note ignition timing and engine speed. Repeat with regular and high octane in the tank, noting any differences in ignition timing and rpm. If ignition timing is higher and/or engine RPM is lower with the higher octane fuel, that would explain it.
As I've mentioned in other posts, I work on the weekends for a NASCAR team and frequently travel long distances to/from racetracks. This year when we raced in Dover De. I drove down on Wednesday night from the Boston area using regular gas I bought in town. Route was I-95/Jersey Pike. During the weekend I filled the Prius back up with Sunoco unleaded race gas (don't tell NASCAR) which is rated at around 98 octane. On the trip home, also at night and similar road/weather conditions my mileage increased about 2 mpg. I averaged between 53 and 55. Not a very scientific test but in general I think it says that there is a very weak (or no) correlation between high octane and increased mileage when driving around sea level. It did seem to run with more pep but I'm pretty sure it was a placebo effect. That's just my experience.
The NASCAR racing fuel is not the same as premium gas, it does not contain ethanol. You mileage gain is due to pure gasoline.
