mpg with 91 vs 87, in your prius c

It looks like you're just getting back to your pre-winter MPGs. Even if its not quite as warm as it was then, you've probably learned to drive the car better. Temperature can make a big difference, especially with shorter trips or if you're blasting the heat. Also, the winter blend of gasoline generally returns lower MPGs regardless of temperature, you may have gotten summer gas at your last fill up (do not know when they switch there).
Not saying the 91 octane isn't making a difference, there's just too many variables to be sure at this point. If it does end up being better, it'll be interesting to see if its enough to justify the extra cost.

 

We are seeing big variations on the warm days. Do both of these fuels conatin E10 or is the 91 E0?

 

I only use 91 octane and my average mpg is 55. I tend to get 55mpg to 60mpg highway and 60mpg to 70+ mpg city.

 

The difference between 91 and 87 octane gas is the component chemicals that are used. gasoline is a mixture of many chemicals, notably hydrocarbons that make up crude oil. Refineries merely extract the ones that usefully can burn to power internal combustion engines. As such the recipe for gasoline changes with the crude oil used to extract the gasoline. Generally,high test gas does have more energy than low test because the refinery has to use more energy components to obtain higher antiknck properties especially true once tetraethyl leasd was banned.
today, 87 octane gas is likely to have greater amounts of ethyl alcohol and that component alone kncks the heat of combustion of gas by 3 -5% or so.
we have run lab tests under the same loads in the lab and find that you can, but not guaranteed, get as much as 3 to 4 mpg more with high test on a Priuc sized engine. Its not guaranteed because there is a wide variety of gas mixtures available in gas stations even day to day. in either case of course, its not economical as most premiums are 10 to 20% higher cost so a 3% gain isn't worth it. And yhe difference in detergents and other additives simly aren't worth considering. The only possible advantage in premiums is there is less likelyhood of alcohol contamination. Yes, contamination.
It is possible to get less mpg with 91 compared to 87 but thats a minority. It just depends on the gas distributed. But on average you should get a 3 to 5% increase in mpg's.
we've had the opportunity to check at least 9 tank refills with and without etanol contamination and find the difference about 4 mpg regularly. low forties on ethanol laced gas and upper forties on gas without ethanol.

 

I did a check of 87 vs 91 last year during the summer. I found no difference. Several times.

I have been buying 91 lately The station has no non-ethanol 87, so I buy 91 w/no ethanol and tack on the car wash to the purchase. My cold weather mileage suffered greatly with either octane, but has started creeping up with the warming weather. I

I vote that the difference is only the warming weather. Try it again when temps have gotten to normal and put a couple of tanks through it.

My Z28 Camaro, on the other hand, gets better mpgs using 91 rather than 87. But it has higher compression and can take advantage of it.

 

i vote no, waste of money.

 

no, just going with historical info reading here. all the best with your testing, try to be as scientific as possible.

 

This question has been asked and answered so many times, I feel like it is 1947.

Once again: 91 Octane will not improve fuel economy in a car in which 87 Octane is recommended.

 

Thank you Mahout...I've been getting criticized for talking about the possible energy content differences.

To expand on what you said, I was reviewing Chevron online info (Google "Chevron Premium"). They have a nicely detailed 120 page report including discussion of the energy content variations. According to Chevron, on average Premium only has about 1% more energy than Regular. However, my question is NOT what is the average energy content difference, but rather, what could the maximum difference be?...that answer seems to be 7-8% (quoted in Chevron report) but your number 4-5% makes sense to me....the 7-8% would be say winter regular vs. summer premium - maximum possible density variation.

The other tidbit from the Chevron report is: those of us living in RFG areas, we should not expect any energy content advantage for Premium due to the EPA RFG recipe does not allow higher energy aromatics etc.

Bottom line for XrayDoug- I don't know if Oregon requires EPA RFG in your area. If not, then there could be bit more energy content in your 91 Octane.

I will post some of the Chevron report later...in general Chevron seems to have about the best public on-line details for consumers. BTW I saw nothing in Chevron info to suggest any advantage from Premium ...they say they put the same amount of Techron in all grades. Chevron and others seem to be heavily down-playing the energy content variations: they are not marketing premium as higher energy content...but I feel energy content variations are potentially of interest to Prius owners...if some of us have access to higher energy content, we'd want to try it. Not me as I am in RFG region, but maybe in my travels.

OK I keep going. I measured 0.719 g/cc density in my winter RFG Regular today. It's pretty easy to measure. Just take a red plastic safety container (eg; for lawn mower) and fill it safely with 1.00-gal or today I did 1.50-gal. Get a weight on empty can. Then get your net weight of gasoline and divide through by ccs. The only pain-in-the-butt was I had to go to Bed Bath and Beyond to get a 5-Kg kitchen scale. Do your measurments at home as the station owner will think you're nuts (may be true but try to hide it). Density relates to energy content.

 

The largest variables in the experiment would be outside temps and ethanol/no ethanol.

Anything with ethanol will give less mpg. Ensure you use regular and premium with no ethanol or there is no reason to do the test.

 

Engines today are designed to; 1) run efficiently on 87 octane, or 2) run efficiently on 91(+) octane, but CAN run on 87 octane. You can use 91+ octane in any engine designed as #1, but it's not designed to take advantage of higher octane that a #2 engine would. All engines today have spark control that will advance and retard based on octane levels, but only engines designed for higher octane can advance the spark control enough to take full advantage of it. One tank of 91 octane doesn't prove anything, but if you were to alternate tanks of 87 and 91 over a long period of time and have consistent fill-ups using the same station and pump, say over 12 months to account for seasonable weather changes, if 91 octane yields higher mileage you should see a pattern over time that proves or not any advantage.

 

I monitored the engine coolant temperature with a SG in our Gen2 Prius today as part of my post coolant exchange routine, and was reminded just how long it takes the Prius engine to heat up to ~ 158F -- the temperature required for the ICE to turn stop spinning when the car is coasting below 42 mph.

Now admittedly the first 8 miles from my home to my first stop was a good 1/2 downhill, but I barely hit 150F!
Hobbit has said in the past that it takes 2 kW to turn the ICE, so P&G below 158F is no where near as fuel economical as above 158F. I don't know about the Prius 'c,' but my Gen2 built-in gauges do not let me distinguish true P&G from P&C (pulse and coast.)

I agree with others that the warming weather explains the improved MPG of OP and not the higher octane fuel. I suspect the quicker transition to true P&G is the reason. If OP used an engine block heater he could finish his comparo quickly, and enjoy high MPG along the way