95 vs 97 octane for MPG


Longitudinal study of 95 and 97 octane MPG<o:p></o:p>

I was interested to see if there was a difference in economy between 95 and 97 octane unleaded petrol and started recording my MPG using the brimming method and the fuel economy.Co.uk web site.<o:p></o:p>

All figures are based on an 07 plate 2.0l sport (soft top) doing an average 20 miles a day of which 75% is on B roads and the rest on A roads, all in the Shropshire area and average driving. (I haven’t thrashed it but also not driven deliberately slow either)<o:p></o:p>

The fuel was purchased form the same Tesco’s garage each time although there were occasions when I forgot to zero the clock or lost the receipt and I have omitted these occasions from the results. Like wise I’ve not included times I’ve been forced to buy other brands of fuel.<o:p></o:p>

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  95 octane unleaded 97 octane unleaded
MPG 32.3 34.94
  33.02 35.3
  35.46 33.61
  34.57 34.07
Average MPG 33.83 34.49
AVG cost per ltr 118.65 pence 123.5 pence

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As you can see there does not appear to be too much difference. I would like to obtain ten results for each fuel type and perhaps do some statistics to find out if it’s statistically significant; however I suspect that the difference is too small to matter.<o:p></o:p>

I realize that there are many factors involved in measuring real life MPG and I can’t account for all of them but then this is just out of personal interest and not a scientific report!! <o:p></o:p>

What I will say however is I personally believe that my car feels more responsive on high octane fuel although I would guess this is only about 10-15% quicker and would love to do some sort of blind test to see if I’m not just imagining this. Maybe I could let the misses fill it up and not tell me which type she uses!!<o:p></o:p>

Interestingly I drove 99 miles to Birmingham and back the other day which was pretty much all motorway driving, averaged about 60mph and using high octane fuel I got 43.8 MPG.<o:p></o:p>

What do you all think??<o:p></o:p>

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I think that Tesco’s super unleaded is 99 RON, not 97 RON. Wink


It is an interesting experiment, the claims are often argued about, but not many people post up results of there tests.

May I ask how empty you run it before switching fuel types and are you switching frequently of sticking to the same octane for a number of fills. I ask as the fuel already in the tank at full up time may have a baring on the results.


I did the 95 octane tests first, I did not record the first tank of 97  I used as wanted to give it a chance to work through the fuel system first, I normaly let the tank get pretty low before I fill it up.

My local store is a Tesco express and only sells 95 and 97 octane but I will double check the label next time I’m there. I was not aware they did a 99 octane fuel which would probably have made for an even better test, alas I would probably have to drive 10-15 miles to find it.

Could be a regional thing then, dictated by the nearest petroleum refinery?

I am pretty certain that the two Tesco stores local to me sell 95 and 99 RON.

A blind test of each fuel using a repeatable experiement is the only way to really tell if there is a difference, and unless you are going to do it, which you aren’t, the data is worse than useless. Comparing two uncontrolled fuels via random testing on a statistically insignificant sample size will generate random results. Any conclusions you make from your testing will be as scientific as flipping a coin. Doing this sort of thing properly is a massive pain, and even the government fuel consumption tests can be fiddled and beaten. If I were you I’d stop worrying about it and just get on with enjoying the car.

Yes I agree there are many variables that I can’t control for and the data is completely specific to my car but I never intended it to be a scientific report I just though it was interesting to look at what fuel economy I was getting. 10 results for each fuel type however should be reasonable to perform a t-test but I take your point more data would be better. As for the car I love driving it!! <o:p></o:p>

Plum, just enjoy the car mate, if you get some satisfaction from conducting these sort of tests then that’s great. I know down to the last drop how many MPG I get from my car, but that’s all on the highest octane I can get, my SC doesn’t like the cheaper stuff

When you come out of the lab let me know the results.  Can’t be bothered with all that myself.  I like to drive Whistling

 On average my 94 V-Special 1.8 manages around an extra 35 miles before I need to re-fill when I use Super-plus` which seems to be the same whether I use BP Ultimate, Tesco Momentum, or the Shell version which I forget the name of.

My recommendation is to use ordinary unleaded but make every 4th tank-fill a dose of the expensive stuff. Occasional use of Redex hill help many areas of the engine/ fuel system.

Also dont forget tyre pressures make a huge difference in fuel consumption, I usually raise my pressures by 6-8 psi for long motorway journeys.Iinternal friction can rob you of MPG and by regular changes of oil in the engine, gearbox, differential and by using the highest quality oil you can get cheaply, then further economy is attainable.

Air-con is a thief and will easily rob you of 5-7 mpg so save it for really hot days and wet days only when it hepls tp de-mist much quicker.