super unleaded - more mpg?

[tgr]

Does higher RON fuel e.g. 98 or Optimax give better mpg and if so, why?

[GravelBen]

Depends on the car - a lot of turbo stuff especially is optimised for high octane fuel so will suffer on low octane. My Legacy is cheaper to run on 98 than 95 as well as having better power, response and smoothness.

As for the why, lower octane = less knock resistance = ECU pulls timing and adds more fuel to avoid detonation.

[sawman]

I used to get about an extra 10% mpg from my mx5 on 98 or tescos special brew 99. It felt more perky too, but I think that may have been my imagination - A bit like when you drive faster after a fill up than just before it!

It used to be better on the 2nd consecutive tankful of super after using regular, I guess the management needs to recalibrate?

[mad4amanda]

I get 10% more mpg on super too assume its got more energy per ml

[98elise]

Its entirely reasonable for a car to get more mpg, assuming you drive at the same speeds. You will need less throttle to achieve the same speed.

[Petrolhead_Rich]

I have odd two week periods where I run on 98/99 stuff.

nothing initially but by half way through the first tank the average is up and the car definitely feels more responsive.

But then I also notice the difference when I have the tools in the boot, slower acceleration, better oversteer

[SturdyHSV]

sawman said:

It used to be better on the 2nd consecutive tankful of super after using regular, I guess the management needs to recalibrate?

I only have first hand experience with how the LS2 ECM works, but for spark timing that has a high octane and low octane table. It hovers between the values in each of these tables depending on knock. As GravelBen said, lower octane means an increased chance of knock, which the ECM will try to prevent by pulling timing and adding fuel.

As such, it should be very quick at reacting to different fuel, possibly the second tank was better because there wasn't any old lower octane left in there.

[Jakdaw]

mad4amanda said:

I get 10% more mpg on super too assume its got more energy per ml

In fact, super unleaded generally has less energy content than 95 RON stuff:

Per http://en.wikipedia.org/wiki/Octane_rating:

wikipedia said:

Octane rating does not relate to the energy content of the fuel. It is only a measure of the fuel's tendency to burn in a controlled manner, rather than exploding in an uncontrolled manner. Where the octane number is raised by blending in ethanol, energy content per volume is reduced.

[ReedyGT]

I try to use Tesco 99 or V Power in the Legacy as it does run a lot better - Definately better performance and economy. The more power the more gains - 5th Gear (or Topgear, can't remeber!) did a test on an Impreza a few years ago and it made big gains.

The BP Ultimate is only 97 RON though, so I stay away from that. Most of the other super unleaded fuels are 99 RON so better.

[kambites]

mad4amanda said:

I get 10% more mpg on super too assume its got more energy per ml

I think it's actually has fractionally less. What it has got, is more octane which makes the fuel less keen to combust when pressurised so the engine has more leeway with its ignition timing whilst still avoiding pre-combustion (knocking).

Hence the answer to the OP's question is a bit more complicated than "yes" or "no". Some engines have no "knock sensor" - these have to be tuned to run on a particular minimum octane rating; if you put in lower octane petrol, the fuel will start to combust before the spark fires. Such car tuned to run on 95 will get no benefit from 98; such a car tuned to run on 98 will run like a complete dog on 95. Most modern engines can detect knocking and modify their ignition timing accordingly - these engines will run fine on any petrol but will produce more power and/or economy on higher octane fuel. Turbocharged engines seem to get particular benefit from it, since I suppose they can also alter their boost pressure to suit the fuel.

[Brilad]

I seem to get slightly better mpg in my 3 MPS but always try to use either co-op super unleaded or tesco because the car runs a lot better on those two fuels.

I have only occasionally put lower rated fuel in it when the tank is almost bone dry and I have arraived to find 'out of use' signs on the super unleaded pumps.

another thumbs up for Tesco Loony Pop from me.

[SirSamuelOfBuca]

Mr r5 gtt is completely different on 99 than standard. Smooth, better power, Definitely better mpg.

[E38Ross]

depends on the car. my mates 1.2 ibiza notices fk all difference. my E38 notices a tiny bit better mpg but not quite enough to make its extra cost back (but i still use it....better for the engine and the owners book says to use it). my dads V8 M3 notices a massive difference....average from 18mpg to 21mpg. 3mpg might not sound much that it's over 15%!!!

[Megaflow]

In my experience it is very variable from car to car. I had a mk3 2ltr Astra Sport, running that on optimax saw an inprovement in mpg that more than off set the extra cost, I tried the same experiment in a Civic 1.8VTi, but can't honestly remember the outcome, this would probably suggest a marginal increase, but not enough to off set the extra cost. Same test in a Volvo S40 T5 made no difference what so ever.

I have yet to try with the current Mondeo 2.5t, but as it is the same engine as the Volvo, there is no reason to believe the result will be any different.

[Riknos]

sawman said:

I used to get about an extra 10% mpg from my mx5 on 98 or tescos special brew 99. It felt more perky too, but I think that may have been my imagination - A bit like when you drive faster after a fill up than just before it!

It used to be better on the 2nd consecutive tankful of super after using regular, I guess the management needs to recalibrate?

Just to pick up on this - You had a 91 MX5, which doesn't have a clever ECU, a very basic one infact, so doesn't have a knock sensor, and wont be adjusting the timing or anything to the fuel, so wont be 'recalibrating' anything, so it was all in your head.

[kambites]

Riknos said:

Just to pick up on this - You had a 91 MX5, which doesn't have a clever ECU, a very basic one infact, so doesn't have a knock sensor, and wont be adjusting the timing or anything to the fuel, so wont be 'recalibrating' anything, so it was all in your head.

Not necessarily - the ignition timing might have slipped off so it's running poorly on 95RON.

[otolith]

I did the maths for 20,000 miles on the Civic Type-R I used to have - calculated a running % of high octane fuel in the tank and correlated it to the miles on that fill. Lots of scatter, you would never be able to say anything conclusive over just a few fills, but over lots of fills a statistically significant correlation emerged between the two. A line fitted through the points suggested that 100% super was worth - I forget the numbers - something like 1.2 miles per gallon over 100% 95RON.

Of course, it wasn't double blind or properly controlled, so it's possible I inadvertently used super for more economical journeys, but I don't think I did.

[kambites]

So that puts you mp£ about the same for the two fuel types in the Civic?

[E38Ross]

kambites said:

So that puts you mp£ about the same for the two fuel types in the Civic?

but then you're running a fuel containing better additives, so (supposedly) better for long term engine health

[Caulkhead]

Brilad said:

I seem to get slightly better mpg in my 3 MPS but always try to use either co-op super unleaded or tesco because the car runs a lot better on those two fuels.

I have only occasionally put lower rated fuel in it when the tank is almost bone dry and I have arraived to find 'out of use' signs on the super unleaded pumps.

another thumbs up for Tesco Loony Pop from me.

Same here, my 6 MPS really doesn't like 95RON and runs best on Optimax or Tesco 99, but then there's a big sticker in the fuel filler saying 'Minimum 98RON' so I guess it should!