C2 Fuel grade / consumption

Hi, My girlfriend has had a C2 (1.4) for about a year here in the UK (from new). We've always filled up with standard unleaded fuel. We didn't think
to try a higher grade fuel partly because it's not a performance car, and partly because a few years ago, I tried my old Porsche 944 on the good stuff, and found that I gained performance but didn't save any money (with 70 and 30 mph speed limits, there seemed little point).
Anyway, we thought we'd try a tank of 98RON fuel instead of the normal 95RON in the C2. It was 2p/litre more (90.9p instead of 88.9p). We are a little stunned by the difference. We do a lot of short trips and stop- start motoring, with the occassional long haul. Our average fuel consumption overall on 95RON was 37.6mpg, with long-hauls returning something like 41-42mpg. We haven't done a long-haul yet on the 98RON fuel (just short runs and stop-start), but the average is coming out at 42.8mpg!
If my sums are correct, we're saving over 1p per mile. Once we've done a long trip, I'd expect the figures to get even better.
My new phrase this year seems to be "here's the rub" - don't know why, I hate it... But here's the rub: What octane rating does fuel in France go up to? When we checked the manual to make sure that 98RON wasn't going to blow the engine up, we noticed that the maximum grade is actually higher than 98RON. So, fuel is not only cheaper in France, but there's the possibility that it's better quality than the stuff available in the UK.
Here's another rub: We're a two-car household. Our 1.4 Citroen C2 runs on high octane fuel. Our 2.7 Porsche 944 runs on low octane.
--
Regards,
Dave
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Dave Ryman (dave snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :

Bear in mind the weather's improved markedly of late, so the car's going to be on cold start enrichment for less time.

95 or 98 are the two standard grades, same as here.

You can get higher than 98 here - BP Ultimate is supposedly 99, Tesco do 100 in some stations, and there's a new (forget which brand) 102 on limited availability.

I've always found the opposite, to be honest - cars that don't pink or run- on on 95 over here do on 95 over there. 98 tends to be a smaller premium in France than here, so we tend to run on 95 here and 98 there.
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(snip)

I'll have a look for that - thanks. I can't remember the maximum mentioned in the manual: I know it's at least 99, so we'll at least try that once we've run for long enough on 98 to be more sure of our figures.
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Dave
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Dave Ryman (dave snipped-for-privacy@hotmail.com) gurgled happily, sounding much like they were saying :

I strongly suspect that you're at the tipping point in the price/benefit curve, though - supermarket 98 tends not to be too big a difference (3p at JS when I filled the XM yesterday - at 1p, I'll go for it; at 2p, I'll think about it if I'm feeling flush and doing runs; at 3p or more it can swivel. On an 85l fill-up, it's a big difference...) but the very-high- number juice tends to get pricey quickly.
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It shouldn't make any difference in theory. The octane rating is a measure of the anti-knock properties of the fuel, rather than a performance indicator. Provided the manufacturer states that your car can run on the lower rated fuel then there shouldn't be a difference.
If it can't, and your engine is pre-igniting, pinking, then you may get a fall off in performance but otherwise I can't see why there should be better mileage with one over the other.
But then I'm not a fuel engineer....
TonyB
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TonyB ( snipped-for-privacy@soxclara.co.uk) gurgled happily, sounding much like they were saying :

Presuming the ignition timing and injection mapping remain the same.
In a modern engine, they won't. The engine may very well have a base ignition and injection map set for 98/99 or higher, which is then adjusted to suit lower octane fuels, as detected by a knock sensor or similar, as well as the results of combustion as detected by the lambda.

Because you'll be working the engine harder to get the same performance. In something like a C2, where you're pedalling reasonably hard to stay with traffic, that's certainly likely to affect economy.
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gurgled happily, sounding much like

That all explains perfectly well why our 2006 C2 1.4 does better mileage on the 98RON, but our 1989 Porsche 944 2.7 doesn't.
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In gurgled happily, sounding much like

That's because the Porsche is meant to run on 98 RON and will use the extra octane to produce more power - thereby using at least the same amount of fuel as it would on 95 RON.
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Pete M - Using the Scouse Side of the Force -
Golf GTi Mk2 (2.0 transplant in progress),
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Thanks Adrian, That makes sense to me too. TonyB
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Adrian wrote:

I tried my car (406 2 litre petrol) on the expensive BP/Esso petrol instead of the normal Sainsbury cheap petrol for about a month (1600 miles+). It maybe made 1 mpg improvement. Serpently not enough to warrant changing onto it permanently as it loses me money using it.
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Malc
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gurgled happily, sounding

Interesting - maybe the economy factor is related to engine size. My 2.7 litre car doesn't gain any economy with the good stuff (hence it's cheaper to run on 95RON fuel, even with a little pinking once in a while). I got the impression that the engine just said "yum yum": My gains were all in acceleration.
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Dave Ryman wrote:

Could be. I can't say I noticed much improvement in acceleration but then I'm not a foot to the floor driver. .
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Seems that, when you reset the on-board computer's figures, and have a full tank, it takes a while before the figures become accurate. After having done some more driving of various styles, and finishing the tank of the 98 RON fuel, the fuel consumption is hardly any different to the figures we got on 95 RON.
I do apologise if I got anyone excited about the prospect of saving a few bob ("Bob" = "shilling" = 1/20 of a British pound).
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Dave
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I always check my fuel consumption by filling the tank til the cut off operates on the pump then divide the litres by 4.55 to get gallons. Check the mileage and reset the odo before driving off. I have found my Picasso computer to be about 2 mpg optimistic using this method, but I do get a genuine 48 mpg which is better than my last car, a Discovery at 28 mpg! They are diesels by the way. Thanks for the info though. TonyB
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