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I have a 2014 Fiat Punto and have been using 92 octane gas. I have never read the manual, but am now told I should use 95. Should I use what is in the tank before filling with 95 or is it OK to mix them.

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It's not a problem mixing them. Just start using 95 if that is what the engine calls for. You'll not create any issues doing so.

Modern day engines have sensors which can adjust for the fuel. If yours calls for 95, but you're running 92, the engine most likely won't be putting out the power it would on the 95. You probably won't notice a difference in driving, but you might. Anyway, its nothing really to worry about mixing the two.

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Presumably since you say "92 octane gas" you are not in Europe.

The whole topic is a mess, because there are different scales for measuring "octane". The most common scale world-wide is RON, but there is a different scale known as MON which produces lower numbers, and in some countries (particularly the USA, Canada, and Brazil) the numbers on the pump are actually the average between RON and MON. The MON number can be as much as 8 points lower than the RON for some fuels.

The Punto was designed in Europe, and none of this is an issue there, because grade are always described in RON and the cheapest "regular" gasoline grade available almost everywhere is 95. If lower RON grades are on sale they are sometimes more expensive than 95 because of the small demand for them. For example a typical UK gas station sells "premium" 95 ROM ("premium" being marketing-speak for "lowest grade", of course) and "super" 97, and possibly a higher grade (e.g. 99, 100, or 102) with a brand name from a particular oil company (Shell, BP, etc).

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  • About 92 not being available in Europe: Denmark, Ukraine and Moldova still had it available a while back and Belarus still had 93 around that time. Not sure if they still do, but that was less than a decade ago. – Mast Jan 20 at 9:13
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    "and in some countries (particularly the USA, Canada, and Brazil) the numbers on the pump are actually the average between RON and MON." At least in the USA, this is explicitly stated at the pump, too. ((R+N)/2 and all) – JAB Jan 20 at 21:07
  • I've seen that "102" thing here and there and it never made sense to me - isn't it just a percentage of one chemical to another? How can a percentage possibly be higher than 100%? Sounds like a marketing scam to me... – Darrel Hoffman Jan 20 at 21:59
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    @DarrelHoffman LPG has a RON of 108 - its not a percentage. – Criggie Jan 20 at 23:09
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    @DarrelHoffman, octane rating is a measure of how much compression it takes to ignite the fuel. It's on a linear scale, with pure heptane having an octane rating of 0, and pure iso-octane having an octane rating of 100. Something even more resistant to compression-induced ignition than iso-octane, quite reasonably, is described as having an octane rating above 100. – Mark Jan 20 at 23:14
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A modern car (say, post-1995) will run on lower octane gasoline with reduced performance (power, economy) and no other issues.

Older cars did a lot of nasty things ranging from occasional "pinging" or "ringing" noise and accellerated engine wear to complete engine failure.

Then again, one cannot easily find these days neither a pre-1995 car nor a gasoline with low enough octane number in order to create problems in a car built for pre-1995 standards gasoline.

p.s. it is OK and pretty much advisable to mix some '95 gasoline into '92 in your tank

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  • 1995 also happens to be roughly where cars tend to handle bio-fuel-added petrol (like 95 E10) better. Between '95 and '09 how cars handled fuel got even more complex, to a point it would surprise me the Punto cares much about the difference between 92 and 95 octane. Performance suffers a bit, sure, but near-empty it's going to drive just fine. Wasn't a fast car to start with. – Mast Jan 20 at 9:24
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    one cannot easily find these days neither a pre-1995 car - I have to disagree. I'm in the UK and see pre-95 cars all the time. On my street alone (about 60 houses) there are 3 pre-95 cars (A-reg, F-reg and M-reg). – Aleks G Jan 20 at 13:10
  • @AleksG in absolute sense, yes. I'm in Bulgaria and I pretty much own two cars that are 1982 and one that is exactly 1995 so on my street alone (about 100 houses) they are at least 3. Then again, most pre-1995 cars today are either unpleasant to use, expensive to own, or both, so you need a good reason to own one. (Well, the reason may be that you are poor and in a desperate need of a car.) What's that "A-reg, F-reg, M-reg" ? – fraxinus Jan 20 at 14:04
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    @fraxinus In UK, for cars registered between 1983 and first half of 2001, the first letter of the registration indicates the year - A is mid-1983 to mid-84, F is mid-1988 to mid-89 and M is mid-1994 to mid-95. These cars are all owned by fairly old people who probably bought them as new. – Aleks G Jan 20 at 14:19
  • Thank you for the explanation. Our (and other that I am aware of) registrations are region-based. – fraxinus Jan 20 at 14:48

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