91RON in car optimised for 95RON fuel

[NickZee]

Hi everyone,

New to the forum, have been hypermiling a 2014 BMW 640i Gran Coupe. Although it is a fantastic sports car, it also has a fantastic Eco Mode that is very effective and and fun to use because of all the data that it provides to the driver. I see hypermiling more as an Eco-friendly challenge that avoids a few stops at the gas station as opposed to any real concern about saving money.

The manual States that 95RON fuel is recommended, and the minimum requirement is 91RON. Any lower than 91 may cause damage according to the manual. So 91 should be safe to use and on my first tank I have not noticed any problems with the smooth running of the engine.

I was wondering if any of you could chip in about whether using below recommended rated fuel is likely to effect mileage when driving at low to moderate loads?

If there is no knocking at low to moderate loads, and the energy rating of 91 is the same, then I should be able to use 91 without adverse effect on mileage?

91 is generally 9% cheaper than 95, and my limited driving so far indicates that fuel economy is about the same on my work run but it is early days in this experiment.

[gone-ot]

Yes, so long as that 91 octane rating is maintained...remember, it's supposed to be a minimum of 91 octane, but tank-to-tank variances are known to lower that number at cheaper gas stations.

[redpoint5]

My Acura says something similar, saying 91 PON is recommended, and anything below 87 PON may cause engine damage.

I've been running 87 PON for years with no adverse affects other than reduced top-end power. The timing is slightly retarded, which may reduce fuel economy ever so slightly, but I don't notice a difference from my fuel logs. I can tell the difference between 87 and 92 PON just by driving the car. My wife used to randomly choose one or the other, and when I went to drive I would correctly identify which grade she choose based on acceleration feel.

Sometimes I'll mix my own, filling about half with 92 PON and then filling the rest with 87 PON. Mid-grade fuel here (89 PON) is simply a mixture of 40% premium and 60% regular, which they charge more for than mixing it yourself.

All this to say, run the cheaper stuff and see if it impacts fuel economy in any noticeable way.

[NickZee]

I have now done three trips to work and haven't seen any noticeable difference in fuel economy, so good times . Will re assess at the end of this tank.

[oldtamiyaphile]

Originally Posted by NickZee

Hi everyone,

New to the forum, have been hypermiling a 2014 BMW 640i Gran Coupe. I see hypermiling more as an Eco-friendly challenge that avoids a few stops at the gas station as opposed to any real concern about saving money.

91 is generally 9% cheaper than 95, and my limited driving so far indicates that fuel economy is about the same on my work run but it is early days in this experiment.

Which is it? The only reason to run 91 is to possibly save money.

If I had a $200,000 car, I don't think I'd take the risk for $0.10 per litre. Wheels magazine lists all BMWs as requiring 98, and all European cars across the board require 95 (apparently our 95 is already a lower octane than European 95 - though I suspect this may not be true).

My mechanic had to rebuild a VW because the owner had been running it on 91 since new and at 50K kms it was dead. I'd stick to the recommended rather than minimum.

I run my Euros on 95 or 98, I'd rather not be paying for a new engine for the sake of a few dollars at the pump. Just seems a bit risky to me. I understand that theoretically you should be OK, but it's seriously weird that BMW allow 91 when every econobox needs 95, hence my wariness.

[some_other_dave]

Originally Posted by oldtamiyaphile

(apparently our 95 is already a lower octane than European 95 - though I suspect this may not be true).

The US uses a different ratings scale than most of Europe does. There is about a 4-5 point difference between the two, so one batch of gasoline would likely measure as 91 RON (European system) and 87 AKI (US system).

Their 91 is our regular-grade, basically. Their 95 is on the order of our mid-grade or the "premium" crap we get in CA, and their 98 is pretty close to the higher-grade 93 that some folks in other states can get.

-soD