You're ECU will correct for the lower grade of fuel (retard timing, etc.)
I could be possible that you might get 38 MPG on premium or more. I've registered better than 40 MPG on my S on long trips if I behaved myself, but I must state it's not as much fun when you're behaving .
Just fuel for thought.

 

I admit to thinking 1, but not 2. I figured octane and hp were related.

 

I've tried that in other cars with similar results, Octane rating did NOT affect the MPG outcome. Driving style, however, did.

 

I thought lower octane would cause lower MPG?

 

It's 93 Chevron or Shell for me. No problems with stumbling, stalling, or whatever.
Gas cap lid says 91 but down here it goes 87,89,93.
I also drop a bottle of Techron in the tank every 3K or so.

 

In a nutshell, the octane in the gas keeps it from detonating at higher compression rates. For a higher HP engine with high compression rates you will need higher octane to keep it from detonating. However, on an engine that needs say 87 octane, running higher octane reduces the energy level of the gasoline at the same compression. Technically running higher than required will generally give less HP than what is needed. It might or might not be measurable but you are running less efficient gas for your car than it's designed for.

http://www.bargaineering.com/article...-gas-myth.html

 

Plenty of opinions and esoterica about MPG and octane on this thread. But if you have a MINI you can do your own experiment and your own Arithmetic and see if the [at today's gas prices] the 7% penalty that you pay for the recommended octane [91] results in more or less than 7% change in fuel mileage. Remember it takes more than just one tank to do this. The engine management software 'remembers' and averages out changes in the amount of correction for fuel quality. Try three or four tanks of one level and then a few of the other. It's not rocket-surgery. It is easy to be beguiled by $0.25/gal difference at the pump but you could be costing yourself money and performance by going the lower octane route.

 

If you really want to avoid problems use premium non-ethonol gas if you can find it. Here in Louisiana its fairly easy to find and is roughly the same price per gallon as the 10% ethonol, if that makes a difference. Once I started using the pure gas I no longer had black carbon deposits on the tailpipes. Ethonol turns to water in the gas, especially if the car sits for a while, so you are burning 10% water in your gas. If you get carbon deposits on your exhaust there must be some inthe engine, esp. the direct injection engines on the MCS. Like others have stated, for a few dollars a fill-up more its just not worth taking the risk. I also run a bottle of Chevron Techron (the same stuff Mini sells in a different bottle for a much higher price) every third fill-up and have had no problems in over 2 years('09 MCS vert.)

 

If you have access to non-ethonol gas I highly recommend using it. If you have a MCS use premium not regular, the higher the octane the better. It only costs a little more and you will get better mileage per gallon and your engine will run cleaner. I started using pure gas two years ago and the '09 MCS runs much better and gets better mileage.Also, once on pure gas the carbon buildup on the tailpipes

virtually disappeared and I now clean the exhaust pipes about every 3 months and even then the buildup is minimal. Ethonol turns to water in your tank and I believe is one of the causes for the carbon buildup in the valves. If you are burning 10% water it can't be good.

Tofind if there are any pure gas stations in your area Google "pure gas". You will find a site that lists by state and city all stations having non-ethonol gas and the octane rating for each grade available.