Gas question: what if there's no 91 around?

[JD10367]

I've already decided I'm using premium, as the car's engine manufacturer calls for. However... they call for 91 octane? Problem with that, at least up here in New England: most stations don't have it. Most have Regular (87), Super (89), and Premium (93).

Now, I suppose I could combine 89 and 93 every time I fill up, to make 91... but that's sort of a pain in the tookus. I'm not going to try to guess how much gas I need, divide that by 2, pump that, then put my credit card in AGAIN for the other half.

So, what I'll probably do is: fill the tank to the brim with 93, then when it gets down to a half tank I'll top off the half-empty tank with a half-tank of 89. Then I'll run it to E and fill it up with 93 again. Thus alternating a tank of 93 and a tank of 91.

This is the best compromise I can think of. I assume the car's engine will figure this out? I understand the whole idea of not wanting to use 87 or 89 because it ignites too quickly in the piston chamber; so what happens with 93? Does it screw things up just as much to use 93 as it does to use 87 or 89, or am I simply wasting a little money?

[NuBiker]

I'd guess that 93 octane would be fine.

[SuperGeek]

It calls for 91 minimum, 93, 95 or 100 are fine. 100+ is like liquid gold, but it probably won't hurt anything.

[2smartArizona]

Originally Posted by SuperGeek

It calls for 91 minimum, 93, 95 or 100 are fine. 100+ is like liquid gold, but it probably won't hurt anything.

[Belleveu1]

$8 a gallon for 100 octane. Wow! That's too expensive. Can't see how you would get 60% more mileage than using the 93 octane (equal cost per mile). You can get 100 octane at any airport for probably less than $8. Out here in the Seattle area 100 octane would cost about $5.50 per gallon and 91 octane is about $4.05 per gallon.

[SuperGeek]

Yeah, even the 93 is 6 bucks a gallon on that pump... when you get to prices that inflated, it's not about fuel economy anymore. If you're paying 8 bucks a gallon for 100 octane, you need 100 for racing or something other than idling at the WalMart parking lot waiting for Ol' Betty Sue to git outta yer way.

[Feral_Collie]

Nice picture! . . . a morbid reminder of things to come.

Ever see the Futurama movie?
"That 100 dollars could have bought me 1 gallon of gas!"
~Al Gore

[Old smart]

The information provided to the rest of the world (ROW) is that the '08 smart takes 95 Research Octane Number (RON) - 85 Motor Octane Number (MON). US pumps list Pump Octane Number (PON) or (RON+MON)/2. ROW is told the equivalent of 90 PON and the US is told 91 PON. Actually, ROW gas pumps don't list PON, they list RON only, and the equivalent PON naturally settles at 5 points below RON.

So, a 50%-50% mix of 87 and 93 will produce 90 PON, and the monetary incentive increases a bit above making 91 from 89 and 93. At the now common +15¢ and +30¢ increases above regular, the difference is a savings of 15¢ and 7-1/2¢ per gallon respectively. It still may be as much hassle as it is worth when you are considering only about 8 gallons at a pop.

Aside: I once went to the station on a fueling mission. I filled the car with 93, then a couple of jerries for the generator/chainsaw with 87 (on a credit card). Then I tried to fill another jerry for the sailboat's auxiliary diesel. AMEX immediately shut me down. With three transactions attempted within a few minutes their system thought someone had pinched my card (I had to pay cash for the diesel). By the time I got home a few minutes later AMEX had already called the house to see what the circumstances were.

[rawlus®]

i've only seen 91 on pumps in NH and maine, in MA and RI i've been using 93. there's a gas station just outside my office that's had 93 for around $3.59-$3.79 for a few weeks now. averaging about 43mpg over several tanks.

[JD10367]

Originally Posted by Old smart

The information provided to the rest of the world (ROW) is that the '08 smart takes 95 Research Octane Number (RON) - 85 Motor Octane Number (MON). US pumps list Pump Octane Number (PON) or (RON+MON)/2. ROW is told the equivalent of 90 PON and the US is told 91 PON. Actually, ROW gas pumps don't list PON, they list RON only, and the equivalent PON naturally settles at 5 points below RON.

So, a 50%-50% mix of 87 and 93 will produce 90 PON, and the monetary incentive increases a bit above making 91 from 89 and 93. At the now common +15¢ and +30¢ increases above regular, the difference is a savings of 15¢ and 7-1/2¢ per gallon respectively. It still may be as much hassle as it is worth when you are considering only about 8 gallons at a pop.

So, what you're saying is, they say to use 91 but that's only 'cause we don't MAKE a "90" over here, and that I could combine 87 and 93 to get that 90? Cool, that's even cheaper. Fill it up with 93, run the tank half-empty, top off the half-tank with 87... Voila, alternating tanks of 93 and 90.