As my userid implies, I'm a Saab 9-2x Aero owner, aka Subaru Impreza WRX. I only have a few hundred miles on it and have been negatively impressed by the gas mileage (about 21). Other than that, it's a great car and the $6,000 rebate plus GM employee pricing eased the mpg (and premium fuel) pain. Question re octane: I usually buy gas at Costco and the two octanes offered are 87 and 93. Does the octane resulting from mixing the two fuels have a direct mathematical relationship? In other words, if I had 2/3 tank full of 93 octane, would filling it up with 87 octane result in 91 octane for the entire tank?

I'm afraid I can't answer your question, but...

Prices are assumed
13g of 93 octane @ 2.60 = 33.80
7g of 93 @2.60 = 18.20, 6g of 87 @ 2.40 = 14.40 ---> 32.60

Is your time and frustration worth $1.20? If you don't care about performance or engine longevity, just put a whole tank of 87 octane in...

Actually, I'm not at all "frustrated", just disappointed that a 2.0L 4 banger gets 5 mpg less than my Mustang GT with an engine (5.0L) over twice as big and does it on gas which costs 20 cents a gallon more. Also, no "time" is involved because I'm at the pump anyway and hitting the "87 OCT" button takes the same time as hitting the "93 OCT" one. Sorry I rattled your cage. Have a nice day. :)

I read somewhere about mixing fuel and calculating the octante rating but I can't remember the whole deal.. My sugestion is just go with 93, it's better for the engine anyway. Turbos tend to detonate, and you don't want that happening.

In theory you can mix them to get 91, yes. But as said its a lot of trouble to save $1.20 per fillup.

Your mileage should improve a bit as the engine wears in. Also try to drive smoothly and stay out of boost most of the time.

Thanks for the responses. I did some more googling and found the following, which answers my original question (from some college study's FAQs as I recall):

6.15 Can I mix different octane fuel grades?

Yes, however attempts to blend in your fuel tank should be carefully
planned. You should not allow the tank to become empty, and then add 50% of lower octane, followed by 50% of higher octane. The fuels may not completely mix immediately, especially if there is a density difference. You may get a slug of low octane that causes severe knock. You should refill when your tank is half full. In general the octane response will be linear for most hydrocarbon and oxygenated fuels eg 50:50 of 87 and 91 will give 89.


Btw, just yesterday I utilized this tactic when I put in over 7 gallons of 89 octane to a tank over half full of 93 octane. I agree 100% that there's not much money involved (in yesterday's case, only about 75 cents). Too bad 91 octane gas is rarely (if ever) seen (at least by me). Perhaps at the old Sunoco, which had 5 or 6 different grades available, but now I see just 87, 89 and 93 and at Costco, where I usually get gas, just 87 and 93.

I'll just rationalize the whole issue by figuring the extra $1,000 rebate I got on the Saab 9-2X Aero (turbo) vs the Linear non-turbo model pays the 20 cent per gallon difference for premium gas over regular over the course of 100K miles. ;)

Before we got our WRX my wife was concerned about the extra cost of premium gas, so we worked it out. Assuming 10,000 miles/yr, 20 mpg, and $0.20/gallon extra, the added cost is only $100

Before we got our WRX my wife was concerned about the extra cost of premium gas, so we worked it out. Assuming 10,000 miles/yr, 20 mpg, and $0.20/gallon extra, the added cost is only $100

Yep, that's how I got my estimate above of the extra $1,000 rebate for the turbo Saab 9-2X covering the additional 20 cents/gallon for premium over 100K miles, i.e. the same $100 per year (10K miles) you got times 10.