Interesting responses, I was mainly worried about something happening to the engine like how they get sludge if you don't spring for the full synthetic oil. (although it sounds like they still might get sludge anyway!) I told the mechanic about the lower fuel mileage on lower octane gas, which of course he said is nuts, he'll admit that the mileage would go down very slightly, but not more than 10%, which would be an extra 40 miles per tank. He might be wrong though, because I remember reading about the "FlexFuel" vehicles they make now that can use the E85 gas. When you run them on E85, the mileage does go down a fair amount.

Well, I guess this means I'm going to have to do an experiment! As soon as I get up the energy to do it, I'll fill my car with 93, drive it till it nearly runs out and refill it with 87 and compare the mileage and how much gas it takes to refill the tank. Now, of course it will make a difference as to how much highway vs. stop and go driving I do, etc. but if there really is a big difference in mileage, I think it should still be pretty obvious. Of course I will report my findings!

 

E85 is over 100 octane, you just need to spray way more of it with much bigger injectors than gas.

 

idk where the hell u getting gas at 2 dollars a gallon but i wanna live there haha... im paying 3.50 for 91 octane at shell

 

i pay $2.80 for 91

wouldnt put anything different in! (well, except for 93... when i can find it)

 

i pay 3 for 93 at shell. and ive done this experiment. for a month or so i ran 93 at a turkey hill with 10% ethanol in it. when i was just chipped i could get like 350 out of a tank. i switched to shell 93 without the ethanol and one time got 420 miles out of a tank. but since then i got lead foot syndrome and now have a k04 so i only get like 320. but where i live, shell is the only station without that ethanol in it.

 

As a teenager owning an Audi, i have researched this topic with great depth. my hypothesis was i didnt know if i would save money using higher octance, or using regular 89 octane. So i broke out the ol' pad and pen and crunched some numbers.

I got 350 miles out of my tank at 89 octance. 89oct was $2.69 p/gal on 16 gallons. SO.

2.69x16= $43.04 to get 350 miles.. 350/16=~22 mpg.. not too shabby

Next time, i used 91 octance, and got 390 miles. 91oct was 2.79$ p/gal. on 16 gallons. SO.

2.79x16= $44.64 to get 390 miles.. 390/16=~24 mpg.. woah, thats better than 89 octance.. hmm.

350/$43.04=8.13197 miles/dollar

390/$44.64=8.736559 miles/dollar

According to my data, it not only is better in the long run to use 91 octance fuel, but you may also get better gas mileage. I havent tried 93 octane yet but imagine that you will only recieve better results.

Therefore, although it may hurt to fork out a little bit more at the pump, you are proportionally saving money and getting more miles out of your dollar.

*NOTICE: I drove the same roads with both tanks, and did not change my driving style with both tanks. I also checked the psi on my tires and ran both tanks with the same psi on all 4 tires. I used BP gas, and the prices were the same at both fill-ups, meaning when i used 89oct, 91 oct was $2.79, and when i used 91oct, 89oct was $2.69