I picked up my 3.0L '97 4x4 about a year ago, and was consistantly getting about 15/16MPG on 75% higway / 25% city driving with 87 octane gas. Even with new plugs, wires, valves, sensors, air filter, etc. I still would get the same MPG. I never experienced any knocking or pinging, and the engine seemed to run smooth. On a whim I tried using 91 gas, and immediately my mileage jumped. After 3 tanks I can say I am consistantly getting 20MPG+ with the same driving routes and style. I have also noticed VERY SUBJECTIVELY that my RPMs seem to run a few hundred lower at any given speed on the highway than before. Is this to be expected? Does this indicate some problem with the engine? I thought the 3.0 was designed for 87 gas...no? Now I'll try 3 tanks at 89 and see what happens...

Thanks,
Aaron

 

The only way you will get a change in rpm vs speeed is through slippage in the tranny. It's basically fixed unless you change diff gears or tires.

 

Good point - I'm an idiot. Although with my luck the tranny is about to explode...

 

How are you calculating the MPG?

 

After filling the tank until the dispensing handle "clicks," I reset the trip odometer. After the tank gets mostly empty, I refill again until the handle clicks, look at how many gallons I just refilled, and divide my mileage by that number. Been doing that for years...is there a better way?

 

Nope thats the way to do it.

 

Yep, thats it and wanted to make sure that is how you were doing it. I've heard some pretty convoluted methods, the worst of which is using the gauge rather than gallons used based on fill-up.

I have no explanation for jumping from 15 to 20+ when the only change was high octane gas. It shouldn't do that, but if it is then I would roll with it. You getting a lot more than your money's worth out of it. That extra 100 miles is only costing you about $3.50 or so.

 

Wish mine would do that. The best I ever get is about 19mpg, with the tuner. On 87 stock I was getting 17-18mpg.

 

Is your truck flex fuel? I don't remember what years exactly. But if yours is flex fuel(you can tell by the green leaf emblem on back of your truck) then it can adjust its timing. I should try expensive gas.

The best Aaron

 

If it was flex fuel, why would it be able to adjust its timing for 91?

 

Higher octane fuel is harder to burn, so I think something else is the reason for this mileage increase.
I ran 3 tanks of 87 in my 91+ octane SVT Focus, and the mileage jumped, going down in octane...

 

Yeah, I really can't explain it either. From the VIN code, my truck does NOT suppoer E85 FelxFuel, so that can be ruled out. I'm continuing to do other stuff to the car, so the whole experiment is about to become multi-variable. I may go back to 87 to see what happens, but getting an extra ~5MPG is definitely worth the extra $0.20/gallon.

 

One of my former employers F-150's was getting about 25% better milage than the rest of the fleet and it turned out to be the 02 sensor. It wasn't running lean enough to detonate , but was getting better fuel milage than the other trucks. That was 4.2 liter and auto's, about 20 vs. 16. They were all equally heavily loaded and abused.

 

I believe higher octane gas gets better milage in my experience. I use 93 and get more power and runs smoother. I've always used higher octanes since my grandfather explained the burning proccess to me. Dont know if this is true...and dont have evidence to back it up...but it sounded believable. He explained that premium gas does not ignite as fast as 87 but premium has a bigger explosion in the chamber. This explained why timing was needed to be advanced on older vehicles with premium. 87 octane ignites faster but does'nt have as big a "boom". I myself notice you dont have to push the gas pedal as far to get speeds you want. Also i think premium burns much cleaner and ive never have carbon build ups.