higher octane = higher mpg for 2002 V10

Generally speaking, if the Owners Manual doesn't tell you to use a higher octane, don't. It gives you no benefit. And costs more money. Trust your automotive engineers.

(Well, maybe not "Generally Speaking". Maybe "Always"?)

HR

I did a trial in my 99 (pre three-valve and crank mods) and found no detectable difference between 87 and 89 and 93, other then what I was speeding on gas

Well, this is not a Ford example, but...

My Saab turbo... From the owners manual: "For best fuel economy use regular gas. For best performance use preimum".

Now, I could give f* all about economy, after all I drive that Saab about 500 miles a year, so I use premium, but were I to consider maximizing my economy I'd sure try regular.

Now, closer to your Ford, my Dodge Hemi says Premium (only). I'm cheap with that truck (gets many more miles) and usually fill with regular. I've not noticed a change in economy in it (if there is, it is nominal) but performance is not as good as with premium.

If your engine calls for 91 octane, and you've been putting in 87 or 89, then the engine will adjust itself for the lower octrane. If it calls for

Your Saab Turbo _wants_ premium fuel, but can adjust itself for regular fuel if you don't mind the performance degradation. Instead of 0 to 60 in 4.5 seconds on premium, it takes 6 seconds on regular. Most people would never know the difference if that's all it is.

If it calls for 91 , Its probably a performance car like a VW, Saab or Corvette or other. And the engine timing may not be adjuistable down to 87 or 89, or adjust to a lower octane as it might be a bumped compression ratio requiring the octane. This is why they say , use 91 only. If it can adjust down then you can use 87. If the car has variable timing for octane then a higher octane should allow higher mpg/ power if the timing adjust for it . If it doesnt adjust then the higher octane might be wasted. Might , because higher octane gas is slightly less volatile then a lower octane with less additives to raise the octane in the first place. In the case of some gas stastions. They just mix their tank of 91 with the tank of 87 in the pump . To pump 89 in your car. However if the car has a minor problem like a worn egr washer, running hot, or a slightly advanced timing. Or just needs a tune up. It might run better on the higher octane. stop pinging, or get better mpg.