Going on my first road trip next week to Colorado. I have an 02 disco with no roof rack, stock tire size, tuned and running like a striped-***-ape.

Was wanting to know about best cruising speed to maximize mpg. I run the 91-92 octane petrol. That's another question. Is running the high octane gas that critical?

 

Yes to the good gas! It's what it's made to run on.

 

Best cruise would probably be about 45 MPH. I normally try to take my time, keep it around 60 on the interstate and seldom need to pass others.

Most will recommend using mid or high test. Previous owners said they used mid-grade and will admit changing to regular 87 octane US equivalent of 91 RON since the engine is designed to run satisfactorily using it. Have never heard as much as a ping and don't notice any reduction in performance. Juries are still out on any increase of combustion chamber deposits we may hear about caused by different octane fuels. One of these days will probably try out high-test for several months to see if increase of MPG is noticeable as many have reported.
Workshop Manual:......

 

I've run a tank of 87 or 91 multiple times. It will run, but never happily and I've always gotten a decrease in gas mileage.

 

At higher altitudes you can get away with slightly lower octane fuel than at sea level.
I accidentally tried a tank of 87 here in NYC once. Misfires and stumbling galore. I'd stick to at least 91.

 

I have run 87 exclusively in my Disco's with no detrimental permanent effect on the engine (such as engine knock or piston or valve damage). Aluminum headed and block engines are more tolerant of low octane due to the better thermal conductivity of the head and reduced hot spots for preignition). Of course the computer pulls timing for the reduced octane and there is less horsepower and fuel economy as a result, but not enough to offset the lower price of the regular fuel. I mean come on, its a Disco, how much fuel economy could you possibly gain with high octane. And performance, give me a break - none of these are going to win any drag races - 5000lb brick with a 4.0 or 4.6 liter engine? And at long highway speeds the engine doesn't need the octane anyway - light load, low rpm. I doubt the computer even pulls any timing with low octane at highway loads, probably only happens at high load conditions like pulling a trailer or heavy acceleration.

Let's say its good for 1 mpg, 2 mpg would be a miracle (20% increase - no way) - that's a 10% increase in fuel economy. Difference in cost of fuel is 10% more for midgrade and 20% more for premium - so best case you are breaking even?

Better to put your money in spare parts, it is a Disco...