I'm sure engine swaps in JK's are becoming more common. When you swap for a Hemi or whatever else, what kind of HP numbers are you trying to hit? Lets assume the vehicle weighs in at 6,500# (loaded down with gear, 5 passengers, full fuel tank (25 gallons) and and extra 20 gallons on hand), and you're running the appropriate gearing for your 37" tires. Sure more is better, but there comes a point where you are making wasted power. One of tasks the DEFENDER will perform is overlanding. So being efficient with fuel is key. To be able to go four to six hundred "trail/road miles", maybe just a whole day, without having to sweat my fuel situation would be an added plus! Opinions? Am I being too optimistic?
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Going off the numbers you provide (weights, fuel capacity) you should be able achieve those mileage numbers. Typically you can plan for an average of around 12mpg (combined hwy/unimproved road/breaking first tracks), that would yield roughly 540 miles per fuel load out.
It doesn't matter what vehicle or engine combo I have run; jeep, land rover, HMMWV, truck, it just always seems to float around the 10-12 mpg mark for combined driving.
In the photo below, this was an extreme case at all ends, mileage needing to be covered, types of terrain traversed, weigh carried. It was all extreme limits. The vehicle started out dry/empty at 9,000lbs, when completely loaded with men, equipment and fluids, we were 13,300lbs. Even with the craptastic Chevy diesel motors, we managed between 9-13mpg over the course of 3 weeks. A 24 gallon tank with 55gals in fuel cans.
I have no idea what this Tacoma weighed, but it was ridiculous.....again it was getting around 12mpg over the course of about a week of driving with that load. Bad low light picture, sorry.