Forum:
put the old thermo i got with the engine back in and the temp has went down but i think i need a new fan clutch the new thermo i got had a smaller opening then the old one but now i got that prob solved i got another 
can you get higher gears for the front and rear diffs in the sportrak/us rocky im looking to lower my rpms at speed right now im geting about 26 mpg
Playing with gearing is
Playing with gearing is always a double edged sword. What you gain on the highway you loose from off-road and load hauling ability. Tyre size is outwardly the simplest place to go for a gearing change a simple increase in tyre section can give a big difference in speed/rpm as long as the tyres fit under the arches.
But be aware that the larger section will put more strain on all the drive and suspension points so savings on fuel will be spent on maintenance of faster wearing parts...
The main problem is getting the speedo/odo to read correctly on the new rubber but overall the savings made will not be that significant as you may not be able to cruise in top gear for long enough to garner the saving you are looking for. I guess 10m.p.g. is your ideal improvement(?)
The first job with a diesel conversion would be to find out if Daihatsu Japan fitted a diesel to home market cars then you have a source for the parts required to do a fairly straight forward engine and gearbox swap.
Although the saving in fuel relative to the conversion cost will mean tens of thousand miles of driving to make it worthwhile in savings terms. Given the difference in the US/UK gallon 26mpg is about the best you could expect. In the UK Sport tracks on review test drive at around 30mpg which is almost the same figure given the different in the fuel volume.
An Auto engineering workshop will be able to fabricate an adaptor plate should you only be able to swap the engine. You may also need to look at the cooling system capacity so a thicker radiator core may be required and this may mean a custom build to clear water and power steering pumps etc
Have you thought about using LPG for fuel? In the UK this is about half the price of unleaded petrol. The car drives the same and can return to petrol at the flick of a switch the conversion costs are moderate and the cost/savings recouped much faster.
Just to give you an idea of UK unleaded fuel cost we are paying around £5.50-5.70/gallon or around $12-15 by the time the volume difference and fiscal slips have been factored in. LPG is currently about £3.00/gallon
On my current pay I work 4 hours to buy the fuel to drive my 1000cc car to work for the week...previously I was working the equivalent of 14 hours to put fuel in my 2.2ltr turbo-diesel (4x4 auto and assisted everything) MPV for the week.