Search up the tdi swaps that people like to put into tacomas. It won't be fast but it could be done for not to much money.

99 Montero....15mpg! Sold (so sad)
02 Jetta TDI....48mpg!
03 2500 Ram QCLB Cummins....18mpg
02 2500 Yukon XL, We out grew the montero, at least it has one locker.....13mpg (sold)
01 montero limited