what would cause my 92 navajo to rev high when i start it and while cluching on 1992 Mazda Navajo

it revs up when i start it untill i put it in gear and start letting out on the cluch the only time it stops reving up is when i pull up to a stop light and put it in first gear with my foot on the cluch the truck will drive its self when i lit off the cluch because it revs high i really need to know what i need to replace cause this is eating up my mpg help!!!!

Asked by for the 1992 Mazda Navajo