With very few pickups in the four-cylinder business these days--the Ranger's gone, finally--the Nissan Frontier is one of the few mid-size trucks to even offer the gas-saving engines.
In that form, the Frontier does a decent job of improving on the gas mileage you'd find in larger trucks. The in-line four gives the Frontier an EPA rating of 19/23 mpg in its most basic form, with a manual transmission. Add an automatic, and the mileage drops to 17/22 mpg.
Stepping up to the six-cylinder Frontier erases the advantage, and the EPA numbers drop to 16/20 mpg at best. At worst, the Frontier checks in with a 14/19-mpg rating for automatic-transmission, four-wheel-drive models--a figure far off the base Ford F-150's 17/23 mpg, and almost on par with the V-8 Fords. The six-cylinder Frontier will save you gas--when it's compared to Nissan's larger Titan, yes, but the Titan is one of the thirstiest pickups.