No, your 3rd gear is actually your high gear, 1:1ratio. The 4th gear, or 'D' is overdrive (when needed) and is .70:1, otherwise it willchoose the appropriategear for you. Leaving it in third will simply keep your RPMs higher and use more gas.

 

I got the v6 automatic LS model...everythin with car seems great so i dobut I got engine or tranny problems....but with the vegas hills its good streth 15 miles keep going up etc 3-5 occasion on the 260 mile route to there from L.A. and I just dont think the blazer is capable of going up there at 75 unless u glue your foot on the gas pedal and even at that point it still takes time...its one of those things, i

 

Maybe use it like a manual. I guess you could put it in third, haul up to 5k RPM or relatively close to redline, and then put it back in OD. Just thinking theoretically though. I would never gothat fast up a hill in my Blazer.