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truck with a 308 has ALOT higher top speed than one with a 373
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but around town performance will suffer. and towing will be all but impossible. i might use 3.08 gears behind a diesel, but a stouter rear gear plus overdrive is really the way to go. on my dad's 87 Suburban c1500 w 350 and 700r4, changing from 3.08 to 3.73 gears was night and day. before that the truck couldn't get out of it's own way. in o-d, we maybe lost 1mpg. the engine no longer working so hard at such a low rpm. a 3.42 rear from a 90's truck might be a better choice. his current 99 Burb C1500 has hit 22mpg with 3.42 gears.
on my 72 Suburban C20 307, 3 on the tree, i changed the rear gears out from 4.57 to 4.10. the 4.57 gears were so low that i barely used 2nd gear. now i can make full use of 1st and 2nd gear and it will still be stout enough for towing. i do shift into 3rd above 20mph under light throttle. i installed a Q-jet at the same time and the combo is perfect for around town and back country roads. a steady trip at 50mph has so far netted me a best of 16.5mpg.
even now the truck turns 3,000 rpm at 60, so i take my time on long highway trips. i have a spare (late 60's) Saginaw with the Borg Warner R-10 overdrive on the back. but i don't have the equipment to operate it and the trans supposedly needs a 1st gear synchro. but the effective ratio in o-d will be 2.87:1. that's 7% better than a 3.08 on the highway, but i won't have to sacrifice the off the line performance of those 4.10 gears.