Advice on gear ratios

[BIGBLU 72]

I have been kicking around the idea of changing the rear end in my 68 lwb from 4:10 gears to 3:07 gears. the truck has a strong 400 small block and a fresh TH 400 tranny. wat are some advantages/disadvantages or concernes about swaping the rear ends? also the 3:07 rear end is out of a 69 swb.

[john02fxdl]

Well, first of all, your wallet will like it. You'll get better fuel mileage and less strain on the drivetrain if driven on the street( I'm assuming it's a street driven truck). It's actually very complex when choosing a rear gear ratio. You have to account for weight of truck, stall speed in torque converter(if any over stock), tire size, cam profile and usage of the truck (main operating range in mph and rpm) A numerically lower gear ratio will be slower on take off, but less rpm on highway (3.08, TH400, 70mph= approx. 2000-2200 rpm). A numerically higher gear ratio will provide quicker off the line acceleration but will scream at highway speed ( 4:10, TH400, 70 mph= approx. 3500-4000 rpm.) You could choose a happy medium that will provide decent off the line performance and decent cruising speeds on highway like a 3.42 or 3.73. I would suggest a 3.55 but IIRC no-one offers it in a 12 bolt truck rear(I could be wrong).

[BIGBLU 72]

thanks for the advice! this is my daily driver and the truck i drive to college so the good highway cruising speed and mpg with the 3.07 sound great to me.