Gear ratio advice

[OregonNed]

My truck is currently getting its rear differential rebuilt (a lot of problems found internally). It currently has open carrier with 3.08 gearing, but here is my chance to change the ratio for a bit more umph! We are switching to Posi as well. I’m running 27in diameter tires, and I’ve studied the RPM charts. Now thinking about 3.42 or 3.55 for it. Any opinions on one vs the other? Running a 327 with TH350 trans as well. Thanks in advance

[Accelo]

I think you will be running right in the sweet spot with 3.42 or 3.55 gears.
Stock camshaft? If so you might consider the 3:42 ratio. With a mild camshaft do the 3:55 and don't look back.
Rick

[OregonNed]

Thanks for the reply. My cam is not stock and I have been leaning towards 3.55

[pjmoreland]

I have 3.73 gearing. With a TH400 trans, my truck was a bit intense on the freeway at speeds over 60 MPH. I had. '67 years ago that had 3.07 gearing. It was great on the freeway, but it was a dog around town. Going somewhere in the middle makes a lot of sense. Personally, I would be inclined to lean more toward a ratio that is better for freeway driving, but it really depends on how you plan on using your truck. If you have a relatively quiet exhaust system, then that will make higher numeric gearing more tolerable at higher speeds.

[jimijam00]

Cam specs?
What is your ratio of freeway/highway to city driving?

[OregonNed]

It’s a mild cam. Have no idea of what went in when the PO rebuilt the engine. Gonna guess city/freeway is going to be about 50/50

[roboticjunky]

If you have a truck 12 bolt rearend, then go with 3.42 gears since they don't make 3.55 gears for the truck 12 bolt rearend.

They do make 3.55 for car 12 bolt rearends.

[OregonNed]

Originally Posted by roboticjunky

If you have a truck 12 bolt rearend, then go with 3.42 gears since they don't make 3.55 gears for the truck 12 bolt rearend.

They do make 3.55 for car 12 bolt rearends.

Thank you good to know that. Appreciate the help

[tim_mc]

I'm also planning to have a spare 12 bolt rebuilt as a posi this year. I really like my 3.08s but also have a 700R4 with lower 1st and 2nd gears than the TH350. I love running 75+ at 2000 RPM, but most of my driving is local. I go back and forth on 3.08 or 3.42, but currently leaning toward 3.42 as I don't think I'd give up too much with 4th + lockup. If I still had the TH350 I'd almost certainly go with 3.42 gears.

[schovil69]

The difference between 3:42 and 3:55 is not likely to be noticeable, darn near identical. Throw a dart at a paper with the two ratios written on it and choose the one that the dart is closest to.

[Wrenchbender Ret]

If you have been running a 3.08 & go anything lower you are going to be disapointed the first time you get out on the highway. You will be waiting for the thing to shift again.
George

[jimijam00]

Originally Posted by tim_mc

i love running 75+ at 2000 rpm, but most of my driving is local.

3.73.