transmission help

[Tylerz181500]

im all for having a whole proper gearing, most things ive just acquired knowing and never really understood why its the "proper" gearing.

basically my 72 has 36's 9 inches of lift and a th350-205 and 3.73s ill probably end up taking the body lift off and running 35's or keeping it on and running 35-37's (the 36's are no good)

trucks mostly gonna be a work truck/cruiser, so i dont want to be too high up in the rpm range if i have to go on the highway, so my thoughts were either what rear end gears would i need? id like to go to a 14 bolt/60/70 or an eaton if i can, or should i swap to a 4l80e-205 so i an have overdrive?

i havent gotten into looking at 4l80e swaps yet

[geezer#99]

Try this calculator.
Your 3.73's and 36" tires show 2263 rpm.
Just right for highway driving.
An o/d will drop you down to about 1600 rpm. Your motor likely won't be happy turning that slow.
http://www.advanced-ev.com/Calculators/TireSize/

[Tylerz181500]

37s would be a bit higher than 2200 then?

[geezer#99]

That's what the math says.
What mods you got to what motor?

[Tylerz181500]

no motor yet, im building a 350 for it, lt4 cam, ls6 springs ported and polished vortec heads carb intake 750 holley to be tuned, felpro 1094 gaskets on a stock but rebuilt vortec lower end. nothing to fancy and then of course exhaust, e fans, and some other stuff. th350 trans and 205 right now.

i was just curious ive always had a hard time understanding gears and tires

the overdrive you said would put me too low, so id fix that by going with a higher gear then?

the calculator you suggested at 75 said id be at 2600 that seems like alot to me. i know my friends 72 with 350 sm465 np205 and eaton ho72 rear with 4.56s does about 2600.

[geezer#99]

I'd put it all togethor first and see how it runs with the gears and tranny you have.
You're right about the o/d.
Sometimes you need a higher ratio rear gear combined with the o/d to keep your motor happy.
I call that the 'sweet spot'.
Every motor is different.
I ran a sbc that got it's best mileage at 3400 rpm and 85 mph.

[Tylerz181500]

i do know about the sweet spot i just always thought there was some common knowledge to things like my gmt400 commonly 3.73s and 33;s go together, 4.56 and 37 etc.

i was just uber unsure on a 3 speed like a th350

now a thing i always get confused higher gearing would be going from 3.73 to 4.56 right or is higher lower and lower is higher????

just wanted to kinda plan ahead yanno, like if i need to regear ill go 14 bolt with some better gearing, or if im good with gears and an overdrive would help maybe id look into a 4l80e swap. id rather regear to find a 14 bolt though because 12 bolts scare me, ive shot the pinion straight up breaking the plug welds on a few trucks playing in the mud. never drove those on the highway though.

[geezer#99]

Easy way to remember is the higher the number the lower the speed.
Higher gearing is lower numbers like 3.08 or 3.42.
Lower gearing is higher numbers like 4.56 or up to 5.13.

[Tylerz181500]

so its backwards allrighty! so in your opinion keep the 350 add a shift kit or something? the truck will probably end up 3 inches lower with 35's

[geezer#99]

Keep the t350, leave it stock. Wait until you have your motor in and tuned with whatever tires and your existing gears. Then if you feel you need more off the line power and better cruise rpm look into an o/d tranny and maybe some different rear gears.