What gear ratio should I go with?

[Big70]

So I have a 70 short bed with a 454 30 over that's pumped with a th400 behind it. My plan the hole time has been to go with 373's in my 12bolt. Good idea or bad? I wanted a streetable hot rod that will go light to light but still be able to jump on the freeway. Your thoughts?

[BlouDon]

I've got a 700R4 behind my 350 with a 3.77 12 bolt rear. Not a hot rod by any means, but its rapid out of the blocks with the 3.06 first (TH400 has a 2.48 1st). Too much right foot and all heads turn my way.

Fantastic on the highway with the 0.7 overdrive 4th and lockup torque convertor.

2200rpm cruising at 75mph (our speed limit here in South Africa). With the Th400 it was 3100rpm...

[Jrainman]

if I was looking to set up that combination and have it do what you are asking for I would be looking in to adding a Gear vendor unit to the TH400

[garyd1961]

It depends on how your motor was built. Does it have a big lumpy cam built for high rpm power or a mild cam built for low end torque.

[Hart_Rod]

Don't forget tire diameter plays a large part in this also...with that being said, I believe most factory Big blocks came with 3.08's unless ordered differently.

[Heavymetl]

A good converter is worth more acceleration than a deep gear. With the broad torque curve of a big block, you definitely don't need a ton of gear to get things moving.

But the converter is everything. The newer nice units from places like FTI and PTC don't exhibit much slip at highway speeds, run smooth and quiet, and let the engine get into the meat of the torque curve quickly.

Anything from 3.23 to 4.11 would probably ET within a tenth of each other on a track.

[HO455]

My Burban has a 350 with 3.73's and 275/60/15's. I'm turning about 3000 rpm at 65 mph. If you are comfortable with that rpm for long trips then no problem. Personally with the generic small block (unbalanced, cast pistons, stock valve train) I have I generally travel at about 60 mph on the highway for long trips. That being said it is my daily driver and I would go to an overdrive before switching to higher gears with a vehicle as heavy as a Suburban. It had 3.07's in it when I bought it and acceleration was measured with a minute hand.
So after all that yes it is a good idea.

[weim55]

If it is a mild cam, dual plane intake 454 I would go with factory 3.07s a good posi And call it done. It’s so much nicer to drive freeway speeds with that gear and the 454 still has plenty of torque to lay rubber anyway. Sure, that freeway gear isn’t quite as much fun as a 3.73 but it’s so much more livable. Been there done that..........

Steve weim55 Colorado

[jocko]

Originally Posted by Big70

So I have a 70 short bed with a 454 30 over that's pumped with a th400 behind it. My plan the hole time has been to go with 373's in my 12bolt. Good idea or bad? I wanted a streetable hot rod that will go light to light but still be able to jump on the freeway. Your thoughts?

What is your rear tire diameter and current gear ratio?

[CastIron]

I have 3.73s on my truck with a manual transmission. Short trips on the highway are fine, but I wouldn't want to take it on any trips where I'd have to maintain 70 mph for an extended period of time. It gets pretty loud inside at about 65 mph, and that is with quiet exhaust and a fan clutch. With a TH400 and no torque converter clutch, the rpm would be a little higher too.
If you are planning on changing gears anyway, I'd say go for around 3.42 or so. You're going to need some sticky tires if you want to get maximum acceleration with 3.73 gears and a big block.

[dennislbrooks]

I say OD also with the BBC. I run a 402 with 700R4(Monster) with lockup, 3.73 gear and 32 inch tires. Mine is a cruiser 4wd. The low 1st in a 700R4 may be to your liking lite to lite. I am not a converter expert however mine is a 1500 stall.

At about 1400 rpm with OD/lockup I am running 60 mph. BBC with stock 350 HP cam does not care.

Gear vendors are expensive - been there. Dump the 400 for the street.

[jocko]

Light to light hot rodding and freeway cruising rpm are mutually exclusive gearing goals that can only “really” be solved, without any compromises, by an overdrive. Increase your ratio for the light to light and you raise your highway rpm. Reduce your ratio for highway cruising and light to light perf suffers. Overdrive retains the same off the line perf you have today (or will have if you swap rear gears) while enabling lower highway rpm. The only ways to introduce an overdrive is by an OD trans swap or an after-the-trans gear reducer (i.e. Gear Vendors overdrive, for example).

Since you have a warmed-over big block and it sounds like you intend to use it, I personally believe the TH400 is a good trans to stick with. The only trans I’d (personally) replace it with (for this specific scenario), to achieve your two goals, is a 4L80E – which is essentially a TH400 with an OD gear (and a bunch of electronics to deal with). It’s an involved and expensive swap. You can soup up a 700R4 and some have luck with that combo, but not everyone - but that’s also an option. The alternative, and a good application for your scenario imo, is the GV OD unit. It’s more bulletproof than the TH400, so it won’t be your weak link when you get on it.

The additional capability a GV OD unit brings, when compared to an OD trans, is that it also allows the gears to be split, which in essence turns your TH400 into a 6 speed. In other words, the gear reduction ratio can also be applied to each gear, not just top gear. But to be honest, it’s probably not something of interest for running between stoplights – gear splitting is more applicable to motor homes and/or towing heavy loads up an incline.

Having said all that, that’s just the gearing aspect of all this – none of us on here knows your big block and what “pumped” actually means – or just how seriously you want to go from light to light. You may indeed find that a low numerical ratio rear gear such as 3.08 or 3.42, etc provides more than enough acceleration and an acceptable highway rpm to make YOU happy. While folks’ input, mine included, may be helpful in your decision, only you know which tradeoffs you’re willing to accept or how much you’re willing to pay to eliminate compromise. Good luck! If you post your tire dia and current gear ratio, can post a pic of all the rpm data at various speeds for you.

[Getter-Done]

There are calculators on-line that might help you.

The T400 has a 2.48 to 1.00 in first;
1.48 to 1.00 in second;
direct drive in third

Link:http://www.tremec.com/calculadora.php

Link:https://tiresize.com/height-calculator/








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[jocko]

here's an example for a 30" tire:
EDIT: and it shows why you would not want an O/D with a 3.08... (too low highway rpm) - and why you really need one with 4.10s if you're gonna spend time on the highway. I probably wouldn't use an OD with 3.42s as highway rpm gets a little low and highway rpm is not that bad to begin with in a 1:1 final drive trans, unless you're doing 70-80 all day long.

[Greasey Harley]

If you want impressive Stoplight--->to Stoplight, 3.73:1 with a Big Block should be perfect
Unless you have overdrive, you may not like it on the open tarmac.

[HO455]

Originally Posted by jocko

Light to light hot rodding and freeway cruising rpm are mutually exclusive gearing goals that can only “really” be solved, without any compromises, by an overdrive. Increase your ratio for the light to light and you raise your highway rpm. Reduce your ratio for highway cruising and light to light perf suffers. Overdrive retains the same off the line perf you have today (or will have if you swap rear gears) while enabling lower highway rpm. The only ways to introduce an overdrive is by an OD trans swap or an after-the-trans gear reducer (i.e. Gear Vendors overdrive, for example).

Since you have a warmed-over big block and it sounds like you intend to use it, I personally believe the TH400 is a good trans to stick with. The only trans I’d (personally) replace it with (for this specific scenario), to achieve your two goals, is a 4L80E – which is essentially a TH400 with an OD gear (and a bunch of electronics to deal with). It’s an involved and expensive swap. You can soup up a 700R4 and some have luck with that combo, but not everyone - but that’s also an option. The alternative, and a good application for your scenario imo, is the GV OD unit. It’s more bulletproof than the TH400, so it won’t be your weak link when you get on it.

The additional capability a GV OD unit brings, when compared to an OD trans, is that it also allows the gears to be split, which in essence turns your TH400 into a 6 speed. In other words, the gear reduction ratio can also be applied to each gear, not just top gear. But to be honest, it’s probably not something of interest for running between stoplights – gear splitting is more applicable to motor homes and/or towing heavy loads up an incline.

Having said all that, that’s just the gearing aspect of all this – none of us on here knows your big block and what “pumped” actually means – or just how seriously you want to go from light to light. You may indeed find that a low numerical ratio rear gear such as 3.08 or 3.42, etc provides more than enough acceleration and an acceptable highway rpm to make YOU happy. While folks’ input, mine included, may be helpful in your decision, only you know which tradeoffs you’re willing to accept or how much you’re willing to pay to eliminate compromise. Good luck! If you post your tire dia and current gear ratio, can post a pic of all the rpm data at various speeds for you.

Well said sir.