55.2-59 another gear ratio question with a bbc

[Spank-a-lank]

I have a 58 shortbed half ton that I am looking at outtin a 402 bbc in. I have had problems finding a manual transmission with overdrive that will work with stock(or close) shifter position and am curious if anyone has used the sm420 with a higher gear ratio in the rear diff.(3.00 or 3.08 depending on axle type) wanting to be able to cruise highway speed ok but also know that the sm420 is a wide ratio transmission. just wanted to hear anyones experiences that has done this or close to it. please do not recommend an automatic as I do not want that. thanks in advance. any real advice/experience welcome.

[geezer#99]

The sm420 is just a wide ratio 3 speed with a stump pulling granny gear.
The only thing good about it is the shifter location.

Is your 396 mild or wild?
What’s your rearend gear ratio?

[mongocanfly]

I'd be looking for a nv4500
But it all needs to work together
Rear gear, tire size, final trans ratio, desired cruise rpm at xx mph

[G&R's57GMC]

What is the diameter of the rear tires ?
What gear ratio do you have now ?

The engine you have is a torque monster . The truck is light and traction challenged .

You won’t need low rear gear . If you were to install a 2.50 gear you won’t need overdrive and a Turbo 400 or a 3 speed manual .

Yah I know not very sexy but it will get down the road quick .

[mr48chev]

This link is off an EV site but is the simplest Speed, rpm, gear ratio, tire size calculator that I have found. http://advanced-ev.com/Calculators/TireSize/

It is figured at 1 to 1 at the output shaft so you have to factor in any overdrive in the final gear ratio meaning if you have an 80% over drive and are running 4.11 gears you multiply the 4.11 by .80 to get 3.288 or 3.29 or 30 for your final gear ratio.

You put in any three factors and get the fourth so that helps a lot. It's great for the "what will a tire size change do to my RPM at 70 kinds of questions.

This one will just give you the rpm after you put in the other factors.
https://purperformance.com/p-29669-rpm-calculator.html Great if you just want to know what rpm will change when you do other changes though.

[_Ogre]

Originally Posted by G&R's57GMC

The engine you have is a torque monster . The truck is light and traction challenged .

You won’t need low rear gear . If you were to install a 2.50 gear you won’t need overdrive and a Turbo 400 or a 3 speed manual .

traction challenged but true
did anyone make a rearend with 2.50 gearset?

[geezer#99]

I think gm had a 7.5 inch 2.29 ratio. Came in late 70’s Malibu’s.

[mr48chev]

I guess before we get too carried away we really have to ask, exactly what do you plan to do with the truck?

Are you going to tow with it?

Are you going to be hauling heavy loads with it?

Are you going to be running around the woods or overlanding with it?

The Transmissons you asked about are all work truck transmissions with a compound low gear for as we used to say "stump pulling".

The SM 420 is strong and still has a big popularity with old school Jeep guys for crawling around the hills. It is non syncro and not really a streat trans behind a big block.

One of if not the best informative pages for the transmissions you are interested in is on the Novak adapter site. https://www.novak-adapt.com/knowledg...ssions/manual/ They are primarily saying how the Transmisson might work in a Jeep conversion, but they do a great job of explaining the plusses and the minuses of each Transmisson including which ones may be weak when you put power to them. Great info for what you want.

As far as overdrive or rear gears go back to that adavanced EV link I posted in post and play with it. Put in your tire diameter, gear ratio the truck has now and the speed you want to run down the road and hit calculate rpm and it will tell you what rpm you need. Put in the rpm you feel comfortable with, tire size and speed you want to run, and it will you the final drive gear ratio you need. ( trans output x rear gear ratio).

I'd go with a good as in good shape, SM465 over a 420 though. No issue behind the 402 and they are synchronized making shifting a lot smoother up and down. You would need the matchng iron bellhousing with the larger hole for the bearing retainer though. That setup is all over the place in 68/91 Chevy and GMC two-wheel drive trucks.

[1project2many]

did anyone make a rearend with 2.50 gearset?

Yes. That's a Blue Oval gear ratio. Somewhat common, too.

I think gm had a 7.5 inch 2.29 ratio. Came in late 70’s Malibu’s.

I had one in a '79 Monte. I used it when I installed the 455 Buick and switch pitch THM 400. Light car, high torque, 18 mpg if I kept max speed to about 70. I had dreams of replacing the THM 400 gearset to get lower first and second gear ratios but that never happened. I didn't abuse the axle and it lasted to 250k miles. The axles wore out at the bearing surface and replacement axles were crazy $$ so I installed an 8.5" axle with 3.08 gears.

If you are looking for a truck transmission with OD I would strongly suggest an NV4500. It takes a fair amount of modification to install it on the factory bellhousing but it can be done.

[mr48chev]

The 2.5 and around there rear ends are normally run in a light rig like 1project2many had with a 3 speed automatic. They are clutch burners behind a stick. Normally no one drives a compound 4 speed starting off in granny low all the time unless you are actually headed up a steep incline or pulling a pretty serious load.

Most guys with a compound 4 speed in a pickup want to run a 3.5 or 3.7 gear for road use and still be able to pull or haul a load while being able to drive down the highway at highway speeds but that is with a 30 inch tall tall tire too. That is a 235-75-16 like I have on my 71with the 500 Cad with the turbo 400 and 3.55 rear that should run 2793 rpm at 70.

Looking back to when I had the 283 and Muncie 4 speed in my 48 in the 70's it had N-50 14 tires on the rear that were 26.84 inches tall, ran 3.0 gears and did 3000 rpm at 80. It loafed along at 2063 at 55 mph.

[burnin oil]

I drove a 76 blazer with the 465 and a 400sb for years that had 3.07 gears. It worked fine, even with 31" tires. The only thing to remember is that there is a 50% drop in rpm between 2nd and 3rd so the motor has to have a little Rev to it. I never understood people's hatred of them. The 420 and 465 transmissions are about bomb proof and scattering a trans sucks. I also did haul the farm tractor a few times with that truck. No problems


Both the 420 and 465 are synroed in 2, 3, and 4 but 1st and Rev are not.
Nv4500 is about the same as above only adding OD. I really like them but are not really worth the extra money. I do have one in my 72 k20 that has 4.10s.

[Ziegelsteinfaust]

Originally Posted by _Ogre

traction challenged but true
did anyone make a rearend with 2.50 gearset?

Yes they did. GM made them factory, but I believe it was 2.43. Aftermarket 9" has 2.50 I believe.

I put a stump pulling 455 Olds into a 68 Nove with Joe Mondello parts. The car was faster with 2.43 gears then the 3.42 it had before.

I am not up on much besides factory GM stuff, but super high gears were fairly common in the later 70's.

[burnin oil]

I remember 2.5 and 2.7 gears in the GMs. I never saw them in a chevy but Oldsmobile. Same cars just different name plates.

[1project2many]

2.73 was common in 8.5" axles.

With start of 7.5" axles in late 70s and lightweight G body cars, GM was equipping passenger cars with small bore, long stroke engines and numerically low rear axle ratios. Under square engines tend to favor low rpm torque peaks. With emissions measured in grams per mile, it made sense that turning the engine fewer times in a mile should reduce emissions. And as a bonus, fuel economy tended to improve.

The option codes actually list a G72 2.14 rear gear. Sounds like a ratio for Bonneville speed runs to me.

[53burb]

I believe the rearend underneath my 53 Burb, IIRC is a 2.29 or 2.31 from a Nova...maybe a Camaro...haven't driven it with it tho....

[dennislbrooks]

Square body 4x4 automatics ran 3.07/8's. I run a similar axle 84 4x4 rear diff in my 4x4 59 (12 bolt) but with a 3.73 gear. I take you want a small 5 lug which can be obtained from an s10 4x4 rear axle as mentioned above. I can see you ripping this axle up with too much traction and a posi.

The sm420 is a good trans and I expect with the bbc you will only start out in 2nd gear for all normal driving. Probably never use 1st. You may want an 11 inch good clutch.

If you go to an overdrive manual or auto, my 67 4x4 402 bbc runs a 700r4 (.78 OD and lockup) with 3.73 gears with 265/70R17 tires and at 60 mph, I run about 1550 rpm. The bbc does not care much.

A harder to find item is a 9.5 14 bolt with the large 5 on 5 which can easily be made 5 on 4 3/4. Search for a 454 SS 1/2 ton pickup. You may have to change gearing. I run this axle with its 3.73 in my 67 but modified it to 6 lug. Perches need modified for this axle.

Put a 9inch Ford in it.

Just info

[burnin oil]

Even with the BBC torque you will still like granny with 3.07 gears. Most time you don't use it but things like drive through or a steep incline it's nice to have. I called it concert gear because it was perfect for creeping inline without slippimg the clutch. 2nd is a 3.57 gear ratio so it is decently low.