Technical rear end ratio

I am running a 454 bb hooked to a Turbo 400 GM trans, no over drive, my rear tire size is P235/70/R15. My RPM's at 60 MPH is abt 3000, too high. I would like to turn 1700 or 1800 at 65 MPH can anyone tell me the rear end gear ratio needed to make this happen?

 

2.47 gears would do it...

 

1880 rpm with a 2.50 rear gear. 1729 rpm with a 2.30 rear gear. Boy would that be a dead player at the green light.

 

Have at it: http://www.csgnetwork.com/multirpmcalc.html

Your tires are theoretically 27.95" tall.

 

Oh, and make sure you have a good, big fluid cooler.

 

That's a Strange Brew of a ratio. Are those Disraeli Gears ?

 

Swapping my 400 for a 700 would be another fix but one without computer assist. How to know if bolt pattern on tran matches 454 bp? I know there are several variations.

 

Two bolt patterns on 700R4 trans if my brain is working right and there isn't one designed to fit one of the odd ball engines that doesn't have a Chevy V8 or a BOP bolt pattern. I don't think they put them behind 2.8 or 3.8 engines though.
My computer isn't letting me save photos to be able to show the difference right now.

 

V8 700s are all chevy bolt pattern, so it will bolt up. But a 454 makes a lot of torque. The 700 is a relatively finicky transmission.

The 2.47 gear is common in late 70s big Fords/lincolns. I have a set.

 

Compromise , 308s will be nice off the line and still....it's a big block...

 

Cool, now I know. A search of "2:47 gear ratio" led me to Creams album Disraeli Gears and the song Strange Brew @ 2:47 play time. Go figure

 

A song I used to listen to on reel to reel tape in the 60s

 

3:31 is a good 12 bolt Chevy ratio to use behind a big block without overdrive. You never said what rear you're using.

 

No computer goes with a 700R4, just one 12V keyed wire, and a ground.

 

I thought BBC was for hauling rocks, hauling big boats, or hauling ASS !!!
With 2:47 gears, you must be headed for the salt to haul ass .

I'm going to take a very educated guess that your BBC shows worse efficiency characteristics at 17-1800 Rpms than at 3000 Rpms.


Here's a post from another thread about switching a Th400 to 4l80e, with controller. He was at 3300 rpm @70mph with turbo 400. There's a 3 mpg increase. How many miles does he need to drive it before the cost of the swap comes close to the fuel savings.
12 mpg vs 15 mpg. - a standard bench rebuild for 4L80e is $1200 and an average controller is $700. Who wants to do the math ? Think it's that he needs to drive 45 miles to save 1 gallon of gas, buts late and I'm beat so I could be off. Gimpy, what do you have here.

Tested it on a 60 mile trip and got a solid 3mpg increase at 15.2 mpg. This was at 70 with converter locked and a nasty headwind I was driving into. Very happy, truck feels great at 2200 rpm on highway

 

I've got a 327 with a cam giving good torque at lower RPM and 4 bbl with a 3.08 rear end. Turns 2200 RPM at 70 MPH and have decent acceleration. That should work good with your larger engine.

 

Remind me in the AM. Heading to bed.

 

I think this will be a cool set up He will need some pull gear and not lug that big block. Alot of big blocks come with those lower number rear gear ratio. But for highway don't they like to have some Rev. Especially trucks for pulling boats or whatever and of coarse areo dynamics

 

Not enough info on the truck, for me to help him figure out what he oughta do...I just answered the question he asked.

I ran 2.75 gears in my 55 with a blown 454 for years, it was still fun. I have the same ratio in a 5000+ lb car with a 460, it's fun to drive, doesn't seem to lug the engine that I've noticed.

But we each have our own idea of how to set up the ratios.

 

Sixty miles, all other conditions assumed equal:

12mpg for 60 miles uses 5 gallons.
15mpg for 60 miles uses 4 gallons.

My local shops get $1200 to rebuild a TH400.
They get $1200 to rebuild a 4L80E, and you need a $650 controller.

Torque converters are close enough to be the same price, rebuilt.

So that is $1200 versus $1850.

Today's national average (I wish) price for a gallon of gas is $2.41.

At a cost differential of $650, that is the equivalent cost of 269.71 gallons of gas.

To break even, you'd have to drive 16,182.6 miles.

If you already have a TH400 in, and it is in good working order, it is a $1850 cost differential.

That would be the same as 767.63 gallons of gas.

To break even, you'd have to drive 46,058.1 miles.

 

Al that said, I am not sure how well a 454 is going to do at 1800 rpm.

That is going to be up to a whole mess of variables that have not been provided.

Gross vehicle weight, drag coefficient, every single aspect of the engine (fuel delivery system, manifold, heads, valves, compression ratio, cam, etc.)

An optimistically efficient 454 pulls in slightly less than 200 cfm at 1800 rpm. You will need a fuel delivery system that can handle doing that delivery at as close to 100% efficiency as possible, and still handle everything else it is expected to do.

Plenty of modern vehicles turn at that rpm at cruising speed, but they all have one thing in common with each other, that they don't have in common with yours, and that is EFI.

 

1800 RPM at 70? It would be a dog around town. Play around with a gear calculator, you may get better... or worse... numbers depending on tire height.

 

down to the penny....amazing!

 

A nice compromise gear for a direct-drive setup (no OD) is to choose a rear gear with the same numbers as the tire height. It gives about 2000 rpm at 60 mph, 3000 at 90 mph. Both those numbers exclude converter slip and are within the roundoff for reading the tach. With your tires, I would say a gear between 2.60 and 3.00 would be excellent.

 

And I thought I was alone at doing useless math just because. Either way, doing something and not doing something is equal.

 

I had it on an 8 track cartridge for my Lear Jet 8 track player.

 

I'm pretty sure I have the vinyl version at home. I think I'll get it out and listen to it the next time I'm there.

 

I have a 700R4 out of a 85 Chevy truck in my 57 with 373 and it gives me the best if bought the worlds, plenty of punch off the line and down the track and a switch to kick the over drive in order n the hyway I run 65 all day long and turn just about 2200 rpm just make sure it's built tough or u'll be having it rebuilt after a few hard runs lol I also run a 3000 stall in mine, lol just a thought in fact I'm gonna move up to a set of 456 gears if I can find them for my 12 bolt cheap enough lol, jmo hotrod Harry


Sent from my iPad using The H.A.M.B. mobile app HotRod Harry

 

Never cared for the 700's ratio between 1st and second 3:06 to 1:62 for that type of action. Your e.t. would benefit with a tighter one two shift of 2:47 to 1:57 along with 4:11's and not lose as much cruz rpm.

 

Boy a lot of info. Guess my info provided was a little sketchy. My concern is not gas mileage but turning the bb what I thought was a little hard [3000 at 57 mph]. My th400 trans is rebuilt complete but seems to quick or stack shift 2nd and 3rd, that's another adjustment issue I'll take care of. I have not been able to determine rear end ratio, axial code is KC G201 1 4 , unit is a 10 bolt 8.2 or 8.5 inch from a early 70's model Chevy Caprice and completely rebuilt. The differential cover is a rounded 11 inches across with bulge for ring gear, it has the 2 square lugs on bottom of housing and from lower center cover bolt to either abjection bolt is 3 1/4 inches. The carrier casting is type "O". The od of my rear tires is 27.95 inches. I know this ratio thing must be determined prior to making any changes. Not sure how bb would act on low end with tall gears but it sure jumps now. The truck is never abused but still it needs to be able to get out of it's own way. Please talk me through this and I appreciate it.