Gearing with TurboHydramatic

[asphaltburner]

Frankentruck is running a TH400 and 4.11:1 front/rear gears right now with 33" tires. 1973 axles. I'd like to reduce the highway rpms by changing ring and pinion fore and aft but would like to ask for opinions from others running a similar setup.
3.73 gears sound OK, but 3.42 may provide the best low end acceleration and still lower top speed rpm sufficiently. My 350 V8 has enough HP and torque to turn low gears!
I'd like to hear from those of you with a similar mechanical setup. In your experience what axle gear ratio has worked best for you with your TH400/350? The truck needs to commute 40 miles roundtrip on the highway, but usually in bad weather. Then it's in 2WD high, 4x4 high, or FrontWD high (twin stick NP205),...low is reserved for terrible situations and mountain FUN!
May I ask your opinion on axle gear ratio? Expensive to do this properly,...I want the final product to be perfect!
I calculate 3000 rpm@70mph with 4.11 gears, 2730 rpm@70 with 3.73, and 2500rpm@70 with 3.42. Sound accurate?
Thanks, Todd.

[Keith Seymore]

Without doing any math, my comments are as follows:

3.42 was always our "default" ratio when spec'ing a 2wd half ton small block truck.

So - for a stock 4wd I'd probably recommend a 3.73 to overcome the inertia and drag of the larger tires (and extra drivetrain).

However - you're at 4.11 now, so a drop to 3.73 might not show much in terms of fuel economy - plus, you're saying your motor is warmed up bit so -

I'd say 3.42.

[James McClure]

Todd, I'm running 3.08's but with a 465 and 32" tires. 350 stock eng and 4WD. If I am carefull with the left pedal I get 17 MPG on the road and about 14 around town. It doesn't seem to change much ( 1 to 2 mpg) even when pulling a 7000 pound equipment trailer. Jim

[asphaltburner]

Yo! Thanks guys, anyone else have input?
I'm leaning toward the 3.42:1 ratio due to the TH400s' lack of OD, and as Keith puts it, my "warmed" 350.
It sports 9.2:1 compression because of the World heads (67cc combustion chambers, 2.02/1.6 valves, and I added roller tipped rockers as an easy, cheap upgrade). Also runs shorty headers, Weiand manifold, 600 Holley on a 2" spacer, and a Crane towing cam advanced 4 degrees. So, Keith, I think you're correct.
Thanks Jim, right now I've calculated 11.9mpg in my first trial,...I do not anticipate EVER getting 17mpg but I hope to get over 12, especially if my gearing swap is "just right". This truck is SO much fun to drive that I'm willing to pay for the gas!!
Thank you very much for the good input!
Todd.

[Keith Seymore]

Originally Posted by asphaltburner

...So, Keith, I think you're correct...

I'm doing an "end zone dance" right here at my desk....

[asphaltburner]

One other quick question,...does anyone know if a 3.42 ring and pinion are available for my axles? Both from a '73 Custom flatbed, the rear is a 5560 rated full floating:



and I believe the front is a Dana 44:



Does everyone agree, and are 3.42 gears available for each of them?
I'll probably have Bullhide 4x4, in Ft Collins, CO do the work,...they estimate $1500 to $1800 total.
Thanks again, Todd.

[Pyrotechnic]

Another thing to consider is a Gear Vendors. Expensive yes, and you can only use it in 2WD, but honestly that's really the only time you will need to use it.

[asphaltburner]

Yo! Pyrotechnic!
I apologise for my ignorance, but what are you referring to?! Sounds enticing! Tell me more!
Are we talking OD?
Todd.

[Prong]

I run 3.42's in my 79 SB 4x4 with a fairly stout 383/T350. I run 32" BFG tires on it, and it gets around 12 if I don't get too wild with it. On the highway I've gotten as good as 14 out of it (keeping the R's around 2000).

[3r!c84]

Originally Posted by asphaltburner

Yo! Pyrotechnic!
I apologise for my ignorance, but what are you referring to?! Sounds enticing! Tell me more!
Are we talking OD?
Todd.

its like adding a transmission on your transmission lol

look: http://www.gearvendors.com/cg2wd3s.html

[b454rat]

Buddy has a truck that has a stock 350, 4-speed 3.42s and 33s. I drove it for awhile, and wasn't too bad. Steep hills needed downshift, but other than that, good all around gear.

The 3.42 is available for the 14 full floater and the Dana 44. But finding those gears might take some time....but think it would be worth it...

[LONGHAIR]

I'm with Pyrotechnic on this one. The GV overdrive will not cost you that much more than changing 2 sets of gears, and you get the best of both worlds.

[68 TT]

Originally Posted by asphaltburner

Frankentruck is running a TH400 and 4.11:1 front/rear gears right now with 33" tires. 1973 axles. I'd like to reduce the highway rpms by changing ring and pinion fore and aft but would like to ask for opinions from others running a similar setup.
3.73 gears sound OK, but 3.42 may provide the best low end acceleration and still lower top speed rpm sufficiently. My 350 V8 has enough HP and torque to turn low gears!
I'd like to hear from those of you with a similar mechanical setup. In your experience what axle gear ratio has worked best for you with your TH400/350? The truck needs to commute 40 miles roundtrip on the highway, but usually in bad weather. Then it's in 2WD high, 4x4 high, or FrontWD high (twin stick NP205),...low is reserved for terrible situations and mountain FUN!
May I ask your opinion on axle gear ratio? Expensive to do this properly,...I want the final product to be perfect!
I calculate 3000 rpm@70mph with 4.11 gears, 2730 rpm@70 with 3.73, and 2500rpm@70 with 3.42. Sound accurate?
Thanks, Todd.

Todd, I had 33x12.50's and 3.42 gears on my 90 Z71. Even with the lower first gear of the 4L60 trans it was a dog off the line.

3.73's are probably your best compromise and will put you right about 2200 rpm at 60 mph with the T-400. Much better than the 3200 rpm I am at at 60 mph with my 5.13 gears in my 78 K20.

Unless you do a lot of freeway driving and not much city driving the gear swap will cost you more than the gas mileage change and loss of off the line performance is worth.

It really doesn't hurt the engine any to spin it up in the 3 & 4 grand range all day long as long as you have proper oil pressure and the water temp stays where it should. Fuel consumption goes up a little but not as much as you would think with the increased RPM.

I ran everything from 2.56 to 5.13 gears in my 70 Chevelle over the two decades I had it. The gearing below 3.31's actually got worse gas mileage than the 5.13's & 4.88's did. It liked the 3.73's the most with the 28" tires so your current 4.10's are actually equivalent to about a 3.6x gear ratio with 28's. What you have is probably the best gear for you.

You could always gather the parts to do a 4L80E swap like I have in the works

[68 TT]

Bigger cams and big valve heads need more gear not less because of the low end torque loss. Look at the cam catalogs, the bigger the cam the more gear and more stall they recommend.

What stall converter do you have in there? You can't go with much to keep the stall speed below your cruise RPM but anything will help.

With your big valve heads and larger cam you don't build much off idle torque to get a 3.42 geared 33" tire truck moving. They may not work so well for your combination.

[68 TT]

Originally Posted by Pyrotechnic

Another thing to consider is a Gear Vendors. Expensive yes, and you can only use it in 2WD, but honestly that's really the only time you will need to use it.

They have been around for quite a while so you can find them used too. A friend just got one for a 2-wd T-400 for $800.

Not sure how easy finding a used T-400 4x4 version will be though.

If forced to buy new the $3000 price tag puts them easily $1000 more expensive than purchasing, completely rebuilding and installing a stand alone 4L80E setup plus with the GV unit you don't get the overdrive in 4x4 mode and you don't get the cool features of the electronic transmission.

If you really want the overdrive the 4L80E is the way to go. They have been around since 1991 so there are plenty of them in the wrecking yards. I just picked one up with the wire harness and computer for under $100 at the local pull-it-yourself wrecking yard. Finding a T-400 / NP205 truck is harder. Unfortunately this is the next thing I need to track down for the conversion to happen. I will have under $500 into the 4L80E swap including having to buy another NP205 and getting my driveshafts modified. I won't know if the transmission needs rebuilt until I get it on the road but the fluid was red and smelled good and there were no metal particles in the pan so I think it will function fine.

[asphaltburner]

Wowzers! Now I've got some contemplation to do!
My ol' '53 Studebaker Commander had OD and it was phenominal!
Gear vendors has the OD for the TH400/NP205 for about three grand, I could experiment with the axles by changing ratios for about a grand and a half, or I could pursue another tranny swap.
I really like my TH400,...it was completely rebuilt a year ago with a new 1200-1500 stall converter, performs perfectly, and combines nicely with the 3/4 ton 4x4, 5560 fullfloating rear axle, twin stick NP205, and the torquey 350V8 to make FRANKENTRUCK capable of some heavy-duty work! So I think I'll stick with my TH400/NP205 combo.
Thanks for the info and suggestions 68TT!
I'm obviously going to need to talk with Gear Vendors about their OD, thanks Pyrotechnic!
Sounds like Prong runs a similar set of gears, the same type of tranny, and in the same environment. So I'll also talk with Bullhide 4x4 a bit about the final result of a re-gearing to 3.42.
Geeze, THANKS guys! I KNEW I could get my questions answered here,...just hadn't expected to find out about a few other options!
Guess that's why I became a site supporter!
Todd.