Hello,

I am attempting to build a rear wheel drive rat rod with the intention of taking it on some light offroading. The vehicle will be build on an old truck frame with a VW bug body. I have read that the NP435 transmission has offerings in both 6.68 and 4.56 and there is an A833 transmission as well. Performance wise I would like to achieve the following.

1. Comfortably cruse at 65-70.
2. Have a "low" gear to craw up loose rock and gravel.

The engine I have is out of a 73-74 Dodge Dart if that helps. Does anyone have any recommendations? I have never messed with gearing before.

Sincerely,

Paul

For a vehicle that ligt you probly dont the the NP435 it's a giant heavy transmission with a Low 1st gear.

Are you doing 4wd or 2wd?

If 4wd.... then get the NP435 with associated case.

If setting up 2wd.. I would go with an A-833 with the OD since its' a custom vehicle the Truck version used up to 87 is cheap and easy to find.

some quick calculations... with say a 5.38:1 Rear axle gear ratio:
1st Gear =3.09*5.38= final drive of 16.62:1
OD Gear ( 4th) = 0.68*5.38=final Drive ratio of 3.65 it's a bit steep for the highway but can be done espically if you add in a tire taller than say 28" in Ø.

You can base your calculations from there.

I have the NP in my truck with a 318 and I can pull out at idle in Granny 1st with that 6.68:1 first gear - It will redline at about 12MPH with a 4.10:1 Rear axle ratio.

Others will chime in too.

Greg

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http://www.youtube.com/hyperpack

For a mixed use vehicle such as this I would look for a 4WD donor so that the transfer case gives you the low gearing that you desire and you can keep relatively close gearing in the transmission and make best use of the limited power. Barring that you need a wide range gearbox which rules out the passenger car gearboxes.

To calculate the gear range of the transmission you divide the 1st gear ratio by the high gear ratio. For a 1:1 high gear the range of the transmission is therefore the 1st gear ratio as N/1=N. So a 6.68 range for the wide ratio NP435 or 4.56 for the "close ratio" NP435 (or the NP445 that I have and like).

The NP445 (NP4530) trans is found in later '70s and newer Dodge trucks. Mine came from a 1978 D150. It is synchronized in 1st gear which the NP435 is not. Gears are 4.56, 2.28, 1.31 and 1:1. With 30" tall tires and 3.55 gears it's still plenty of torque multiplication (16.2:1 in low). And 2nd through 4th are closer together than the A833OD so it's far easier to keep the engine in the torque curve.

The A833OD as found in pickups has a 3.09, 1.67, 1:1 and a 0.71 high gear. 3.09/.071 = 4.35 which is close to the overall range of the 4.56 1st gear NP435. It's a smaller transmission that also uses a smaller clutch, but in a light vehicle it's fine. My NP445 weighs in at a stout 150 lbs and the A833OD is only about half that.

Have you read the 4-speed article? http://www.slantsix.org/articles/4-spee ... speed1.htm
Did you find this long discussion on gearing? http://slantsix.org/forum/viewtopic.php?f=20&t=63627

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Joshua

Thank you Greg and Josh,

The gear calculation was a big help. I am actually thinking to do 2 wheel drive to keep it simple and keep the costs down. I have an old Pathfinder for true 4x4 driving, and would mainly use this is Gambler 500 events. My rough plan (you know how those change) is to get the engine running, purchase a transmission, then get a frame. I have a lead on a 1980 Toyota 2 wheel drive pickup (sans engine, transmission, and bed) for $75 with title. My goal is to keep it cheap enough that if I have to push everything out to the curb and walk away I can do so, lol.

It looks like the A833OD may be my best bet due to weight and cost. Josh, I will also read up on the articles you have sent. Thank you for that resource.

Sincerely,

Paul Barrow

PS: What models would be good candidate to search for the A833OD manual transmission?