1971 K20 Gearing Query

[NZBurb]

Hi everyone.

Firstly thank you for the great active community here - I've learned a bunch from the numerous threads.

I've recently imported a 71 K20 (3 speed auto with a 350 GM Goodwrench) to New Zealand and now we've got through compliance we're finally on the road! I'll get some shots for you guys shortly :-)

Overall she drives really well, however at 60mph on the motorway she's really working at around 3,000 rpm. I'm currently running some 33s which leads to a gear ratio of 4.9 which seems quite high compared to the 4.1 I understand they were built with (dodgy tach?). She previously had 37.5s (I had to swap them out for compliance and didn't drive her with them on), so am looking for some advice on which would be the best option to work through:

1. Change the gear ratio (keen to hear what you guys are running)

2. Look at a new trans, e.g. 700r4 (keen to hear if there are any major challenges to keep an eye out for).

Thanks in advance for your hel[p,
Vaughan

[NZBurb]

Sorry guys - forgot to mention I have an Eaton axle (as per those in the thread
below) if that helps in anyway.

http://67-72chevytrucks.com/vboard/s...d.php?t=339732

Thanks,
Vaughan

[NZBurb]

Well after all that it looks like the Eaton was shipped with a 5.14 for the 3/4 ton trucks (http://www.stovebolt.com/ubbthreads/...&Number=870449), so that explains that 4.9 I came up with :-)

[LockDoc]

-
First off, welcome to the board. Sometimes there isn't much traffic on here on the weekends.

Not much available for the Eaton's as far as the gears go. The 4.10's are about as high as they go. The 700r4 would probably be your best bet. Here is some reading on it while you are waiting on other members to chime in with their experiences....

http://67-72chevytrucks.com/vboard/s...ad.php?t=33266

LockDoc

[Bigdav160]

I'm pretty sure 4.56 or 4.10 were the only choices of gear ratio from the factory. It doesn't mean that someone didn't install something else.

I agree with the Doc, an overdrive transmission is going to help.
It's easier, on a 4WD, to move the engine forward to accommodate the longer transmission than to push the transfer case backwards.

[vectorit]

Welcome to the forum!
The Eaton HO52 rear axle in these trucks could have been ordered with either 4.10, 4.57, and 5.14. Being that yours is a K20, chances are it is 4.57 ratio unless swapped out.

There isn't much of a market on Eaton HO-52 & 72 axle parts, let alone new ring and pinion kits.

If you are looking to go to a taller gear for better highway manners, then I would start looking for a GM full floating 14 bolt rear axle. This axle housing will have more options for gears, and the replacement parts will be much easier to source.

[franken]

Rather than guessing, pull the cover off the front diff and get the numbers off the ring. The gear oil needs changed anyway.