[holdenrules]

Ok, so I have a 84 lwb that had the factory 1.0. Converted it to 1.3 with the 5 speed of a 88 sierra. I run standard diffs (4.11) and standard transfer it came with being the 1.0 tansfer case.

I want to be able to sit on 100klms comfortably with out it revving over 3500rpm. I have 235-15 tyres on it.

Question 1- should I put the 3.9 donar car diffs in it.

Question 2- if I do this what transfer do I put in it?

Question 3 - any one got any transfer or diff gear combinations to achieve what I need to achieve. I can buy aftermarket but what is best transfer gears or diff gears?

[jono165]

Check out www.lepayne.com for a gear calc. Just note that later 5 speeds in pretty sure have a .865 5th gear. It's in mph but its easy to convert back.

To be honest though 3500rpm I reckon is pretty spot on for cruising rpm, a g13 will sit on that all day no worries. It needs to be revving at about that to produce enough torque for hills etc, you don't really want to be dropping back to 4th all the time.

[SierraDan]

what jono said.

[shep]

What Joni said except I reckon ya need 3800 to 4000rpm @100kph. Also are you using a GPS to work out your speed or using the speedo? If speedo then it will be wrong.

[jono165]

With your current setup I work it out to be about 4000rpm @ 100. If you switched to a stock WT sierra case you would be close to 3500rpm. 4000rpm isn't too bad a jimny does this from factory, my 1.3 Sierra does 110 at about that rpm and is happy to sit on it.

If you changed to 3.7 diffs you would have roughly the desired rpm with the 1L case. Or you could do 4.16 gears in the case and 3.7 diffs which gives you about the same cruise rpm as the 1L case and 3.7 diffs but a much better low range.

[sideways]

shep wrote:

What Joni said except I reckon ya need 3800 to 4000rpm @100kph.

This x 1 million.

If you've got the 0.8 top gear (which you probably do if it came out of a NT) then you're only doing 3600 rpm at 100 (assuming your tyres are 235/75/15). If it's the 0.864 then you're doing 3900. Dropping 100-400 rpm isn't going to make the car particularly better on the highway, Sierras make so much wind and road noise that you probably wont even notice anything but having to use more throttle all the time and having to drop back to 4th more often.

Embrace the revs, your car will love you for it.