6.2 & Axle Ratios

[rd durham]

I've slowly been pulling the parts together to get my 6.2 installed in the '79 Jimmy. It's beginning to look like it will be next spring before any thing major gets done. One item that has cropped up has been the axle ratios.

The axle ratios on the truck have been a mystery as the axle tags were missing when I bought it. Last weekend I managed to find the time to jack up the front and rear axles to check the brakes, and while I had the drums off the back, and the calipers removed from the front axle I spun the axles to get an idea of what the ratio might be for this truck. It looks like the Jimmy has 2.56 axle ratios. Definitely less than any 3.00 or any higher ring & pinion. I'm thinking that while this might be good for economy with a gas engine, it might not be so good for a diesel. I have another truck that I use as my primary hauler and off road vehicle, so I'm not too concerned about off road performance, though I'd want to take it occasionally. This truck will be driven around town more as my daily driver, or for the wife's occasional use when she sees patients as a home health care nurse and needs four wheel drive because of the weather.

Any thoughts as to good or bad with these gears?

 

[Pookster]

You didnt say what tranny you had in your truck. Most of the diesels I believe came with a 308 rear gear. 256 is pretty numercally low. If your happy with that 256 in a gasser application, you'll be happier with it behind a diesel application. Granted it aint gonna be a great highway merging gear, but if you have a 700R4 with the low first gear, it helps a lot to get things moving.

 

[rd durham]

The 1979 Jimmy came with a 350 V8 gas engine and 350TH automatic transmission. I never drove the truck, but got it after the engine, transmission and transfer case were removed. The 6.2l diesel that's going into the truck came out of a 1984 CUCV Blazer and has a 400TH automatic and 208 transfer case attached.

 

[6.2puller]

My C10 6.2l 700R4 came with 2.56 from the factory (AFAIK) Problems I had were not in the low RPM merging and start up area but in the high RPM. My truck had to be going 75plus to get into OD. Not great on the highway around here where the speed limit is 55mph. I swaped to 4.10s and now I am in OD at 55mph if I am cruising but shifts nicely to 3rd for hills and passing power. The only problem I have now is that 1st gear runs out quickly around 7-10mph and shifts hard if you are on the gas.
With the higher first gear of the TH400 you may not like the 2.56s much but they will be nice in 3rd at 60MPH range.

 

[rd durham]

Thanks. I guess that I will give it a try, but probably keep my eyes open for some 3.08 to 3.73 gears for later.

 

[tRustyK5]

I'd gear it so that it runs down the freeway at around 2000-2200 max. With 31" tires I'd run 3.08's...making the cruise rpm approx 2200 rpm at 65 mph.

Much deeper than that and you'll really hate it on the highway...at 2500 rpm mine is louder than hell!

Rene