5 spd. and 3.08 gears. Bad idea?

[rscam]

I just did an NV3500 swap and I have 3.73 gears in the rear. I am thinking of going with 3.08 gears. I have 275/60-15's. Will the rpm's be to low at highway speeds to be feasable? I am not worried about take off since 1st gear is 4.03. I think high gear is about .73-.75.

[piecesparts]

A change such as you suggest will make your RPMs drop by 546 revolutions. The 3.73 gears run out at about 3134 RPMs at 70 MPH and if you drop your gear ration to 3.08 gears, you should be around 2588 RPMs. I see that as a good exchange for your situation. The GM SBC likes that area of RPMs, as long as you have not done something to change the overall engine components---IN OTHER WORDS changed the cam profile or any other major engine work.

My 383 stroker motor runs down the highway at 2500 RPMs at 70 MPH and I did this with an OD tranny and 3.73 gears. It loves the pwoer curve at thsi speed and responds real well.

[67_C-30]

Originally Posted by piecesparts

A change such as you suggest will make your RPMs drop by 546 revolutions. The 3.73 gears run out at about 3134 RPMs at 70 MPH and if you drop your gear ration to 3.08 gears, you should be around 2588 RPMs. I see that as a good exchange for your situation. The GM SBC likes that area of RPMs, as long as you have not done something to change the overall engine components---IN OTHER WORDS changed the cam profile or any other major engine work.

My 383 stroker motor runs down the highway at 2500 RPMs at 70 MPH and I did this with an OD tranny and 3.73 gears. It loves the pwoer curve at thsi speed and responds real well.

You're not figuring in the OD. With 3.08 gears and a .75 OD, the RPM will be 1940 RPM @ 70, and would be 2350 RPM @ 70. I personally would stay with 3.73's for the low end acceleration. If you frequently run 75-80 MPH on the freeway, then the 3.08 set-up would be a better choice. A 3.42 would be about perfect IMO.

[67_C-30]

After a little research it seems that the majority of NV3500's have a .73 ratio OD, so make those numbers 1889 with 3.08's, and 2287 with 3.73's @ 70MPH. (not a whole lot of difference).

[Wrenchbender Ret]

You didn,t say what engine you are running. I,ve got a 70 stepside with 350 engine, 700-4R trans, & 29" dia tires. It drives great in o-drive & will pull long hills on the freeway with A/C on without downshifting. If I had something heavy to pull I could just use 3rd & still not over rev.

[piecesparts]

Originally Posted by Wrenchbender Ret

You didn,t say what engine you are running. I,ve got a 70 stepside with 350 engine, 700-4R trans, & 29" dia tires. It drives great in o-drive & will pull long hills on the freeway with A/C on without downshifting. If I had something heavy to pull I could just use 3rd & still not over rev.

You didn't give your gear ratio, that is the deciding factor in this whole conversation. My guess fro your description, you are geared low.

[jamie72]

send huck a pm he did this swap in a 72 and was happy with the results.

[jamon8]

I have the same set up

Its great and all you have to do is shift into 5th when you need it

any more questions feel free to ask

at 1500 rpms in 5th gear I am doing 68 and getting about 18-19 mpg with a lopy cam

[Brad]

You don't mention what size engine you have, but regardless, you should really keep 3.73's in the rearend for best mileage which is what you're going for.

[Huck]

I had the 3500 pulling on a 308. My recommednation would be a 342 rear as it give you just a little better pull then the 308 but not the mpg issue of the 373. It really depends on how you want to compromise as that is inevitable. will you tow? Want a strong pull from a stop? MPG?

[rscam]

Sorry for the lack of info. I have a 355 with about 320 horse. This truck will never pull anything. It is just to enjoy and drive once or twice a week. I probably won't drive it 5k miles a year. Gas is always an issue now. So gas mileage would be a little important. I was just thinking with the 4.03 first gear and 3.73 rear and the tire size I have that 1st gear would be almost useless. I just didn't want to go the other way and make OD useless on the other end if I used 3.07's

[rscam]

Also correct me if I'm wrong but I am probably in the 350 ft./lbs of torque range and I would think that would be plenty to hold 60-65 which is about as fast as I would run.

[67_C-30]

If you are going to be going any faster than 60-65 there is no way in the world I'd go with 3.08's. You're engine is not making anywhere close to 350 ft./lbs at 1600, which is where you will be in OD at that speed. Pushing one of these bricks at that low of RPM with a SBC is going to like to eat more than it would with the 3.73's in my experience.

[Brad]

Also, if only 5000 miles/year, why even spend the money? You're stepping over a dollar to pick up a dime.

[special-K]

I agree 3.42 is as low of gearing you`d want with o/d.3.08s may be ok for flatland,but you`d chew up more gas trying to pull hills in o/d.To me,running 3.08s is like having o/d.That ratio loves the hiway.

[Huck]

thinnking it through---most stock 350--auto--came with 308 and pulled just fine. But they also got like 14mpg. My 72 crate with the 3500 trans and 308 pulled ok highway. Would downshift on long steap hills to maintain 70mph. When towing a trailer, I was in 4th most of the time.
Used stock 235x75R15's. My 2002 2wd extcab has the 5.3 auto and 373 tow package with 17" rubber. I get 18-19 mpg highway. neighbor has similar setup with 342 gears. he can get 20-21mpg and occasionally tows also with no big deal. I like 373 gears but with the price of gas ---heading towards $4 a gal, I would prefer the 342's but as mentioned, the price to change would probably take 10 years of driving to justify!! So--I'll just keep air pressure up a bit and keep my foot out of it a bit. Huck