Thoughts on 3.08 rear w/ lsx

[kevmac]

Anyone run a 3.08 rear with a 5.3/4l60e swap. My 11 gmc has 3.42 in a heavier ride. Looking to make a driver with mpg in mind. Thanx

[67ChevyRedneck]

You'll want a steeper gear. Something in the 3.40s would be good but a good ole 3.73 is a great combo with od in these trucks. I have a 3.73 and 700r. The 700r has a .7 od gear. I think the 4l603 has a .67.
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[MARKDTN]

I wouldn't go 3.7. I have an '82 K20 with 700R4 and 3.42 and it is a pretty good combo to me. With a Corvette L98 TPI engine, I get 18ish in town and 22ish on the road.

[Wrenchbender Ret]

I have a 70 with 350 & 700R4 & 3:08 rear end. It drives great. If I am pulling a load I can drop it down to direct drive & itr is still not over reved.

[NVBUYER]

I'm running the same combo as Wrenchbender, although it likes to bog a little bit I really like being able to go 70 without feeling like the engine is spinning up too much.

[68Stepbed]

As long as the engine is stock, it will be fine. If you run a cam and stall, you will definetly need lower gears. 3.42-3.73 gearing is optimal with that combo.

[burnin oil]

I bet the 3.42s get better MPG thn a 3.07 rear gear. The 3.07 would have the motor lugging more and the 3.42 is not a high rever once you add OD. I never felt that the LS motors made good low end torque but shined in the mid to upper RPMs. Great for muscle cars but terrible truck motors.

[k_n_browning]

wrenchbender do you have any idea what you get on fuel milage with your combo.
With two bad knees my standard trans is getting painfull.

[69SCSS]

I had an '02 Silverado with a 5.3 and 4L65E, and a 3.42 rear, and I used to get 21-22mpg on the freeway on long trips even going over mountains between here and California. With my current 4.8/4L65E and 3.73 I get 18-19. I'm not sure a 3.07 is going to be much better than the 3.42 honestly, but if you don't tow you'll probably be fine. I'm kinda on the fence with that too, I have a 3.73 posi in my truck and I'm considering dropping to a 3.42 or 3.07... probably 3.42 because I will tow. Good question! it always depends on what you're going to do with it.

[kevmac]

Thanx for the input guys, I've read a lot about 3.08 vs 373 with OD. Trying to get a feel for the 5.3. I love the 5.3 in my 11 gmc which is heavier than the 70 swb I'm putting togather. I haven't seen 342 in the old rears and don't want to rebuild cuz I already have one of each[3.08 and 3.73]I'll never pull anything with this and my ol 72 350 auto w/ ac never bogged even when pulling. But its always great to here from everyones experience. What did we do before the net was invented?

[burnin oil]

Originally Posted by kevmac

What did we do before the net was invented?

Guessed!

The turbo 350 didn't bog because the RPM was high in direct gear. I had a 6L with 4.10s in a 2500 4x4 and it got terrible milage. 12.4 with or without a load. That was my last newer Chevy.

[eldogg]

The question no body asked is what size of rear tire? That makes a huge difference in gear ratio. I think MPG between 3:08 and 3:73 would only be 1-2mpg but I think the truck will drive better with 3:73.

[Wrenchbender Ret]

Originally Posted by k_n_browning

wrenchbender do you have any idea what you get on fuel milage with your combo.
With two bad knees my standard trans is getting painfull.

I get between 16-18 MPG. It has a q-jet carb & HEI ign. I built it for 20 MPG but it just won't do it.

[N2TRUX]

Originally Posted by eldogg

The question no body asked is what size of rear tire? That makes a huge difference in gear ratio. I think MPG between 3:08 and 3:73 would only be 1-2mpg but I think the truck will drive better with 3:73.

Finally, someone actually puts some thought in to the answer. No one can recommend a gear ratio with out knowing your tire diameter.

For reference, a late model truck with a 305/40/22 (31.50") will see a 1 MPG decrease from each gear set. Going from 3.08, to 3.42 will drop 1mpg, and down to 3.73 will loose yet another MPG.

The opposite effect is the lower the gear, the more torque you gain. This equates to more SPG (smiles per gallon) until you pull up to the pump.

[burnin oil]

A quick glance at his avatar says stock tire size range. Pretty sure he was sticking 38's under a 2wd either. That is why I didn't ask. Either way I would want to see no less than 2k RPM at cruise. No lugging on hills and enough RPM to keep all the accessories happy.

[see ten]

I literally just walked back in the door from my first trip with my 5.3/4L60e. I have a 3.08 gear and a 27.8" tall tire. I drove 120 miles through the foothills/mountains of north georgia as well as through friday evening traffic of a couple small towns (stop and go). I shoot for keeping the speed no more then 10mph over the posted limit... But the first third of the trip I kept catching myself running 70+mph in 55 zones... Did better as I got used to the throttle and the "sound". So most of the trip was ran between 55-65 mph. Pulled 20.8 mpg.
I still haven't driven this setup enough yet to have all the bugs worked out just yet. Electric fan temp is still set up for the old first gen small block... probably needs raised a bit. CEL came on when I had the o2 senors repositioned last week... it's still on... not sure what's going on there yet... but. maybe my mpg will increase... maybe it'll get worse. Who knows.

[see ten]

just a btw... according to the calculator on Randys ring and pinion, I should be turning about 1860(ish)rpm at 60mph.

which I've slept since I looked that up...

[rs74]

I was kind of wondering what kind of RPM's everyone was pulling with their gear and tires sizes at around 55-65 mph.

[kevmac]

Originally Posted by see ten

I literally just walked back in the door from my first trip with my 5.3/4L60e. I have a 3.08 gear and a 27.8" tall tire. I drove 120 miles through the foothills/mountains of north georgia as well as through friday evening traffic of a couple small towns (stop and go). I shoot for keeping the speed no more then 10mph over the posted limit... But the first third of the trip I kept catching myself running 70+mph in 55 zones... Did better as I got used to the throttle and the "sound". So most of the trip was ran between 55-65 mph. Pulled 20.8 mpg.
I still haven't driven this setup enough yet to have all the bugs worked out just yet. Electric fan temp is still set up for the old first gen small block... probably needs raised a bit. CEL came on when I had the o2 senors repositioned last week... it's still on... not sure what's going on there yet... but. maybe my mpg will increase... maybe it'll get worse. Who knows.

Great info, Keep me posted if ya could!!!!!