3.08 gears and 20s?

[Smokin72on20s]

I have a 72 chevy pickup, it has 3.08 gears with posi. I am going to put a 4l60e trans and 20inch wheels on it. Assuming i am going to drive this everyday should i change the gearing to say a 3.73? or ?

[Hotrod70C10]

That is up to you really... You will get better gas milage with the 3.08's but you will get better acceleration with say, 3.73's. If it was me I would just keep the stock gears in it and drive it as it is..... then again I don't race my truck.

[crazy4chevs]

(3.73) that sounds about right with a 4 speed auto.

[crazy4chevs]

I must type slow u beat me to the punch. Most of the newer vehicles ive seen with a 4speed at have had 3.73 or close to it.

[bpmcgee]

I don't recall if that tranny has an overdrive. Here's my experience with my SM465 (4 speed stick, 1:1 top gear):

I've got a 3.08 posi rear end and stock tires on my 4x4 blazer. I find that in the 70-75 MPH area I'm doing between 2500 and 3k. My feeling is that this is probably near the sweet spot for these 350s. I would say that if your goal is to drive it every day, stick with the 3.08s unless you have an overdrive.

Brian

[Josh]

4L60E is an electronically controlled overdrive (700r4 thats electronic basicly)

Depending on what size tires you run, and what speeds you want to cruise at, a 3.73 would be a great choice.

[Slammed67]

I have 3:08 with a 700R4 (18" wheels/27" tall tires) and it has pretty damn good accelertion. The best of both worlds..... good launches and highway cruisin! It'll chirp the tires when it hit's 2nd gear. Might as well try out the 3:08 first and see if you like it. I think you will

[68 Stepside]

I have a 4L60 and 2.56 gears in my roadmaster It has no balls from takeoff, but the 21 MPG average combined city/highway makes up for it

My Z28 had a 700R4 backed by a 3.08 posi. I was more than happy with it, still had good takeoff acceleration, and it would flat out fly. I'm with Jason, stick with the 3.08's for now, you'll probably like it.

[Chad]

My Dad has a 71 SWB with a 350/700R4 setup. It has 3.08 rear gears and gets right around 20 MPG. Not too bad for a huge of steel. Oh yeah, it flys out of the hole too!

[N2TRUX]

What size 20's are you going to run? The overall diameter means more than the rim size. Your 3.08's with a turbo 350 is a bad combo, but not with a 4L60E. The 4L60E has a much lower first gear than the turbo 350, so it pulls better out of the hole. The overdrive might be a bit much on the top side.

I say get you tires and tranny in, then decide if you need gears. 3.73's are a great gear for low end, but a 3.42 might be the best of both worlds...

[Richard]

As said earlier the tire diameter is what matters. If I am not mistaken the 4L60E has a 1st gear the same as the TH400 which is just about the same as the TH350. The 700R4 has the lower first gear but a bigger gap between 1st and 2nd. IMO the nice thing about OD trannies is the ability to have a much lower rear end ratio for performance and still have good driving characteristics on the freeway. I had bought my 94 with 3.42 gears knowing I was going to run a 285/40/17 in the back (about 26.5" tall) it was perfect. Got tired of the low look and raised back to stock and have a 275/60/17 now, a bit taller than the stock 15" tires (28"-29") It is a dog as far as I am concerned and my milage actually dropped because the motor chugs in OD now. My 72 will have 4.88 or 5.13 gears with the 38" tires and 4L80E tranny, I expect 17-18 mpg with the new motor on the highway. You can check out www.700R4.com for their calculators to give you a better idea.

[O'l Buck]

my K-10 has 3.08's and 31x10.50's, th-350 and a 350.......I hate my gears, this thing is bad doggy and has alot of trouble pulling much of a load.......3.73's are on my wish list.

[N2TRUX]

Originally posted by Richard
As said earlier the tire diameter is what matters. If I am not mistaken the 4L60E has a 1st gear the same as the TH400 which is just about the same as the TH350. The 700R4 has the lower first gear but a bigger gap between 1st and 2nd......

-It looks like we are both wrong. The 350 and 400 have a much lower first gear than the 700r4 or 4L60E, and the 700R4 has a lower second gear.

That is according to Stan Weiss whoever he is...

GM TH-350
2.52:1 first gear, 1.52:1 second gear

GM TH-400
2.48:1 first gear, 1.48:1 second gear.

GM 4L60E
3:08 first gear, 1.88:1 second gear

GM TH-700R4
3.06 first gear, 1.63:1 second gear

[JayDubBlazer]

has anyone put a 4L60E there truck or seen it done? all I've seen is 700r4s. I bought a wrecked 95 fullsize truck and would like to use the tranmission but need more info. By the way I bought the truck for 300 bucks I couldn't pass it up.

[Josh]

The 700r4 and the 4L60E are the same tranny, the 4l60E is jsut electronically controlled, where the 700r4 (or the 4L60) is hydraulicly controlled. The gear ratios for the 700r4 should be identical to the 4l60e.

The 1-2 gear split is pretty bad, really noticeable in my caprice w/TBI 305/4l60/2.56:1 rear gears. But behind a mild 350 with the right gears it shouldnt be very noticeable. The low (high numerically) first gear is nice to have, even if it has a slight split between first and second. I assume form your sig you have an Lt1? Or do you have the 4.3 V8?

The RPM calculator at http://ring-pinion.com/calculators/calculatorsind.html should give you an idea of how different gears and tires will affect engine RPMs in various gears and speeds.

JayDubBlazer, you would probably be better off selling the 4L60E and going with a 700r4, it'll be easier and cheaper. To use the 4L60E you'd have to buy an aftermarket computer to control it and would wind up spending more than its worth to run it.

[Iron-Butt]

Thanks for the 700r.com info , Richard.

[Richard]

I will see if I can locate the page I found that has all the GM trannie ratios. As I said before the 700R4 is the lowest. Remember numerically higher is a lower gear, so according to your list the 4L60 is the lowest followed by the 700R, 350 and the 400 highest.
I just found this list http://www.chevellestuff.com/generic/rpm_calculator.htm
It says the 4l60 and 700r are the same as well as the th400 and 4l80e, with the exception of the 4l80 having overdrive. I really dont care for the large jump inbetween 1st and 2nd on the 700 and i guess my 4l60. With lower gears it is not as noticable but the higher the rear ratio that gap becomes more noticable.

I guess I pretty much repeated what Josh said

My brother has a 700R4 from Bowtie in his tpi chevy powered Jeep. Been great. He had it installed by them and I cant say I was impressed by the workmanship that they did on the install. They kind of butched his crossmember and did not reinforce it, the modified exhaust had the tailpipe just fall off and the speedo was not hooked up for the test drive. He had spent quite a bit on the whole install and I expected more. When he inquired about the problems they were very defensive. I believe the difference in a good company and a bad one is how they handle the problems, they did not shine here. As I said the tranny is fine.

[Denny]

I have a 4L80E trans in my 69 C-10. It had 3.08 gears aand I changed it over to 3.73 with posi just recently. I was never happy with the performance with the 3.08 gears.

Wes you can use the Compushift computer with the 4L60E trans.

[Tx Firefighter]

Originally posted by Denny
I have a 4L80E trans in my 69 C-10. It had 3.08 gears aand I changed it over to 3.73 with posi just recently. I was never happy with the performance with the 3.08 gears

Them boys over at Pheonix took good care of you didn't they?

I'm very lucky that they're local to me. They build every automatic I use.

[JayDubBlazer]

Denny:
Compushift computer seems pretty neat. How much wire did the system add to your engine bay. I noticed you have to hook up Throttle Position Sensors on your carb. that didn't end up being to finicky did it. I may PM you as I learn more about the system.

[JayDubBlazer]

here is a site I found that has a good explaination of compushift and a 4l80E install

http://www.ocmhz.com/33_coupe_trans1.htm

[/<eystroke]

Tire size will be a large factor in this decision.

If you keep the 3.08 rear, you will need a very low rpm stall converter
because you will be idling the engine at 1200rpm while doing 55mph
with 31" (285/50R20) wheels. I doubt you will ever fully lockup the
torque converter before 55mph, which will cause it to slip and generate
more heat which will wear out the fluid sooner.

In my opinion, you will be happier both with the performance and
longevity of the higher 3.73 rear when you get an overdrive
transmission.

Here is a graph of your rpms/speed with 31.2" tires comparing a 3.08 to
a 3.73 rear. Let me know if you want a graph with a different tire
size.