The Stovebolt Site Forums The Shop Area General Truck Talk Diff. Gear suggestions?

Hey guys,

The question is, should I go with a 3.42 or a 3.73 behind my T5.

I've run the numbers for 65mph, 30" tire diameter,.72 5th with 4th gear @ 2490rpm and 5th @ 1793rpm. Engine is currently stock 55-235 but will be getting a cam, headers and dual intake.

If I went to a 3.73@65mph, 4th@ 2715rpm, 5th @ 1955rpm.

Any suggestions or comments on where my engine would be the happiest? I don't want to be lugging down and downshifting at overpasses but also don't want to turn unnecessary revs. The specs show the max torque developed at 2000 rpm but specs can't compare with personal experience from someone who has driven one of these combo's.

Any input you can offer sure will be deeply appreciated.

Dave

I would lean towards the 373 so you'd still have good pull out of the hole, and yet, 2K is not that wound up. If you are not hauling big loads, then you could go with the 342 and then downshift to 4th when you had the occasional load if it seems to lug.

Depending on what rear-end you have in your truck the ratio choices may be limited. If you have the stock 54 rear-end in your truck, I believe the only choice you will have is a 3.55:1 ratio.

Jim

He's swapping rear ends as the T5 will only work with an open driveline.

I'd go for the 3.73s. That way won't lug down if you are in 5th running 55 and encounter any kind of load (hill).

With an overdrive tranny, I'd go with the 3.73 for sure. I have a 3.90 with an overdrive and like it just fine.

i have nova rearend with a 2.73 yes 2.73 and i like it just fine with my t-5. ron btw it is a daily driver.

Thanks guys...I really appreciate the input.

Dave

this should set you straight. I think your numbers are showing a much lower rpm than reality.
although florida is pretty flat but you have a couple of things to consider; elevation, style of driving habits, towing, road conditions, terrain (mountains vs flat).
I'm running a 292 in a 3100 w/T5 and 3.40:1
see:
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=010333;p=1#000000

Jim,

Why are his numbers showing a much lower rpm than reality? The math is simple, based on his inputs, his calculations are correct.
He is using 30 inches for his tire height
He is using .72 for his fifth gear

You show some smilies with a link to some calculations that you made with 28.5 inch tires and a .74 fifth gear (effective running a lower gear if the rear ends were the same in your two trucks).

And, you are running a 292 which is a lot more power across a wider range than a 235 so you can get away with a higher (numerically lower) gear.

Not sure how that will set Dave straight or that he even needed setting straight, he asked for suggestions and personal experiences.

I'll take a stab here and say that based on the info provided by Jim and assuming he is happy, he thinks should go with the 3.73s since his truck uses shorter tires (lower gearing) and has a more powerful engine than your 235.

Chris

Hey Jim & Chris,

Thanks for the response.

What I'm looking for is someone who can tell me the rpm range where the 235 operates the most efficiently. I guess you could call it the "sweet spot rpm" where the vacuum comes up and the little engine just kind of purrs along.
Every engine has this spot someplace over the power band. I was hoping to get a 235/T5 driver to give me some specific RPM's so I can make the adjustment before I lock in on the diff.

With max torque at 2000 rpm's I guess I'll just go with the 3.42 posi w/discs and keep the 3.73 in reserve.

Thanks again,

Dave

You upped the ante! Disc brakes and a posi on the 3.42 or open and drums on the 3.73? I'd have the 3.42 with discs and live with having to downshift occasionally.

Like you said 2000 rpm is your peak torque, that is why I suggested the 3.73s with 30 inch tall tires, you can still run in 5th at 55. 65 mph will be a nice smooth cruise and should hold speed just fine.

I ran a t5 in my coupe with 27 inch tall tires and 3.38s in the rear with a 54 235, HEI, 2 bbl carb and split exhaust. It would lug in a little bit in 5th around 55 miles per hour if you needed to climb a hill or pass someone. Went to a V8 and automatic.

I'll give you that 4.3
these calculations are from that site i listed (see link) and had nothing to do w/ an engine (they do not list/ask your preference of engines) only transmissions and a rear end ratio. You could plug in any combination you want, hit the calculate button and make your own determinations. if you do not know the exact tire dia, they calculate it also.
my 6 lug 235/75/15 calculated to 28.75''. pressure and tire maintainance made a difference also.

I have a blazer rear end, s-10 transmission and 2:73 rear gear,,,235 engine,848 heads,etc in my 52.My area is mostly flat,, I run mostly in fourth around 2,200 or a little above, this is close to 60 mph,, I shift into 5th about 65,,works fine for me,, on a real steep hill I may have to shift into fourth, depends how fast I hit it and how steep.. fourth is good crusin for me.... a 3:73 would give you more ump..... I believe when in 5th and 65 or so I am running about 1650 to 1700 rpm. a little higher if I stay in fourth,, I do it both ways,, my tires are 205/75/15......6 lug... good luck with your choice..Fred

Hey Guys,

Thanks a bunch!!!

Great numbers! They say "experience is the best teacher.....but it gives you the test first and the lesson after." As you might have guessed this is my first Chevy rebuild and when you get down to the smaller CID engines and the aerodynamics of a cinder block (effectionately stated) you don't have a heck of a lot of leeway!!! LOL I really appreciate your help and expertise....my 54 went to the paint shop yesterday and I hope to have some pix to add to the site soon!

Thanks again, Dave