The Stovebolt Site Forums The Shop Area Engine Shop T-5 w/2.73:1 rear diff?

I've been reading all the threads I can find on the t-5 conversion and rear differential ratios to figure out what to put into my '51 1/2 ton at the same time I put the re-built 261 w/M4f, fentons, offy w/2bbl progressives.

It seems like the most common t-5 tranny that bolts right up is the non-world class found in the chevy s-10's (or s-10 blazer) with 4.06/2.37/1.49/1/.86ratios. With 235/75r15 tires the tire height is 28 inches (27.9).

I'm trying to build a daily driver that'll get decent gas milage and be able to move in and out of traffic as necessary in city driving, and still get decent gas milage.

MY QUESTION: With the most common t-5 and 2.73:1 gears am I gonna acheive what I want, or am I missing something?

Here's how I figure it:

Using an RPM calculator it looks like in 5th gear you'd turn about 1,850 rpms at 65, and 2,100 at 75. The M4F is supposed to be good between 2,000 and 4,000, so if you had to pass someone on the freeway you'd be at the start of the powerband and could either downshift from 65 or punch it at 75 and have the 2 second barrels of the progressive 2 barrels open up.

At low speeds, in city traffic for example, with the 4.06 first gear, 2.37 second and 1.49 third, combined with the 2.73 rear end, you'd be doing 25-30 mph at 3,300 rpm in first, 2nd at about 40mph at 3,100; in 3rd at around 60 mph you'd be at 3,000; at 60 in 4th at 2,000 rpms with the powerband ahead of you, at 75 in 5th you'd be at 2100 rpms (or 1,800 at 65), cruising with power to accelerate.

Sounds good. I run a 2:73 rear in my 52. The 261 should move it down the road real nice and with the overdrive you'd be better than 2:73. One thing thought we are still driving bricks, we are pushing a lot of air. You may also consider 3:08 rear gears or 3:30's which would make it easer on your clutch around town and still give you lower rpm in od.

Sounds like it will be geared a little too high to me. Iv,e got 3.08 gears in my 70 stepside with 7004R o-drive auto. & 350 engine same size tires. It,s just right. I wouldn,t want it any higher. It,s not fun driveing something geared too high. It will bog down on hills or head winds.

46 pickup with 261, 3:08 gears likes the T-5 in overdrive. Now, that is on very flat and fast Calif highways any grade or slow down, it's back to 4th gear. If I had a choice now having driven this set up for a year, I'd want to be around 3:55/3:40 range for gears.

On my 235, 3:55 27" tires, T5 from a Camaro - It likes the freeway at 60-65. Actually it is more comfortable in the 65-70 range but don't want tickets and would not like to do any tricky evasive maneuvers at that speed even with disk brakes. I can pull a fair grade before backing into 4th. I wouldn't want any taller gears than 3:55 for that reason and like I say, it cruises just fine at freeway speeds.

I have the 2:73 in mine with a s-10 five speed,,works well, however in Over drive at about 70 you will lose pick-up speed, and on high unlevel grounds you might at times depending on traffic have to shift to fourth, in fact I run mine more in fourth at about 60 plus,,in overdrive above that is about 1600 rpm,, once I got use to it , I have no problem, a little slow on initial pickup but not bad,,I am running a 54/235 with 848 heads,single carb, fentons, dual exhaust,,,,,so if you have a 261 with the duals , you should not have any problem, I chose this because it was available at the time and I did not want to wait long to drive it as it was/is my daily driver,,,I can always changer ring/pinion at any time but I have driven it this way for over 2 yrs and it runs smooth as a baby butt.........good luck,,,,Fred

i am driving a 54/3100 with a 2.73 nova and orginal 3 speed here in central indiana it does very well. have a t-5 which will be installed just as soon as i am ready not to have the truck for a few days. i have thought of going to 3.11 gears but will drive it for a while as is. at 1.1 in town it stays up with traffic just fine. no passing on highway but i usually hold a speed that no one needs to pass me except the junior racers. btw 235-70-15 tires dual carbs and dual fenton headers, pertronix ign. i have been driving it this way about 20 yrs. yes i would like to pass and fly by but that is what i have my pt crusier for. i am going to my sons house in fishers, indiana about 55 miles saturday and have made the trip b/4 just fine, good two lane road out here we have 12 foot lanes. it always feels like it is going faster than it is, even with the lowered springs and new shocks, power steering also. ron

NM51 -

Well, you wanted some opinions and it looks like they are coming atchya!

With your low gear of 4.06 and your overdrive of .86, I think you are right on the money.

I'm using the 3.76 low gear and the .72 overdrive with a 3.50 rear end and it seems to be right.

You just don't want to spend your trips shifting back and forth from 4th to 5th gears.

At the worse, if it doesn't work right, you can always change the third member to a lower gear, but I think you got it!!

Joe

Thanks for all the input!

I'm going to buy the '78 camaro 2.73:1 rearend I've been looking at and try it out, mainly 'cause it's right in front of me and I'm itchin to get on the road!

If it turns out I don't like it 'cause I'm down shifting too much on the freeway or going up hills, I'll either swap it for a 3.08-3.40 rear end or replace the ring and pinion (and bushings/seals) in that one.

Thank you again for all your help!

i am not trying to hijack this thread but i have a 2.73 with 235-70-15 tires and the stock three speed and i had thought the gas mileage would be better. i put 130 miles on the truck and used 14 gals of gas last week all in town drivin 35 and 40 mph. granted the odometer is off but how much could it be off? RON

TRY this site !!!!

http://www.smokemup.com/ ... go to auto math

perfect for calculating ALL kinds of important ratios like :: sizing tires, rear end swaps, transmission types input, highway speeds and RPM's .....
airflow engine, building and tons more

http://www.smokemup.com/ ... go to auto math

THANKS Ji
I'm about to install in my 51 a 292, T5 (camaro gutz built into an S-10 housing) not just the tail cone put onto a V6 T5 setup, and a 12 bolt 3.43 rear. i have been asking so many questions and a friend said he has a 235 w/ a 3:73 rear end and wished he had put a higher rear end in. i've been toying w/ the 3:43 over the 3:08 and like the 3:43 idea much better. i went to this site and did the calculations and have always like the 3:43 w/ the T5. you just answered it for me. the 3:08 is too high. there are only these three choices in 6 lug for our trucks. anything else like a ford 9'' you would have to redrill for 6 lug or go w/ 5 lug. anybody else i've talked to always has a V8 or a much smaller 235 w/ the T5 whan i asked these questions.
you're the first that has a closer size engine to what i'm doing. my 292 is really torque-ie so i'm hoping the 3:43 is best
thanks again for listing your sumation in the bolt page... i just had to look to find it

My 37 Chevy pu has a 302 GMC,3.36 gears and a Saginaw 4spd with an overdrive unit.2000 rpm in OD fourth is around 70 mph.The truck doesn't slow down on hills at all.My wifes 51 Chevy pu has a mostly stock 261,3.40 gears with a NP883 4 spd OD tranny.2000 rpm in OD is around 67 mph,same story on the up hill.Both these trucks can pull a 1200 lbs travel trailer in OD no problem on relatively flat roads.
Maybe in hilly areas more gear might be necessary with a heavier truck
Personally,I think a 3.50 or so first gear with lower than 3.08 gears is a pain in normal driving,first gear winds out too quick.

NM51,

If yuor truck is runnin' now, try it out on La Bajada goin up to Santa Fe or Monticello canyon goin' to T or C. Figure you might be down to 3rd on those grades with a 2.73. A 3.08 probably pull those in 4th at a decent speed. My 72 nova has a 2.73, does o.k., but engine is a well-worked 427, torque no problema. Buena suerte.

We are running the T-5 with 3:55 gear and 225-75-15 tires. RPM @ 65 is 2350. It is driven in mountainous roads at 4-5k ft. elevation. The car will accelerate in 5th gear up hills! Acceleration in all the other gears is great! We run the same set-up you have, 848 head, offy with H/W progressives, Fentons with 1 1/2 in. head pipes (lowered the peak RPM for torque!)and the M4F cam.

Fuel economy runs 17-18 mpg!

Accerlation is awsome and it runs the Interstate all day long! Even pulls a 26 foot travel trailer to and from car shows!

Had a 2:55 gear for a while but is was no fun to drive. 3:73's would also work well with the T-5.