The Stovebolt Site Forums The Shop Area Driveline 3600 T5 Differential Swap

I have a 52 Chevy 3600 that I replaced the original 4 speed with the T5 per Lugnutz page. He has a lot of very good info and everything went fine from the input shaft swap to making a short throw shifter. The rear end on this truck is a 10 bolt GM at 4.57. It has the full float axles and 8 lug rim which I have 245-75-16 mounted. I really need to change this out. Research says I may be able to get down to 4.1 but a 3.73 seems to be the preferred rear end. I want to keep the 3600 look of the 8 lug. So has anyone with a 3600 ever changed out their rear end and what did you use.

I replaced my 4.57 with a 4.10. A real simple swap. I don't know about a 3.73 but the 4.10 gets me going fast enough (45-50 MPH for me) at a comfortable RPM.

What is the ratio of 5th gear?
How tall do those tires measure?
The reason that I ask is because I too, was bitten by the Lugnutz bug.
I have a '50 3100 with a T-5 with .73 overdrive, 4.64 to 1 rear end gears, and 30 1/2 inch tires.
I could not be happier with this setup from zero through 65 MPH through all 5 shifts. (I do not want to ever go faster than 65 MPH for safety reasons. These babies do not stop on a dime and you will never win a high speed slalom course.)
Carl

Fancyt, Carl's questions are spot on. You can go too high when making rear gear ratio changes if you have a .72/73 5th gear.

I have a .72 OD T5 with 31" tires (235/85/16) with 4.10 gears from a 67 Chevrolet C20 in my'51, 3600 with 216. It works fine as long as I'm running in flat country which is usually the case unless traveling. Well, got to add head winds to the list of when 4.10's are a bit too high. Of course all you have to do is drop down to 4th and all's well. With current gearing my truck will now run comfortably at 60-65 with a 75 mph max. But any hill or headwind and it more like 50/55 mph.

Fact is the original 4.57's were good with the .72 OD. Now if/when I get around to swapping in the 261 that's holding the floor down in a corner of the shop I think the 4.10's will be just right. And if I find a set of the hen's teeth 3.73's that were installed in a select few Eatons I'd be up to trying those. But no way with a 216 would I go with anything higher that 4.10s.

Good Luck,

RonR

Fancyt,

I have a 48 3600, with a 292, and T5 with 0.86 OD. I have 4.11 gears (14 bolt out of an 04 2500) and 245/70/16 tires, which are 29.5" tall. I think I would like 3.73 gears or 0.72 OD better with this set up, seems a little slow for me. It does just fine in the hills of western PA, rarely do I have to shift out of OD.

Jon

My tires per a tire calculator are 30.5". On a T5, direct drive is 4th gear for a 1 to 1 ratio. Using a speed calculator, http://advanced-ev.com/Calculators/TireSize/ , I find that with a 30.5" tires, 4.57 differential, and 2500 RPM, the truck will do 50 mph. In OD, 0.72, the drive ratio would be 3.29, and using this number, in 5th gear at 2500 RPM, the speed would be 69 mph. As I don't have a true speedometer setup yet and no tach, I am going to use a GPS and portable tachometer to do some testing. I don't know if 2500 RPM is too high a revolution for the 235? Plus am I correct in using the 5th gear drive ratio as .72 x 4.57 = 3.29?

Your calc's look spot on.

The 235 used in big trucks had governors set at almost 4000 rpm, so you running it at 2500 is probably on the bottom end of the power curve.

Mike B

I have a 62 one ton panel that weighs in at 5800 lbs before bondo , a warn out 235 with a t5 with .73 o/ d and a 4.57 rear end with 30 in tires. I run the truck between 2500 rpm and 3000 and seems to be the perfect set up for Me right now. I did have a 3.90 rear end in it which dropped the rpm to the point where it wouldn't climb even a gentle grade at high way speeds with out dropping a gear so I put back the 4.57. This not only improved drivability but improved my mpg as well.
At one point a few years back I lost over drive and drove home ,a 600 mile one day trip at 60 mph and 3000 rpm for about 12 hrs that was with the 3.90 rear end,.and the truck seemed to have been running at its best on that trip. I learned on that trip that rpms are your friend.
Once I repaired the o/d I went back to the 4.57 rear as I like to travel at the 70-75 mph on the longer trips of which I do many .
I don't think over gearing it is the answer.
I had 5 different ratios for the rear end here and the 4.57 is the one that worked best with an o/d trans. There is only a couple of mph difference in high way speed between a 4.10 and a 3.90.

John

Setting these drivetrains up correctly for optimal performance fro 0-70 MPH is dependent on the optimal powerband RPM of the engine being used.The 235 seems to just begin to power up at 2500 RPM and hits the sweet spot at between 2800-3000 RPM. At or below 2500 RPM and they lack what it takes to accelerate up even a slight hill.

Have a modern turbo diesel in class C RV ...............same sweet spot on power and actually getting work done at 2800-3000

They made a 0.63 OD T5 for both the Camaro/Firebird and Mustang. This mated to a 4.57 rear gives a 2.88 final which is too shallow for most engines in trucks to handle on anything but flat road and no wind. The 0.68 OD gives a 3.10 final which is probably still iffy without short tires. Keeping the engine in the middle to top of it's torque range at the desired speed keeps both it and you happy.

I have a T5 with 0.76 OD in my 1963 with a 3.73 rear, and 29” tires, and a great running 230. It will cruise along at 2100 RPMs at 62-65 MPH. I rarely need to downshift. I hooked up a vacuum gauge to the intake manifold to watch performance. The conclusion was that lower RPMs will not necessarily give better MPG. I have all the power I need but my engine is probably not in the ideal RPM range for power. So the lack of HP at lower RPMs translates into lower MPG than you might expect. There may be less engine wear, but no actual fuel economy.

Fancyt,
If I were you, I’d do the math and keep the RPMs above 2500 in 5th at highway speeds. Your engine will perform properly with the needed HP and you will not need to downshift on hills as often.
The 0.63 OD can be found in V8 Camaro T5s but the 27% drop in RPMs is a HUGE drop and usually only works well for lighter vehicles with V8 horsepower.

I replaced the rear end in my 49' 3600 with the Dana 60 out of a 69' F250. I had to weld spring perches which was simple enough and have the drive shaft shortened with a yoke welded on to mate up to it. It has 3.73 gearing. I have a 235 with the stock 4 speed and I can cruise 55 at 2500 RPM.