Can my 261 run this tranny and rear end gear?

[garry mould]

Hi

I am planning to install a 261 engine in a 56 GMC pickup.
The truck currently has a 3:38 rear gear, and I am wondering if it has enough power to run a T5 .73 overdrive transmission from a 83 5L firebird.

Thanks

 

[Rich 5150 69]

Garry guessing that you where to run 29 inch tires still would be too much gear, you really need a ( ideally ) 3.73 ratio but here is what the numbers would look like with 29 inch tires...approx..transmission in 5th gear

Engine Speed (RPM) MPH 3.73 Gear
8000 RPM 253.48 MPH
7500 RPM 237.64 MPH
7000 RPM 221.79 MPH
6500 RPM 205.95 MPH
6000 RPM 190.11 MPH
5500 RPM 174.27 MPH
5000 RPM 158.42 MPH
4500 RPM 142.58 MPH
4000 RPM 126.74 MPH
3500 RPM 110.90 MPH
3000 RPM 95.05 MPH
2500 RPM 79.21 MPH
2000 RPM 63.37 MPH
1500 RPM 47.53 MPH
1000 RPM 31.68 MPH

 

[Lakeroadster]

Looks like a workable combination to me.

3.73 vs. 3.38 rear gears... makes very little difference in the usable RPM ranges.

Your question about too little power is a good one, not sure the motor will have enough HP to cruise in OD. But that's the beauty of the T5, just shift down and your good to go, literally!

Data below shows MPH based on 0.73 ratio fifth gear and 29 inch tall rear tires:

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Data below shows MPH based on 1.0 ratio fourth gear and 29 inch tall rear tires:

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[Rich 5150 69]

Given the available HP and torque 135/140 all at 2000 rpm, and just guessing that tire diameter is 33 inches, 5th gear will always be just out of range, he`ll be lugging the motor at 65 mph, never having the engine in an rpm range to drive it. we really need the rest of the details to better answer the question...

 

[Lakeroadster]

Given the available HP and torque 135/140 all at 2000 rpm, and just guessing that tire diameter is 33 inches, 5th gear will always be just out of range, he`ll be lugging the motor at 65 mph, never having the engine in an rpm range to drive it. we really need the rest of the details to better answer the question...

Agreed, more data would be helpfull. The fact that he can simply shifts down into fourth on hills etc. makes the T5 an excellent choice. Also keep in mind that on a relatively flat road, at cruising speed, the truck will only need about 55 HP at 60 mph. http://www.ajdesigner.com/phphorsepower/horsepower_equation_trap_speed_method_horsepower.php

How did you go from assuming a 29 inch tire to assuming a 33 inch tire? Stock tire for a 56 was a 6.70-15 which has a rolling diameter of about 27-1/2", FWIW.

 

[Rich 5150 69]

Gear RPF* RPM Speed RPM Speed
1st 1.46 1500 12 1800 14
2nd 0.88 1090 14 1800 23
3rd 0.60 1218 23 1800 34
4th 0.45 1343 34 1800 46
5th 0.30 1224 46 1800 68
Your RPM at 70 MPH is 1864 (5th gear)

These figures based on a TKO 500 .073 fith gear 29 inch wheels, RPF revolutions per foot on the wheel, rpm shift points at 1800, I still think he will be just below the torque band to keep rolling in fifth gear, sure he can always drop to fourth but that might get cumbersome after a while to have to keep shifting to keep it moving along, he really needs minimum 3.73 to be happy. Been there done that with a CJ2A four cylinder...!

 

[66 Truck Drive]

With that gear ratio and small motor you would be best off to purchase a non-overdrive richmond 5 speed. That will give you a close ratio of gears so you don't have to rev it so high between shifts and also has no od. Tom.