Gear quest

Depending on tire size you could go anywhere from 3.25 to 3.73. Even up to 4.10 if you’re doing Offroad

 

Why a 15 speed ? Are you planning to do heavy haul ? The first 5 gears are useless, they are deep reduction. Your basic gonna have a 10 speed. 13 is the way to go.

 

Yea 13 or 18 or 8LL over a 15 imo. I personally would rather have a 8LL than a 15.

What size rubber do have? My dad recently swapped out a 9 over (old style against the dash) to a 13 double over .62 ratio in the big hole. With 22.5lp and 3.91 rears @65 was roughly 1675rpm and with the .62 I'd now he's at 1375 at 65 and gained .5mpg.

For your rears pick your tire size and find out what you need to cruise in the optimal range. 1750ish I would assume for ole ntc.

 

I liked the reverse in deep reduction, the regular reverse was too fast for tight maneuvers.
The 15 over has a final gear of 0.78 vs 0.73 of a 13 or 18 speed.
Another forum said to run it at 1550 rpm at your cruising speed.
So we need to know what size tires and speed you want to run at.

 

I think 1550 is on the low side for a big cam. That's right on the verge of hurting mpg and engine life. They don't lug

 

I run a Big Cam in my truck. Transmission is a 15613 transferring to Rockwell 3:70's with 24.5 tall. If I run around 1600 RPM when cruising I get my best mileage, but am below the rated speed limit in this area of 70mph.
Personally, a 15spd trans is a good choice for the low lands, but in my area with constant up and down the 13, 18, are 2 of the better choices. At some point I will be going back to my roots, a 6x4 is in my plans.

Now I have a friend who runs a A model KW with a B model Cat that loves his 15, big hole against the dash. But he runs a lot of Texas and California so no need to split gears on the HI side, at least not where he goes.
Be safe out there.

 

Is your 15 a direct or overdrive? I paired my RTO-15615 to a 3.73 rear. I plan on running tall 24.5 and IIRC based off a gearing chart it should put me at around 1500ish mark at 100km/h (62mph). I know for sure the calculator said I could hit 96 mph at governed 2100 RPM lol.

 

Tall 24.5 with a 15 speed and 3.90 rears I turn 1650 at 70 mph.