ISO Tallest OD 13/15/18 speed.

I am seeking some alternatives on finding one of the following in the tallest gear ratio. Mostly hauls heavy locally and within the state. It needs speed to be able to move along or atleast get out of the way.

Truck is a 1979/2008 rebuild AM General M920 8x6, Currently running Rockwell SUHD 6.17 ratios with Detroit Lockers in the rear. Standard walking beam. Has a Hypoid FDS-1807 6.17 in the front. The hypoid has alternative gear ratios but they are hens teeth. So swapping gear ratios isn't really an option.

Current transmission is the CAT 7155 16speed with .83 OD. Works as it should.

Transfer case is a Oshkosh 18000 direct drive single speed. I have an Oshkosh 55000 with .98 high gear so it gets a little OD speed but not worth the price to swap it with new yokes and modified drivdlines for a whole 2mph increase.

Tires are 12R24.5 Goodyear G286s and G177s on the rear. 445/65/22.5 Goodyear 296s on the Steer and pusher. I want to stay with dual tires of commercial availability. Not the 1400 or 1600R20s that will increase speed, yes but single tires aren't optimal. And they only last about 40,000 miles. I have plenty stock of 12R24.5 Goodyear G177s

Engine is a Big Cam 2 NTC350 according to the ESN however it was rebuilt by the military in 2008. I would assume to CPL531 like others in the fleets are.

Running HT3B for now. #10 button. 255/260 pressure. I have an HT4B that I am planning on swapping on with a #5 button. Engine is set at 2200rpm on the governor.

Top speed at the moment is 58mph at 2200rpms. Even with a 2400rpm gov it would only net 63mph.

I know the spicer 18 speed pro shift is a .70 final. The Rtoo14613 is a .62 final but they are getting harder to find etc. I found a few but they state that the countershafts are getting hard to find for them.

Looking for some other options in the .60 to .66 range. Or even taller but I don't think there is much beyond .62.

Will need to run a PTO as well as the truck is equipped with dual 55,000lb winches and a hydraulic welder unit. Truck weighs close to 33,500 with both tanks full. It is 54,000lbs with truck and M870A1 lowboy empty.


Any help to find other options would be appreciated

 

We have a few trucks with WTEC 5 Allison 4700sp 7 speeds that are .74 and. 64 OD units but they have huge holes between 4/5 and 5/6 that the engine is either above the power or bogging.

These run off a throttle cable system. They are pretty pricey to swap over but our others just bog pretty easy with a heavy load. They also use a tiny shaft coupler that has a tendency to break easy and vibration is constant issue. The only cure for this is moving the transfer case back which is a good bit of modify work as well

 

The transfer case to front tandem. Shaft is around 4ft long so possible as long as it was in direct with the front end engaged.

Front axle has desplined front hubs so the internal doesn't spin while moving anyway unless the flanges are swapped