Lower first gear trans

I really like our 2 speed rears @4.10/5.62 for just what you describe, but it is a rather expensive solution.
I don't know enough about trans to offer anything, but I wonder if a good transmission shop could change your low hole gearing for something lower? It would be a huge split to the next gear, but once on a hard surface it may be manageable.

 

The old 15 speed deep red. trans with hi gear up at dash instead of back came in the year your truck was built and will give you 3 gears lower then you have now?

 

According to the information I have the 18 speeds have either a lowest gear ratio of 14.71 (6-series, single overdrive at 0.85) or 14.4 (7 and 9 series, double overdrive 0.86 and 0.73).
The higher the number the lower the gear.

15's have either 15.52 (6 series), 15.4 (7 series) or 16.94 (9 series). 16.94 is 18% slower than 14.4, 15% slower than 14.71.
BUT these are direct drive in top gear, so you will loose your (1 or 2)overdrives. And rpm in top gear will go up by some 15 or 27%. I don't know if that is acceptable.

Overdrive 15's all have a taller 1st (lowest) gear than an 18. (12.2 - 13.3)

If you want a MUCH lower 1st gear you would have to go to something like an 11 speed, for example RTOF-16909ALL.
It has 11 forward ratios. Lowest is 26.08 (almost twice the reduction as an 18) next up is 16.3, and it has 0.73 overdrive on the other end. But all the steps between the gears are much larger than on the 18.

9ALL Heavy Duty Manual Transmission, Manual High Reduction Transmissions - Vehicle - Eaton

Hope this helps, but maybe it just adds more confusion.

 

You Need an Old Mack 18Speed Box. they have a deep Low & reverse gear........

 

Which 18 speed do you have? The A or B ratio. The B ratio has a lot lower low hole than the A. They both end at .73

 

The B ratio is 15.06 verses 14.47 on the A. Which isn't as much different as I thought. The B ratio 18 speed does have the deepest low gear, it would be quite a costly swap for the gain. Another option is Eaton offers a 2 speed Auxiliary that has an underdrive and direct only. So it wouldn't offer any more road speed but would give a deeper gear.

 

KVB is right the 9ALL is the lowest and still an overdrive. Otherwise you have t go to a two speed axle.

 

Not even the "old" Mack gearbox. I have the 2180B in my truck. The 2180 has a stupid deep low low and reverse. The 2180B is the same gearing as the 318, and the 2180's gearing is identical to the 318LR. If you need the high torque version of either, add a 21 to the end...318LR21 has the low ratios in the low low & reverse, and a 2100 ft lb rating.