The transmission in your truck is one of its most important components. Its overall strength and health impacts fuel mileage, trail ability, and durability. Over the years manufactures have produced trucks with weak, undergeared trannies to meet low price points and increased sales. Fortunately, they also built heavy-duty trucks with super-strong, granny-geared trannies that you can swap in to beef up your truck. Most of these were just heavy-duty three-speeds with an important addition. That addition was a compound-low, or granny, first gear for added reduction when towing or trail-running. Transmissions such as the NP435, T-18 and 19, T-98, SM420 and ’465, and even the non-U.S. Toyota four-speed, all offer the user super-strong transmissions with low First-gear ratios.
In addition to needing a lower first gear you may also want to consider an overdrive. This magic gearset allows you to reduce the engine rpm by an average of 25 percent, which will improve your fuel mileage. Improved mileage allows you to keep a little more of the money you might spend on gas in your pocket. Overdrive trannies generally are found in late-model trucks. They are available in both manual and automatic configurations.
The most popular of the manual transmissions for conversion is the New Venture Gear 4500 (NV4500) tranny. With the NV4500 you get both a lower first gear and an overdrive too, the best of both worlds. (See page 76 of this issue for a full story on swapping one of these into an early Bronco.) Another option that gets you overdrive for your manual is to look into a Ranger overdrive unit from Advance Adapters. These units go between the motor and transmission, and will fit many of the popular four-speed manuals we’ll cover here. But, that’s a story for another issue.
So far we’ve talked about manual trannies. On the automatic front, you can swap in a stronger unit but you have fewer options. Dodge owners can swap a 727 in place of the 904, Ford owners can drop in a C-6 for their weak C4 (note that small-block and big-block C6s are different). If you own a Chevy or GMC then it’s easy to upgrade to a TH400 or TH475 from a TH350. These are all non-overdrive upgrades. If you’re looking for an overdrive automatic, things get more complicated.
Overdrive automatics are generally computer-controlled and are troublesome to put into non-computerized trucks. Sure, you can upgrade to a 700-R4 if you have a GM product, but basically this is just a TH350 with an overdrive. This means it’s not the strongest for rough use in heavy applications. However, they are great for use in light-trail rigs such as Jeeps. The overdrive version of the TH400 is the electronically controlled L480E and is the only overdrive automatic for which you can get an aftermarket kit that will allow you to use it in a non-computer-controlled truck. Ford owners can get an overdrive automatic by swapping in an AOD. Other automatics such as the Ford electronic AODE and E4OD, and the Chrysler A-500 (OD 904-999) and A-518 (OD 727), are not worth the trouble unless you’re swapping to a late EFI motor and harness at the same time.
So that’s the overview. Read on for the details.