Not too many of us get to try the same truck and loads with both the five speed and the six speed so I figured an overview of my impressions might be helpful to some. I put 187,000 on the five speed pulling 30,000 plus gross weight. Never had a problem with the nv4500 but lost a clutch at 126,000 and the pilot bearing froze at 187,000.<br><br>The six speed is far more compatible with the work that I am doing so is a great improvement. The extra gear keeps the rpm’s down so the mileage is somewhat better. With the 4500 I had to wind up to over 3000 on a heavy load to shift at a comfortable engine speed in the lower gear. This meant that on some hills it stayed wound up for long periods.<br><br>The six-speed gearing is just perfect for the engine with any load. That being said the syncro setup is not so good. I will do fine with it but it is sensitive and easy to hurt with a rough shift. The 4500 was effortless shifting, rarely missing a shift but the 5600 is both difficult to find gears without thinking about it and it has poor syncros. While I am disappointed in the shift quality, the overall conversion was a good deal, glad I did it.<br><br>To make the conversion, I had to have the drive shaft shortened six inches also the front shaft retubed six inches longer. I had to make an adaptor plate for the rear mount to bolt correctly. The transmission exhaust mount does not fit without cutting and rewelding. The shift plate on the top of the transmission is different but the shifter fits fine so the interior is no different, still has a five-speed ****. The stock ’99 cross member fit fine.<br><br>Going down the road with a load it feels like a different truck. Smooth and seems to have better power in OD although I realize that is just a feeling not actual. There is a definite advantage when slowing to have the forth gear closer in ratio and it does give better power because of it.<br>