I put a 4 speed(don’t know if 420 or 465) in my ‘61 plus 300 hp ‘vette engine with 4.56 gears and big tires, kept it below 60, 12 mpg w/o load a few years back, worked well.
Ed
I ran an SM 420 in my other '65 C20 from '87 to '05, actually it was already in it, so I don't know if grandpa put it in there, or what. I suspect grandpa installed it, because there was a giant hole in the floor and a piece of plywood with a hole cut to slip on the shift tower...lol... Not secured, no sides, just a piece of 3/4 plywood resting on the transmission. Many years later I finally found a high hump, but lacked the flange that goes between the hump and the firewall. It was nice not having a u-joint spinning right next your legs and an open floor, let alone the extreme heat in the summertime, and extreme cold in the wintertime. That truck had the 4.57 HO52, so the engine was turning 4,000 rpm on the tachometer to keep up with traffic on the open road. In '91 I found a '68 Dana 60 with 3.54 gears. What a huge difference! I have no idea what my mileage was while turning 4,000 rpms, but I got 14-16 with the 3.54s while cruising 70 at 2,600 rpms. Back then I didn't have overdrive, and going up steep hills or towing heavy items, the truck struggled on inclines or into strong headwinds. Other than that, those 3.54s are awesome! Now I will have a gear splitter and will have gears between gears, so that's why I generated those spreadsheets.
