[ngoma]

M46 1st gear, at 4.03:1, is granny gear low. 4th is 1:1, and OD is a pretty good 0.79:1 OD reduction. The M46 cars came with the 3.31 rearends, the highest (lowest numerical) available. Didn't you say your car has a much lower-geared rearend? That would affect the driveability, making it great for towing at lower speeds but too busy on the highway. Probably don't even have to use 1st, could start out in 2nd. The 3.31 rearend would make it more useable but 1st is still a stump puller. Larger diameter rear tires would also help. The 740s came with 185/70x14, which are maginally sized for these large cars. I had better results running 195/70x14, and more recently 195/65x15 (even better).

The ZF is seemingly better suited to the D24T, strong, with good ratios, logic, and smooth shifting. 1st is 2.48:1, 4th is a healthy 0.73:1 OD, lockup occurs at ~57 mph. It had a known defect that would destroy itself if revved in neutral or park after having been in gear. TSB retrofit solved this problem, otherwise do not rev in neutral or park, or if necessary turn off the engine and restart, then can rev in P or N *before* placing it in gear. There is another common failure mode with the governor in the tailshaft that causes it to hang in 1st gear too long first start off in the morning. I think the ZF cars came with the 3.46 rearend.

I have one of each, and personally prefer the M46, simply because I have always preferred manual over automatic, but would really prefer a real 5-speed over the kludgy OD, and with better ratios. Too bad the M90s are so hard to come by here in the US.