Ah...I see, it must shift with a solenoid like the factory units. I guess I was thinking it was more like an auxiliary tranny with a mechanical shifter and linkage. Do you know the ratio of the overdrive? I used to run one of the old Borg Warner 3sp. + od units in a '57 1/2 ton. I found that 1st + od was virtually identical to 2nd, and 2nd + od was pretty close to 3rd. Splitting gears wasn't really practical unless accelerating up a hill with a heavy load. For racing, I think the extra shift would lose more speed than it gained you. Depends on the ratios though. If the o.d. splits the gears more evenly it could be a big plus.