Tremec RTS / T170FT (TOD)

A four-speed with overdrive built in Mexico by TREMEC — Transmisiones y Equipos Mecánicos — for Ford, and fitted only to the 1984–1987 F-150 and full-size Bronco, in both two- and four-wheel drive.

Its full designation is T170FT, and the trade calls it the RTS or the TOD. RTS is the shifter arrangement and TOD is Top-shift OverDrive; the honest description of the gear layout is the third name it carries — 3+1. Three working ratios and an overdrive, not four evenly spaced gears, and that distinction is the whole of how it should be driven.

Three overdrive ratios were built and the code is stamped on a steel tag on the case. The B set gives 27.6% overdrive, C gives 37.9% and D gives 41.1%; C is the one usually found in Broncos and in four-wheel drive. First is 3.01 or 3.25 depending on the set, third is direct at 1.00, and fourth is the overdrive. Ford's own door label calls the option transmission code B, which — confusingly — identifies the Tremec 3+1 rather than the B gear set.

Its one real weakness is a driving habit, not a part. These were towed on hard by owners who stayed in fourth — the overdrive — instead of dropping to third as Ford instructed. Overdrive puts a continuous offset load through the gearset and the bearings behind it, and a sustained diet of that is what accounts for most of the failed ones. A unit used the way the handbook describes is a robust gearbox, which is why it remains a popular overdrive conversion for pre-war Fords.

Use SAE 80W GL4 and never GL5. The synchroniser rings are brass, and GL5's extreme-pressure additives attack them. It holds about 1.7 litres.