I am running 255 x 60-15" BFG's on rear with 391 SG gears with A 833 4 speed. My FSM chart does not list P- metric sizing because it is for 1971 cars. There is not enough reliable info on the internet to convert the tire size. It looks like a 39 tooth gear should be close to correct my speedometer so does anyone have real experience so I can dial this in. Classic lists all gears but at $60 a pop I like to get this right the first time? Also the FSM lists two A-833 trannie's a "Standard" and a "Heavy Duty", slight difference in 1 st and reverse ratio's and also lube capacity. How do you tell the difference without a dismantle? My trans has a 1" x 23 spline input shaft, if that helps.