Hi, I'm new to the forum, new to '37 Buicks, but an old hand with cars. Does the engine feel like it's misfiring at 35 mph, or does it just not want to go any further? BTW I would doubt that the oil in your gearbox or diff is causing this problem. Does it keep going at 35 mph or does it start to slow down? As suggested previously - does it feel like it goes OK in the lower gears but once in top gear it "runs out of puff"? If it feels like a misfire but keeps going at 35, then I would be looking at closely at the ignition - leads, cap, rotor, points, and coil. If it just won't go any further but doesn't feel like it's misfiring, then I would look at the exhaust. If it starts to die at 35 at then gets worse as you keep going, I would look at fuel - pump and carb. If it stops going any faster (particularly on hills) but the engine sounds louder (revs harder)- then look at the clutch. Hope this helps. Rob