Depends on a lot of things.
No matter the transmission you won't have the power to go over about 45 towing 8,500 lb. at least not where we live in the mountains.
Yes, an overdrive transmission will let you go faster when empty, but the limiting factor on your truck when towing is the power and brakes, not the lack of an OD gear.
The same 235 engine and 4 speed was used in 2 ton trucks, but they had bigger brakes, and really low gears in the rear axle, so were slow.
Do you have a 3 or 4 speed?
Grigg
