updating questions?

[55 Fargo]

My (fresh and beefy) flathead Desoto (255CI) engine pulls the overdrive well with a 3.55/1 differential and 28" tires. I can run 70 MPH all day long with the engine spinning at about 2200 RPM's. Pulls all but the higest of hills without down shifting. I do not know enough about the hydrive to comment on it so not sure how it equates?

At 70 MPH your current set-up minus the T5 trans, say using a regular 3 spd without OD trans at 70 mph your Tach would be spinning at 2973 rpm.

At 2200 rpm, 3.54 diff, 28 inch tire, and 3 spd trans no OD, you would be cruising at approximately 52 mph.

At 2200 rpm your engine would cruise beautifully all day long, at 2973, it would be running quite a bit harder, may be tougher on the engine thats for sure.

Probably why for the most part, these cars were driven in the 50 to 60 mph range during there hayday, most P-15s might have had a 3.73 or even a 3.9 or worse 4.11, the RPMs screaming down the road at 65 mph

[Reg Evans]

How may blades are on your fan? I've had a couple of these oldies that had 6 bladed fans and the extra 2 blades made a lot more racket than the standard 4 blades.

[Young Ed]

At 70 MPH your current set-up minus the T5 trans, say using a regular 3 spd without OD trans at 70 mph your Tach would be spinning at 2973 rpm.

At 2200 rpm, 3.54 diff, 28 inch tire, and 3 spd trans no OD, you would be cruising at approximately 52 mph.

At 2200 rpm your engine would cruise beautifully all day long, at 2973, it would be running quite a bit harder, may be tougher on the engine thats for sure.

Probably why for the most part, these cars were driven in the 50 to 60 mph range during there hayday, most P-15s might have had a 3.73 or even a 3.9 or worse 4.11, the RPMs screaming down the road at 65 mph

I think they all had 3.9 or 4.11s. The 3.73 came along later. Might have been optional in the p15s but if it was its uncommon.

[Plymouthy Adams]

in regards to Hy-drive equipted vehicles, the multipled torque offered by the convertor made using the 3.73 ratio ideal thus ALL Plymouth P24/P25 rear gears with Hy-Drive is 3.73..there is no exception for this application per the manual.