Jerry, Thank you for the education. I hope others are learning something from what has been shared here as well. Maybe the MPG will improve somewhere close to 14% as well. If so, that would be 28 more miles per 20 gallons of fuel if it was getting 10 MPG. I don't recall the tank size on the truck. I'll check on that. I'm guessing that a lot of other variables would need to be factored in. As they say a little knowledge can be dangerous. It seems that a 14% reduction in RPM's would generally speaking help extend the life of the engine. Although the engine / transmission might need to downshift more frequently and for longer durations especially when towing, pulling speep hills etc.
I have a '66 F250 that had 4.11's, turning nearly 2800 rpm at 65MPH. Pulled a set of 3.54's out of a '68 F250 and swapped the ring and pinion out. That was a great move in my opinion. RPM at 65MPH dropped to 2300. I have a 390 4V and a NP435. Unloaded it is getting 14mpg driving 65MPH.
I pull a 7000 lb trailer from time to time...the 3.54 does great.
Krh427, It sounds like the 4.11 to 3.54 swap is working out great for you. Your experience will give me a good basis for comparison. I don't know how a NP435 compares to a cruise-o-matic. My speedometer will be off until I replace the gear. When I get my truck back I'll drive next to my wife or one of my children at 65MPH on a level road and see what my RPM's turn out to be. I'll report that back here. Hearing about your experience is very encouraging. Thank's for sharing!
If you went with a rear end from a 1970-72 Camper Special, you could go for a 3:73 and also get
better drums (I think from 2" to 2.5"...). The yoke spline count is a bit different, I just did a conversion
Where can you buy the correct conversion u-joint? I'm thinking of doing the same rear end swap to my '66 f250.
I got my conversion u-joint for my 1972 Dana 60 / 1966 F250 Driveshaft from a drive-line shop that services
transmissions and diffs and builds drive-shafts-the original diff was a 10 spline and the '72 diff. yoke is a 29, so this
does the job nicely.
I'm sure you could find one at any driveline/tranny specialist shop. Mine was a sealed (no grease fitting) "Brute Force"
model number 1=0134BF.
Mine likes 55mph. I realize these were made for work/hauling and I love my 250, just wish it had a bit more cruising oomph. With the 3 speed cruiseomatic tranny it always seems like it needs to shift into 4th, but never does(:
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