With your 3.54 gear, the driveshaft turns 3.54 times to your tires turning 1 time. In top gear (not overdrive).. your engine is turning the same speed as the driveshaft.

 

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 have a gps it will give you a pretty close call on what your speed is and then you can adjust until you have the propper setup.

 

Buzz44, Thank you for the suggestion. I don't own a gps but my 20 year old daughter does. I'll, first have to talk her into riding in my truck!

 

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.

Here's one I found online...$15
http://shop.advanceautoparts.com/web...orce_5271134-p

Jason

 

Somebody should tell my truck that.

 

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(: