I just finished my flathead and adde a T5 transmission and I need to know what the best option is to swap the rear end for better highway performance. My new flathead is 250 hp. Thanks
Rudy

 

I have a built flattie in my 39 with a T-5 also and an 8" Ford rear with 3.25 ratio. The flattie loves both the T-5 and rear. Ken

 

Thanks for the quick reply. The rear end you used came from what car? I'm pretty new at this so I'm learning as I go along. Thank you

 

There is a sticky in the 'Forum Tips and Tricks' thread: 1948 through 1960 Ford F-1 and F-100 Rear Axle (differential) Swaps - Ford Trucks

Welcome! Pics?

 

A 250 HP flathead?! It must be blown? Pics / details please!

 

I will post more engine pics

 

Don't know what the rear came out of - I bought it from a junk yard and the dealer couldn't remember what it came out of. With 250hp you might want to upgrade to a 9". Lots of ratios available for them. Ken

 

thanks for your reply

 

With a T-5 transmission, your stock rear should work just fine. In overdrive (5th gear), your effective final drive ratio would be right at 2.75:1, which should give you a top speed faster than you want to go in that truck safely, anyway. You don't really want to be much higher (lower numerically). Especially at that HP level. You'll be hard pressed to keep the rpms in the power band.

 

thanks. that good news

 

8" were not installed in trucks (unless you include Ranchero's).

Were only installed on some 1962/80 mid-sized Ford/Merc Passenger Cars and misc 1963/79 Ranchero's.

The wheels & bolt circle patterns are either 5 lug 14" or 15" 4.50" bolt circle, or 5 lug 14" or 15" 5.00" bolt circle.

 

AGREED, thats why you go for a T5 the 5th gear is most likely a .67 ratio. I have the original Dana 44 in my truck with Ford Racing 306/340HP butr with a AOD, ie same final drive of .67 and beat on it now and then and it holds up. I did install a posi unit. Original gears are still there which are a 3.92, go down the highway good!

With a 250HP Flatty it should cruise with the T5 (Flatty and T5, nice setup!) your rear end should hold up nicely as long as it is in good shape.

 

You should have a Dana 44, the OP should have a Dana 41. That said, I drove my 49 F-1 with the Dana 41 with a Nailhead Buick in front of it without issue for about 10 years. In that the Buick initially was running thru a stock light duty trans, the trans was the "shear pin". A HD 3 speed fixed that problem.

 

Where is the build thread on your truck? 250hp flathead hell ya!!!

 

If you want to change it out, with I think Bill (Numberdummy) is saying is that the 8" rear ends will have a different wheel lug pattern.

A 9" rear end from a 65 to 72 F100 is almost a bolt in deal. Almost in that you will still have to deal with mounts for your shock absorbers, and the parking brake cables will be different. You'd have to figure out some way to adapt the cables to the lever in your truck. Ford kept the 5 on 5.5 lug pattern in the F100's from those years, so your wheels would bolt up. The width is also correct, and the spring perches would be in the right location to bolt to your leaf springs.

I still want to see pics of that flathead (and the rest of your truck)!!

Dan