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I have a 49 GMC with stock running gear. SM420 4-speed with the stock rear differential which I believe to be 4.11 - 4.56? I have picked up a good Ford 9" rear axle assembly in good shape but it has 2.75 gears in it. Calculations show with a 27" tire I would be ~2000 rpm at 60 mph. Are these gears to tall for a daily driver with the 4-speed. It has the lowest granny gear I think they ever put in a GM truck? Something around 7:1.
Thanks

I have what's left of a '49 or '50 Chevy 1-ton where the guy replaced the original rear with a 14-bolt Chevy with 3.70 (?) gears. He said that the stock 216 would no longer pull the truck.

I assume your GMC is a 228, but that's not a lot more motor then the 216, so with those gears you might have some trouble.

On the other hand, I have a GMC 270 and SM420 in my '55 1st Suburban with 3.08 gears, and while its not a race car, and struggles a little pulling a loaded trailer, just driving it around it does just fine.

dwg49gmc

yes your 1st gear is 7:1 but you don't need to worry @ freeway speeds your final is 1:1.
a 2.73 works on flat ground, find a hill (not a mountain) and you may be in trouble climbing it.

you'll now have to carry tow spare tyres
one f@$?
one chevy
you could simply re-drill the axles to chevy five lug.................

my.02
after doing a search on this site about rear ends that could work find
a 6 lugger or change everything to 5 lug chevy. A lot depends on what you will do w/ this truck later on .... an engine change? tranny? hotrod? street it? daily driver? lots to consider.
mine is a 292, T5 and 3:40.

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welcome to the bolt
enjoy your stay

THe truck has a 270 in it and runs great. I was going to change out the front brakes and get the same 5 bolt pattern as the Ford. Have to buy new wheels only and I am told the 9" F%@# is a good differential. PLus it is an exact fit. No Cutting. ThTanks for the help.

What model ford is it from?

Do even the spring perches line up, is it truly an exact bolt in replacement?

You could get an entire 3rd member for the 9inch and change it out in a day in your driveway if it doesn't work with those gears. The 270 is a good engine and should be able to pull through those gears, but it won't be a stoplight racer!

Will you be able to use that granny low transmission? Does your truck have a torque tube? I'm just trying to envision the whole driveline setup.

I'm using a full floater 9 inch in my truck. I had the new hubs drilled for the 6 on 5.5 bolt pattern. I will have heavy duty 4 wheel disc brakes... 12 inch rotors that are 1.25 inches thick front and rear.

I love my 270 they are great motors.

When you do the front brakes the readily available disk conversions either go to the 5-lug Chevy pattern or the stock 6-lug pattern.

While it is probably possible to change the front to match the Ford rear, it may be easier to change that rear to match either of the readily available options on the front. Redrilling the rear axles and drums is often easier than redrilling the front hubs or disks.

Although if you are going to a Mustang II IFS that makes it simple to go Ford on both ends.