Please look at my rear end...

err, differential I mean.   As my H code 351c has 4v heads, 4 bbl intake and who knows what else I am now looking at the rear.   I have read (on the internet so it must be true!1) that the easiest way to tell the difference between an 8" rear and a 9" is that the two lower bolts, at 6 o'clock and 7 o'clock need to be removed with a wrench as a socket head will not fit straight on.  if that is true I think I have a 9" rear?????  I know that I have posi as I leave 2 foot prints when I punch her and while the Marti report shows a standard 2.75 it feels much more to me then that gear wise.  I assume a 9" is more desirable?

Yes, that's a 9". The 351-2v and better cars came factory equipped with a 9". 302-2v and 250-6 had an 8". 

thanks, so that part is what the car should have been born with then.  Still have a posi in place of the peg leg so someone has been in my rear.....

Yes, it came with a conventional (non-locking) rear end. The code for a locking axle would have been 'K', not '2'.

A quick and dirty way to find out what ratio you have, is to lift the back end and turn the wheels and count the revolutions of the driveshaft, after marking it of course. This is approx of course. i.e. if it is 3.50:1 the d/s should rotate 3 1/2 turns to one of the rear wheel.

By the way a "possi" is a Chevy term. Ford term is a Traction-Loc (there was an earlier name for it but the old memory isn't working too well.) OR if it is a Detroit Locker, then it will be like the axle is solid, as for drag racing. I'm sure you'll get a better description from others.

Another way to tell a 9" diff, is the bump on the case is off-set.

Stanglover said:

A quick and dirty way to find out what ratio you have, is to lift the back end and turn the wheels and count the revolutions of the driveshaft, after marking it of course. This is approx of course. i.e. if it is 3.50:1 the d/s should rotate 3 1/2 turns to one of the rear wheel.

Click to expand...

well I did this it it was pretty much 3 exact turns of the driveshaft to one of the wheel, this was with a 15" wheel and the car was born with a 14" so probably the stock 2.75 it came with?  If  so kind of disappointed, but on the other hand it will cruise better??

The axle ratio is independent of tire diameter. 3.00:1 is a fairly common ratio for a 9 inch. but so is 2.89 and 3.10.

Higher numerically means better acceleration, lower numerically means lower cruise RPM. But you probably knew that...

jscott said:

well I did this it it was pretty much 3 exact turns of the driveshaft to one of the wheel, this was with a 15" wheel and the car was born with a 14" so probably the stock 2.75 it came with?  If  so kind of disappointed, but on the other hand it will cruise better??

Click to expand...

The diameter of the tire / wheel is irrelevant as far as gear ratio.  You probably have 3.00 gears.  It isn’t too difficult to swap a third member in a 9”- so don’t be disappointed.  Save up a slush fund and get a third member set up with a new ring and pinion of your choice along with some sort of traction enhancing differential.  There is nothing wrong with 3.00 gears though...

Is there still an axle tag on it? If so, can you picture it or add exactly what it reads. I've got a feeling someone may have swapped the axle, but as long as it's the correct one, i.e. Mustang, no harm done. 

no axel tag I can find, but then again I did not look hard.  No take on the diff

jscott said:

no axel tag I can find, but then again I did not look hard.  No take on the diff

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Good chance it's been swapped already or at least been out at some time. It seems thing like tags are just tossed and not returned. 

If you do find it, it might be for what's in there now or before the gear swap.

..........well you said "please look at my rear end".........