i have 1971 fastback originally a 351 2v. I was wondering what rear end and gear ratio it has. It is totally encased so i can't even see bolts :/ I know its stock though.

 

Its a 8 inch the 9 inch's where in big block cars and 4 barrel cars.Is your door tag still there if so give me the code under axle.

 

some 2v cleve got 9 inch rears and even 302 cars....

i had a 73 vert that was 9 inch with a 302 and a 73 mach 1 with 351c with a 9 inch

 

as andrew said if it's the original the door tag will tell the ratio. otherwise lift the back of the car and turn one wheel until the driveshaft took a full turn. if it's open rear end you have to double the value

 

unfortunately the door tag is no longer there :/

 

You may also have a tag on the axle. From the factory, there was usually a metal tag affixed to one of the pumpkin bolts that told the ratio. My book is showing 14 possible axle ratio's for '71-'73 from 2.75 to 4.11, but for a '71 351 2V it only lists the following ratios:
2.75
3.00
3.25
3.50

 

you can also do a VERY rough way of getting the ratio. jack the car up, put the rear end on jack stands and put the car in neutral. one of the tires slowly ONCE and count how many times the driveshaft turns

 

Both 2V Mustangs I have owned had 9" rears in them. The POD is a 2v and it has a 3.50 geared 9".

I jack up the car, put it on stands, mark the driveshaft and count the revolutions of it as I rotate the tire. I use the valve stem as the marker, start with it on the bottom and take it around one time. Its much easier than getting confused about the spider gears ratio and having one tire on the ground. Besides if it has a locking type differential it would take some effort to turn the one tire, I have a couple where you would need a pretty long bar to break them loose.

 

The easiest way to tell a 8 inch or 9 inch is try putting a socket on the lowest bolt on the 3rd member.If the socket fits its a 8 inch if it don't its a 9 inch.

 

My 72 came with the 351c 2v and AC, It had the 9" non posi came with the 2.73 ratio.