I'm so confused on my rear end ratio!

[mrtleavitt]

Okay, the more and more I research my rear end ratio with ratio caltulators the more I confuse myself and doubt what I have in there. I don't think the original ratio is in it so the tag on the door doesn't do me much good. I have a '69 F100 with a 390 and 4 speed NP435. I run 3000 RPMs at 70 mph. What ratio would that be?

[FORDification]

We'd need the tire diameter to calculate that, but it would still only give you an estimated ratio. If the ID tag is missing off the pumpkin, the only way of determining for sure what you've got is just to remove the pumpkin from your 9" and count the teeth.

[fordman]

arent the number stamped on the ring also. or is that brandx that does that?

[FORDification]

The number of teeth are indeed stamped on the ring gear and the pinion gear. It'll look something like this:



In this case, you'd simply divide the number of ring gear teeth (41) by the number of pinion gear teeth (10) to get the ratio....in this case, it would be 4.10:1.

[mrtleavitt]

I forgotto add, I run 31 inch tires

[theskytoucher]

im thinking you have 3.89 in the fancy little calculator thing on the web it says 3.95 but that could just be a little off on rpm 50 rpm lower makes the ratio 3.85!!

[basketcase0302]

Isn't it much easier to just jack up the rear end and spin the wheel one turn?
Count the driveshaft revolutions. Not exact science but always been close enough for me to tell what ratio.

Basketcase

[heep70]

You guys are doing to much brain work

[Darrman]

LOl, I went through this mess when I put 4:10's in the Explorer, very confusing. My '69 decodes a 3:55 open rear end, but given that everything else has been modified, there is no telling whats back there. I will find out pretty soon tho!

[71PA_Highboy]

It is somewhere between 3.55 and 4.11.

The reasons we can't be more specific are:
We are assuming the speedo is correct.
We are assuming the Tach is digital and exactly 3000 RPM
We are assuming the tires are an exact 31" dia while loaded and on the ground (33's fit that profile pretty well).

If you take all the variables as exact then I think 3.89 is pretty darn close...

Sorry to not be of much help,

eric

[Thunderfoot]

Isn't it much easier to just jack up the rear end and spin the wheel one turn?
Count the driveshaft revolutions. Not exact science but always been close enough for me to tell what ratio.

Basketcase

You guys are doing to much brain work

Yeah, that's the way I do the easy check...

Just mark the location of the drive shaft or pinion and the tire on the ground, then roll the truck straight forward rotating the tire one revolution and count the turns of the drive shaft / pinion... 3 1/2 turns = 3.50 gears, just a pit over 4 turns = 4.10... Tire size makes no differance doing it this way.

[19674x4]

sounds about like 4:10's, i run 2500 rpms at 55 with my 4.10's so 3000 at 70 would be about right