Thread: what ratio do I have???

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I posted a question a couple of weeks back regarding a rear end gear ratio. Have a stock 56 rear end. Am getting ready to replace and wnated to know if anyone can help with the code numbers stamped on there or what would have come with it. Aprreciate any comebacks and Happy New Year!

Phil...

Phil, chock the front tires and jack the rear of the car up and put jackstands under the differential housing tubes so that one of your buddies can slide under the car safely. Have him make a chalk mark on the driveshaft and make a pointer to point to the chalk mark out of a clotheshanger. Put the other end of the clothes hanger under a brick to hold it in position.

Make a mark on one of the tires and do the same clothes hanger trick to mark the spot on the tire. Now, turn the tire slowly for 10 full turns while your friend under the car counts the number of complete turns of the driveshaft. Have him guesstimate the fraction of a turn that he has left when you reach 10 full turns.

Let's say that the driveshaft turned 37 3/4 times. Divide 37.75 by 10 and you get 3.775, which is probably a 3.73:1 rear.

Thanks Richard,

Questions, Does the tire size matter doing this? Have 15's.
And does it matter what the trans is?
The reason why I want to know is that I am ordering (finally) A new rear from Currie. I want 373's, so if I have a 290 or so this will make a nice outa the hole difference...I really dont know what a 210 came standard with. Phil...

This exercise has nothing to do with tire size or transmission. It only determines the final drive ring and pinion ratio. If you're using a Powerglide, TH350 or TH400, the transmission ratio in high gear is 1:1

If you're using an overdrive trans such as a 700R4, you will have to multiply the final drive in the trans times the final drive in the differential to get your overall ratio in high gear.

Phil and Richard, turning one tire will only work if you have a posi (limited slip) rear end.

If you don't have a posi, you have to turn both left and right side tires together.

Great, good thing I have two kids, cause now its a three man operation! Thanks two the both of you. I will let you know how the car runs with the new rear! Phil...

Originally posted by Nomad Mike
Phil and Richard, turning one tire will only work if you have a posi (limited slip) rear end.

If you don't have a posi, you have to turn both left and right side tires together.

Thanks for watching my back, Mike.

If you are only interested in a ball-park ratio, stick the car in neutral, roll it ahead one full tire rotaion while counting the number of revelutions the driveshaft makes. If the driveshaft rotates 3 and a quarter turns during the one tire rotation, you have 3.25 gears, or there abouts. This can be done in a few minutes with a friend.

Originally posted by TyphoonZR
If you are only interested in a ball-park ratio, stick the car in neutral, roll it ahead one full tire rotaion while counting the number of revelutions the driveshaft makes. If the driveshaft rotates 3 and a quarter turns during the one tire rotation, you have 3.25 gears, or there abouts. This can be done in a few minutes with a friend.

Well, there you go. It just gets easier and easier.

OK, so doing it the way typhoonZR said, in one full rotation of the tires (on the ground), the shaft spun 3 and about an inch which would in that theory be approx. 3:10 ratio, however, because both tires are on the ground and the car originally had 14's not 15 inch tires, wouldnt that put the rear gears at about 2:90? That would seem about correct, assuming that ratio was avail then.
So, if all this is true, and I go to 3:73's, this then will be a noticable difference off the line, True? And one more thing. If at 60mph Im tachin about 2400 rpms, what will rpm's go to at 60 with the new rear???

Phil...

Originally posted by fiftee6
OK, so doing it the way typhoonZR said, in one full rotation of the tires (on the ground), the shaft spun 3 and about an inch which would in that theory be approx. 3:10 ratio, however, because both tires are on the ground and the car originally had 14's not 15 inch tires, wouldnt that put the rear gears at about 2:90? That would seem about correct, assuming that ratio was avail then.
So, if all this is true, and I go to 3:73's, this then will be a noticable difference off the line, True? And one more thing. If at 60mph Im tachin about 2400 rpms, what will rpm's go to at 60 with the new rear???

Phil...

have fun!!
http://www.richmondgear.com/101032.html

Originally posted by fiftee6
OK, so doing it the way typhoonZR said, in one full rotation of the tires (on the ground), the shaft spun 3 and about an inch which would in that theory be approx. 3:10 ratio, however, because both tires are on the ground and the car originally had 14's not 15 inch tires, wouldnt that put the rear gears at about 2:90? That would seem about correct, assuming that ratio was avail then.
So, if all this is true, and I go to 3:73's, this then will be a noticable difference off the line, True? And one more thing. If at 60mph Im tachin about 2400 rpms, what will rpm's go to at 60 with the new rear???

Phil...

Hey 56;

You are close, however wheel and tire size have absolutely no bearing on determining gear ratios in the method given. One revolution of the axles is one revolution of the axles. So even if you have the wheels and drums off, if you turned the axles one rotation, it’s still all the same, even though the diameter of the axles are only about six inches, it just isn’t a factor..

I believe where you are getting confused is in this. If you install larger diameter tires, your effective gear ration will drop. What I mean to say is that the rpm of your motor will be less, given you are driving the exact same speed (not according to your speedometer) on the highway. So say you have 3.08 gears with a 24 inch tall tire, and then you install 26 inch tires, your rpm will drop just as if you had installed a taller gear, such as a set of 2.73. That is just a guess. I am sure, someone on this site might want to bother calculating the exact effective ratio, but all I am doing is giving you an example.

Likewise, say you have 3.08 gears with 24 inch tall tires and then went to a 22 inch tall tire, your effective ration would be steeper, as if you installed a higher numerically gear ration, or as they might say in automotive land, lower gears. I know, go figure.

So, 56, from what you stated, my guess is that your gear ration is 3.08, maybe 3.23. In any case, you now know that your gear ration needs to be changed to a numerically higher number, such as 3.73- 4.10. This will make a HUGE difference is your “seat-of-the-pants-o-meter.”

Your rpms would jump from 2400 rpm to about 3100. Just guessing

A quick way to figure what RPM change will result from a rear gear ratio change is this---a 4:1 rear in place of a 3:1, the engine revs 4000 at the same speed where it turned 3000 before. 3.08 to 4.11, means 4110 rpm instead of 3080 rpm. So you can half the numbers, in the first case, 2000 where it formerly turned 1500.

Thanks to all!

Cant wait to rip the treads off. Why won't winter just end already!!!

Made it up to 35 here today, feels like is spring is just around the corner !!!! NOT...