what is my gear ratio

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trudysduster

trudysduster

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can anyone tell me what the gear ratio is in a 69 Valiant 100 slant 6 from the factory. I am trying to figure out what speedo gear I need in it and this is one rearend I have not had to go in yet. When I am running 45MPH in my truck, the Valiant is showing 51MPH so I guess I need a gear with more teeth then to slow it down. Right. It has 165/80/13 tires on it so according to the chart it is about 24" in diameter. I am thinking a 30 tooth but need the ratio to make sure. This is just a plain granny grocery getter car. Thanks
 
It could be a 2.76. Jack it up and check it. Put stands under rear, mark the driveshaft and corresponding mark on the carrier, put a small mark on the right rear tire and one on the driveway/garage floor. Slowly spin the tire one full rotation while watching the marks on the driveshaft/carrier. Count the driveshaft revolutions, I'll bet it will be in the neighborhood of 2 and 3/4 turns.


What about the tires, are they the same size as original.
 
I do have it. I will check that and see if it is on there. If not, I will do what you said Bruce. I think it would be a 2.76 but didn't know what else they had in these.Thanks,Guys
 
where would I decode this. under the axle column, it is either a 1 or a 51. the tires has a 162 in that column. It just so happens that a spring mark goes right through the column of the axle and hard to read.
 
I never trust build sheets, axle tags or what. I ALWAYS turn a wheel around and count the driveshaft. These girls are just too old. Too much water between 'then and now."
 
I would argue it doesn't matter what gear set you have or don't have. You are right that you need a larger gear to slow the speedo down. I have used a rule of thumb of 3 mph per gear tooth for decades with good results. Since your trying to correct a 6 mph error, I'd remove what you have (the tooth count will be clearly marked on it), add two to it and buy that. (have a 30 tooth, buy a 32) This is not an exact science but a simple rule of thumb to get close(r).
 
Well, unless you take down the rear cover and read the ring gear, you may have a mystery on your hands. LOL, when was the last time you changed the rear gear oil????
 
Keep in mind, unless it is a sure grip, you will need one person to hold the opposite wheel still while you turn the other and count the drive shaft revolutions. Otherwise, it may be inaccurate.
 
Post a pic of the build sheet, I'll decide the axle code and Suregrip code as well as the tire code! If there is any character under the Suregrip box, it came equipped with one! The axle code will determine what ratio the car came with, but as mentioned that could've been changed out!

D51 in 69 denotes a 2:76 in a 7 1/4" rear!
T62 in 69 denotes a D70x14" tire
 
this car belonged to an old man . he bought it new. It has 69000 original miles on it. I doubt he messed with the gears.
 
well Geof, here it is. kinda hard to see but under the axle section you can barely see 551 for that code. let me know if you turn up anything. Thanks,Bill
 

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Forget the marks and counting while turning stuff. Use a long piece of masking tape to tape some string to the drive-shaft. Place the tape parallel to the length of the shaft with one end of the string under the end closest to the trans and the other end of the string laying on the ground. With both rear tires off the ground turn one tire a full revolution. Count how many full & partial times the string crosses over the tape. E.G. if it crosses the string twice and gets about 3/4 of a revolution then the ratio is about 2.75:1; if it crosses the string 4 times plus about 1/2 of turn then the ratio is likely 4.56:1
 
ntsqd said:
Forget the marks and counting while turning stuff. Use a long piece of masking tape to tape some string to the drive-shaft. Place the tape parallel to the length of the shaft with one end of the string under the end closest to the trans and the other end of the string laying on the ground. With both rear tires off the ground turn one tire a full revolution. Count how many full & partial times the string crosses over the tape. E.G. if it crosses the string twice and gets about 3/4 of a revolution then the ratio is about 2.75:1; if it crosses the string 4 times plus about 1/2 of turn then the ratio is likely 4.56:1
Click to expand...

I'm confused, the method you are describing sure sounds like "marking and counting while turning stuff".
 
No counting marks on the drive-shaft while also turning the tire and keeping track of how far you've turned it too. Was a suggestion of a shop owning friend of mine and I've it easier and more consistently accurate way to do this.
 
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