Hey folks Used the "jack up a wheel and rotate the tire 2 turns and count the driveshaft revolutions" method to check the rear ratio. Have an '86 buick rear in my rod. Coming up with a little more than 2.25 driveshaft revolutions. Just looking to figure out which speedo gear to get. Am I on the right track? That number seems really low. Thanks j
Is there a tag bolted to the cover on the rear with a couple of numbers on it? If so devide the large number by the small number and thats your ratio. Other wise you can pull the cover off and count the ring gear teeth and count the pinion gear teeth and divide the ring by the pinion?
No need to pull the cover if you marked the tire and yoke very good. If you are positive it is 2.25 turns, then find a site that gives all the ratios for the donor car the rear came from. There will be a very close match. but you forgot to say what type/make of trans. A hamber has posted a site that gives the chart for GM A/T trans speedo gears.
What you need to do is do what you have done but very accurately with the drive shaft turns count since they are low. Be sure you are happy with you accuracy and then double that number and that will give a darn close idea of the ratio. If you truely have 2.25 turns then X 2 = 4.5:1 ratio After that you will need to know your tire diameter etc.
The 2.25 turns was on 2 revolutions of the tire. Don't have to double then right? I am thinking I have a 2.29 to 1 gm rear. Have a 200r4 tranny. You bet on the diameter. Just trying to get to that point Thanks Jim
Look on the RH axle tube, usually on the front side, sometimes toward the top, for the production axle code stamping. It may take some scraping and wire brushing to reveal it but it's usually there. Mainly you're looking for a 3 letter code like "CKM" or similar. If you know the year, make and model that the axle is from, any GM dealer should be able to tell you the origunal factory ratio. Or sitemail me the info and I'll check it when I get to work.
Yes, mark the tire and floor, turn tire two turns exactly. (start to turn the tire before marking it, to remove any slop in the gears). The pinion turns will give the exact ratio without any math; then it's just matched to known gears for that rear end. If you think it could be a 2.29, then the pinion should have gone just a tick further than that last 1/4 turn after passing the 2nd full turn mark. How little or much money? Some guys do this: A gps to measure true road speed, at like 65, then look at the current speedo reading. If it is off say 10%, you can look on ebay under "speedometer adapters". These are OEM mini gearboxes with countless ratios listed in plus or minus pecent. You can usually get it perfect that way, without having to deal with your "biggger tires than stock charts" Don't have a gps? you can use highway mile markers and a watch with a second hand...., go 5 or 10 miles at what your speedo shows to be 60mph. Check the odometer error at the end of the 10th mile, to get the percent of error to look on ebay.
Now wait a second. If you turned the tire 2 full revs and the driveshaft turned 2.5 turns, you are very close to a 1:1 ratio .8:1 to be precise...this is not probable or possible. You might want to mark the driveshaft and the tire and try it again. You should be, at the very least, 3:1....just thinking that the numbers are not matching up here....
No it would be 2.5:1 if you turned one tire 2 times. Remember turning one tire twice is the same as turning two tires once
read somewhere online that this works: jack up one side of rear without one tire touching ground and other side touching. use jack stand for safety. put a big piece of tape on drive shaft facing you and one on tire at 9 o'clock on same side. while turning tire be at clear view of driveshaft. rotate tire 10 complete rotations while counting driveshaft rotations and stop at exactly 10 rotations of tire. if driveshaft rotates 30.5 times then you have a 3:50 rear gear ratio. worth a try
If you can see the driveshaft with the car on the ground, mark the shaft, roll the car one revolution of the tire and count how many times the pinion turns.If you can't, jack it up and have two people turn the rear tires together one revolution in the same direction and count the pinion. Unless it's a posi, then just turn tire one revolution just make sure the other tire is spinning at the same speed and direction.
No no, turn the wheel one full revolution and count how many times the drive shaft turns and then multiply the drive shaft turns by 2.
Why would you do that? Gear ratio is the amount of times the pinion turns to one revolution of tire.3.50 would be 3 and one half turns of the pinion to one turn of the ring gear.
Because you're turning one wheel, not both. Unless you've got a limited slip rear end, and then you're not gonna be able to turn either wheel with one jacked up. Just reread it, it's a lot easier just to turn the wheel 2 full turns and count the number of times the pinion turns. 2.29 with overdrive would be a little tall. Larry T
That's fine, and I'll bet you get close, but only if the axle gear has twice as many teeth as the spider gear. The correct way to measure is have both tires spinning at the same time,whether up in the air or down on the ground.
I think what you are actually comparing is the number of times one axle gear turns compared to the other axle gear. The spider gears just link them together. Since the spider gears are turning opposite of each other, I don't think the ratio between the spider gears and the side carrier gears matters. My head hurts. Larry T
The best was is to check the teeth and count them all, the other ways are; Read the tag and hope it is correct, if it has one at all. Jack up both wheels off the ground and chalk mark the ground and a corresponding mark on each tire and with a buddy rotate both wheels simaltaneously one whole turn, while doing this count how many turns the drive shaft does and that is your ratio. If you are by yourself then jack one wheel turn the tire one complete turn and again count the turns of the drive shaft and multiply it by 2 and you will get the same result. I sure know which way I'd be doing it, but it kinder depends on your circumstances.
No shit... 2 turns of a non posi rear tire will ALWAYS ACCURATELY tell the ratio. period. Anybody that needs to pull the cover at that point...well...
I just turn the tire ONCE. I make a chalk mark on the tire and leave something close to the ground under it so I get an accurate turn. I mark the driveshaft yoke in 4 places 90 degrees apart, best I can, and number it 1,2,3,4. I make a mark on the rear end housing. Agree with taking the backlash out before doing the rotation. Then I count the times the driveshaft goes around. That number is the gear ratio. Counting the 1/4 turns is where the accuracy comes in, those can be split into 1/8 turns if needed. Bob
Sounds like a few of you guys need to get off here and go back to math class. And you call yourselves Gearheads, Sheeesh. Sorry, haven't had my coffee this am.
Yep. I would love to help but I have to inventory the cows. Count the legs..divide by four...count the legs...divide by four. Hope the bull isn't humping one of the cows..that always throws me off count.
Last one I checked I turned the yoke one turn and counted how many times one drum turned. It was a little less than 3 1/2 times. Looked up online and figure it's a 3.41.
Agreed. First item of business for some of you would be to go here, and learn how a differential works. http://www.youtube.com/watch?v=K4JhruinbWc And what's the difference between spinning the wheel once and doubling the driveshaft revolutions, vs spinning the wheel twice and just counting the driveshaft revolutions, which represents the gear ratio? If you're just using eyeballs and not a degree wheel, the latter is going to be more accurate. Of course a posi/sure-grip/track-lock or whatever your preference is requires both wheels be jacked up, and both wheels spun one rotation. The number of driveshaft revolutions represents the gear ratio.
It's real basic shit like this that makes me wonder if half these guys built their rods. He did it correctly. Turn wheel twice, pinion revs = ratio.
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