Gear ratios

I would like to know if anyone could help me with finding a useful formula for calculating final drive ratio? Meaning from transmission input to rear tires on the ground. How to calculate it.
Thanks for any help..
 
You need to know the teeth counts of each gear set that is involved with the power flow of the tractor and find ratios of them individually. For instance, 1. the constant mesh (if your brand is equipped with), 2. the ratio of each gear speed in the transmission, 3. the ratio of the ring and pinion 4. and then the ratio of the bull gears to the bull pinions or whatever final drive your machine is equipped with. 5. If your machine has some type of shift on the go device, ( torque amplifier, power director, etc) then the ratio of each side of it needs to be figured. Those are more difficult since a planetary assembly is generally used for the low side but high is usually one to one. Once you have all these ratios as the power is sent from the input shaft to the rear axle, you multilple each group together. That will give you the total reduction. Its not fast but it is not difficult once you have all the information. A parts book can sometimes be helpful with the teeth counts per gear set but some brands you have to have the lid off and count everything manually. Good luck, Jeff
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You did not state what the tractor is. If it is an antique tractor you might check out: The Antique Tractor Pull Guide Gearing by Zack Peterson (link_disallowed). As I recall the book gives the individual gear ratios and computes the overall ratio.

If you have the gear ratios, post back in terms of "driver and driven"...and we can do it here.
 
By adding the tire into the equation you will need to change to MPH. The final ratio is going to tell you how many revolutions of the engine it takes to acheive one turn of the axle. You can either measure the circumference of you tire or use the loaded radius to calculate the size of tire. To calculate the mph you need to take your engine RPMs times 60 (to get from minutes to hour), take that number and divide it by your final ratio and then times it by your tires circumference. That number will be how many inches your tire will travel in an hour. The last step is to divide that number by 63,360 which is how many inches are in a mile and your answer will be the MPH. Hope this helps. Jeff
 

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