Easiest way to know gear ratio?!!!
 

What is the easiest way to know the gear ratio of my truck. '72 C-20 402 bbc th400. I want to change them if they are not what will give me best performance.

Please help me do this all over the place with my newly rebuilt Edelbrock throuhgout 402 :burnout:

What sort of rear end do you have? If its a C20 you have either a dana 60 or an eaton. 90% of Danas are 4.10's but there should be a tag on the rear that says the ratio and gear count. A big block auto may have 3.54's. If its an eaton you'll have to have someone turn the wheel while you count driveshaft revolutions. Remember to chock one wheel if its a open rear ( non posi ) If its an open rear multiply the driveshaft revs by 2 to get your gear ratio. If its a posi or locker than just count revs. Its more precise if you do ten wheel revs. make sure to mark the driveshaft with chalk to make it easier to keep track of revolutions.

If it is the dana 60 that nobody has messed with, it should have a tag bolted to one of the bolts on the inspection cover that will tell you what the ratio is. Most trucks with your combination came out with 3.54 Dana's in them, but that is not always true. Especially with the custom ordered trucks. If it is the eaton rear end (best identified by the round inspection cover and the drop out front gear set), there will be a set of numbers stamped in the front housing on the top cooling fin on the right hand side of the gear housing. If you have the first three letters I can get you the ratio by looking in my GM parts catolog. Then last but not least, if you have a possi, or track lock rear end the ratio should be posted on the glove box option sheet. WES www.ClassicHeartbeat.com

If you want to figure out a ballpark number, just mark the pinion & turn the tire & count the # of turns. If you want to be exact pull the cover & get the #'s. Your truck could probably use the gear oil change anyway.

I vote on pulling cover and get numbers off of gears, and treating rear end to new oil.

I second the oil chainge. And if you can't find the numbers on the gears, counting teeth is pretty easy.

Thanks for all of the great help. This board is the best. You guys are all great. I will do it the "hard" way and pull the cover. I don't know what weight to put in... 80w-90????

80/90 is good.