Gear Ratio
 

I have a 72 C10 long bed with a 350 and auto transmission. I need to rebuild or replace my rear end and want to match existing gear ratio.

Is there any identifying marks on the rear housing or codes on the vehicle somewhere that could tell me what the gear ratio is without having to remove cover and count teeth?

Thanks for any help.

Re: Gear Ratio
 

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Check out this thread I posted on the subject:

http://67-72chevytrucks.com/vboard/s...d.php?t=578793

Look at posts #8 & #19 for possible code locations.

LockDoc

Re: Gear Ratio
 

If you do pull the cover, the ratio will be stamped into the gear. It will be something like 11/41. Divide the big number by the small number to get your ratio. Or use LockDoc's chart, much easier that way.

Re: Gear Ratio
 

Stamped numbers (as well as the SPID) will tell you what gear ratio was in the truck when new. Over the years, many trucks have had their gearsets changed out. If you want to be sure what's in there now, you'll still need to verify by counting teeth -- or at least counting driveshaft rotations.

Re: Gear Ratio
 

Seems like it takes me way longer to find and wire brush the stamped numbers and then find a chart that is 100% accurate then it does to just pull the cover.

Re: Gear Ratio
 

Using a tach should get you close, or jack up the rear and spin the tires one full revolution, count the number of driveshaft rotations. But you cannot be certain without pulling the rear cover. Probably easiest to use marking compound so you can keep track of which teeth you've counted.

Re: Gear Ratio
 

Bingo! ;) And hey, you get to inspect gear condition and freshen up the gear lube...

Re: Gear Ratio
 

you say you need to rebuild is needed? I honestly don't know how you'd do that without removing the coner.. why don't you dive in and get it done?

Re: Gear Ratio
 

just guessin here, but op must want new gears - thus the "want to match my existing gear ratio" comment - and would perhaps like to order them before rippin it all apart so as to do it in one ssession. i.e. a driver.

Having said that - why do you (tx72c10) suspect you need a rebuild (if you haven't inspected it yet)? Odds are that you won't actually need new r&p gears, but you never know. People usually only replace the r&p gears if they want to change the ratio or if they know what they currently have has grenaded (it would be obvious) after a 16 year old's burnout (don't ask).

Lockdoc's charts are great - but if you have no reason to believe the rear has never apart - then you can probably go with what is on the SPID. If you have nothing on the SPID, then you got the standard gear for your configuration (3.73). 3.07 and 4.11 were optional.

And btw, WELCOME to the board, glad to have you here. Show us some pics when you can.

Re: Gear Ratio
 

jack up the rear of the truck,it takes two people to do this..make a mark on both rear tires and the driveshaft.then both of you at the same time slowly rotate the tires forward til you make one revolution,while turning the tires,one of you need to count the number of revolutions of the driveshaft....for a 3:73 the driveshaft will rotate 3 3/4 times, a little more than 4 times for a 4:10 ratio and so on...

Re: Gear Ratio
 

By the numbers.

Re: Gear Ratio
 

Is the truck still driveable? If so, can you get the RPMs that you are turning when driving a set speed, such as 2300 @ 70 MPH. With this information the rear end gearing can be calculated. If the transmission is a TH350, then the output shoft on it is counted as a 1.0 % and if it is an OD tranny, then the output would be a 0.70%. If you have the tire diameter to include into the calculation, then you can complete the calc.

Rear end gearing = ( RPM x tire diameter ) divided by (MPH X Trans output X 336)

Re: Gear Ratio
 

I have been looking into this as well. I have a 3ott (1:1 final gear), a 3.08 gear and 23/75/15 tires. At 70 mph I am turning 2500 rpm. If I changed my tire size to 265/65/18, the new wheels and tires from a 2013-2015, I would drop below 2300 at 70. I would also lose a little takeoff power. Not a big drop for the price of the wheels. It's really hard to beat an overdrive with a 4th or 5th gear.