Rear-end ratio formula?

[slategrove]

I am trying to figure out what speedometer gearing I need to get my speedometer to read correctly...I think it's 15-20 mph faster than I'm really going. I've done the searches on here, and read a lot of posts about this, but I need someone to tell me if there is a formula to figure out my rear end gearing?

For 10 revolutions of the drive shaft, I get between 5.25 and 5.5 revolutions of my tires. The two ways I could think to do this math straight up give me either a 1.90 rear end or a .525 rear end...neither of which I believe to be anywhere near correct.

PLEASE Help

[BADPURPLE70CHEVY]

Originally Posted by slategrove

I am trying to figure out what speedometer gearing I need to get my speedometer to read correctly...I think it's 15-20 mph faster than I'm really going. I've done the searches on here, and read a lot of posts about this, but I need someone to tell me if there is a formula to figure out my rear end gearing?

For 10 revolutions of the drive shaft, I get between 5.25 and 5.5 revolutions of my tires. The two ways I could think to do this math straight up give me either a 1.90 rear end or a .525 rear end...neither of which I believe to be anywhere near correct.

PLEASE Help

Do you have a way to remove the cover? If so it should be stamped on the gear. You can always count ring gear and pinion gear teeth and divide if you have the rear cover off.

[leddzepp]

Rotate your driveshaft once and count the number of times your tire makes a complete revolution. This will give you the gear ratio. I did this on mine, and my tires turned 3 3/4 turns to one turn of the driveshaft, leading me to the conclusion I have 3.73 gears.

[Rooster's 67]

Uh that would be near a 2.0 gear ratio, not too realistic, but not impossible. Was only one wheel spinning? You need to raise both wheels off the ground, and they must spin in the same direction at the same speed to get an accurate count.

use white-out or shoe polish to make a mark on the tires/shaft for reference.

Or jack up one wheel and count the number of revolutions of the tire in the air for one revolution of the driveshaft.

If you have both wheels in the air and can get an accurate count (posi diff or spool).

Divide the number of shaft revolutions into the wheel revs for Ratio

if only one tire in air(can only do with an open carrier, or non-posi):

Divide number of tire revs by 2
then divide shaft revolutions into that figure

For instance, one tire in air, 2 driveshaft revolution & approx. 18 revs of the tire in air.

18/2=9
9/2= 4.5 or 1:4.5 or most likely a 4.56 gear

Hope this helps.

[Rooster's 67]

divide the ring gear tooth count by pinion tooth count, or Tire revs by shaft revs.

without pulling cover and counting teeth, see my post above.

[BADPURPLE70CHEVY]

If you have never had the rear end cover off this is a good time to give oyur rear end some fresh gear oil as well as count teeth to be sure. Divide ring gear teeth by pinion gear teeth. example 37 ring gear teeth divided by 9 pinion gear teeth is a ratio of 4.11

[slategrove]

The first time, I only had one tire off the ground, which I understand from the above posts is wrong with a posi rearend...

tried it again, both tires off the ground and turning the tire (both turning same direction at same speed) while watching the driveshaft revolutiuons...10 driveshaft revs = 2.6 or 2.7 tire revs

Im wondering if turning the drive shaft would produce the same results in opposite (10 wheel revs & 2.6ish driveshaft revs)???? Because above posts say 2 revs of driveshaft and 9 revs of tires

Am I correct in thinking that my rear end ration is 3.73?

[lakeroadster]

The method for a posi -vs- an open differential is different....

To check rear end ratio jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

[C-10 simplex]

i went thru this fiasco myself, and, in my usual anal style, i made it real complicated, drawn-out, but thourough:

http://forum.73-87chevytrucks.com/sm...p?topic=3785.0


1) Is there a sticker in your glove box or inner fenderwell---it should state what gear ration you got.

2) you can also go to your local gm dealer(preferrably chevy) and they can type in your vin# and find out what gear ratio you got---ASSuming the gear is still stock. From this, they have charts to match up whichever speedo gears go with your combo----don't forget tire size. In other words, which tires came stock on your truck and what size do you got now? This is what screwed me up.

3) i think the easiest(but not nessesarily cheapest) way is to get a GPS speedometer.