How to figure gear ratio

[bigtendude]

I know it's somewhere on this site, but don't know where. How do I figure gear ratio without taking things apart, what is the formula, something about marking driveshaft and one of the tires and rotating the tire and counting how many time the driveshaft turns? Can you please post this if at all possible? Thanks in advance........Larry

[bigtendude]

Found it, thanks anyway!

Oh,
I got the numbers off my rear axle tube finally, after I found out that they were only 3/16th. of an inch high. Was looking for huge applied ( raised ) numbers.......D'oh! Had to read them upside down with a mirror no less. What I can tell is that it's made by GM of Cananda, on the 6th. day of the year, first shift. Axle ratio is AD. whatever that is. Will have to search internet further. The code as I see it is: A D K - M 0 0 6 - 1

[Pontiac1976]

kool good show

[Pontiac1976]

[shadow=red,left,300]Here a pic that should help[/shadow]

[Pontiac1976]

GM RPO Descriptions - "G"

RPO FY LY DIVISIONAL RPO DESCRIPTION

GH2.. 76 -- 87 .. AXLE,RR 2.29 RATIO
GH3.. 86 -- 88.. AXLE,RR 2.77 RATIO
GH4 .. 76-- 81 .. AXLE,RR 2.28 RATIO
GL1 .. 76 -- 81.. DIFFERENTIAL CARRIER,RA AXLE,ALUM
GM1.. 87-- 87 .. AXLE,RR 2.59 RATIO
GM3 .. 85 -- 88 .. AXLE,RR 3.45 RATIO
GM7 .. 82-- 83 .. AXLE.RR 2.39 RATIO
GM8 .. 76-- 88 .. AXLE,RR 2.56 RATIO
GN3.. 76-- 81.. DRIVER CONTROL LEVELING
GQ1 ................ REAR AXLE - STANDARD
GS4.. 85 -- 86.. AXLE,RR 3.70 RATIO
GT4.. 76-- 88 .. AXLE,RR 3.73 RATIO
GT5.. 76 -- 88 .. AXLE,RR 4.10 RATIO
GT8.. 76 -- 88 .. AXLE,H.D. RR 4.10 RATIO
GU1 .. 76 -- 87 .. AXLE,RR 2.41 RATIO
GU2.. 76-- 88.. AXLE,RR 2.73 RATIO
GU3 .. 82 -- 85 .. AXLE.AR 2.93 RATIO
GU4.. 76 -- 88 ..AXLE,RR 3.08 RATIO
GU5 .. 76-- 88 .. AXLE,RR 3.23 RATIO
GU6.. 76 -- 88 .. AXLE,RR 3.42 RATIO
GV4.. 82 -- 87.. AXLE,RR 3.36 RATIO
GV8 .. 82 -- 85 .. AXLE,RR 2.72 RATIO
GW4 .. 84-- 85 ..AXLE,RR 3.31 RATIO
GW6.. 85 -- 88 .. AXLE,RR 3.27 RATIO
GW9 .. 76-- 88 .. AXLE,RR 2.93 RATIO
GX3.. 82 -- 88 .. AXLE,FRT 3.33 RATIO
GX4.. 76-- 83 .. AXLE,RR 3.75 RATIO
GX5 .. 76 -- 83.. AXLE,RR 4.09 RATIO
GY5 .. 76-- 87 .. AXLE,FRT 3.65 RATIO
G44 .. 85 -- 87.. AXLE,RR 3.07 RATIO
G50.. 76 -- 88.. SPRING,H.D. RR(VAR 1)
G51 .. 76-- 87.. SPRING,SPL H.D. RR(VAR 2)
G52.. 82-- 87.. SPRING,EXTRA CAPACITY RR
G60 .. 76 -- 88.. SPRING,AUX
G66 .. 76 -- 88.. ABSORBER,AIR BOOSTER RR SHK
G67.. 76 -- 77 .. ABSORBER,AUTO LVL CONT SHK SYS
G67 .. 78 -- 88 .. ABSORBER,ELEC LVL CONT SHK SYS
G72 .. 80 -- 80.. AXLE,RR 2.14 RATIO
G72 .. 81 -- 81 .. AXLE,RR 2.14 RATIO (EXPORT)
G72 .. 82-- 87 .. AXLE,RR 2.14 RATIO
G75.. 82 -- 87.. AXLE,RR 3.62 RATIO
G80 .. 76-- 88.. AXLE,RR LOCKING TYPE
G83.. 82-- 83 ..GEAR,RING 8.75 IN
G87 .. 76-- 87 .. GEAR,RING 8.50 IN
G88 .. 76-- 83 .. GEAR,RING 8,875 IN
G89.. 76 -- 87 .. GEAR,RING 7.50 IN
G92 .. 76-- 88 ..AXLE,PERFORMANCE RATIO
G95.. 85 -- 88 ..AXLE,ECONOMY RATIO
The code on the center housing

FY = first year. LY= last year.

[Pontiac1976]

See the gear to the left with the gear run up and down dead center is the #'s you may have to turn the wheels a bit get the #'s to face you .
I can send the pic to you then you zoom in on it and see it good.

the big long # is the rear axle case casting #

[bigtendude]

is the GM 71 number, the one I should be looking for? If so, how does it correspond to the numbers list, that you have listed? I see no 71 after GM code on yours, only either a one or a seven. I am talking about the code I see in the pic, it would be to the left side of iron housing.

[Pontiac1976]

The code is on the tube should be GT4 as the drawing the other places to see what you truck should have had new from factory
would be glove box or the inner fender or maybe top of air cleaner.
Should be blue white 4"x 6" sticker that sticker will tell you all the option your truck came with.

[bigtendude]

Well, since I had the money, I went and paid for a rear axle with unknown gears. I paid 100.00 for everything including rear leaf springs. Whole axle assembly. Will pull rear diff. cover in a couple of days, if I end up with another 3.73, I imagine I can sell for more than I paid for it. The numbers on axle tube are very weakly struck, so that's no help, I may put a little acid on them to see if they appear, if not, no big deal. I got a good deal. Only 50,000 miles on it, and it's from Texas, from another 1978 heavy half, with same gas caps on fleetside as mine. Can get really lucky and actually get the original 3.08. Thanks for all the input and info.....Larry

[bigtendude]

Well here goes........ I took cover off rear punkin today and found these #'s on the ring gear; 381/779, and 14:43 if I done this correctly divide smaller number ( 381 ) into ( 779 ) I get 2.04 something. If correct, will this work better than 2.56, also is this bigger than 3.08? There were some other almost invisible #'s on the ring gear '9/'2 ? Help!.....lol

[bigtendude]

oops..If I divide 14:43 number........ 43 divided by 14, I come up with 3.0714285 = 3.08 correct? Since using the other numbers gives me ratio that is unheard of, also my rear end gunk is dark red and smelly, no filings at all in there, is fluid supposed to be red, ( maybe factory )

[BowtieGuy]

Yes you ave a 3.08 gear ratio. As far as the red fluid sounds like you have a synthetic fluid in there. Hard to say with out seeing it first hand.

[bigtendude]

EUREKA!!! and only a hundred bucks............

[Pontiac1976]

Kool If you got the springs & the spring are good shape and the bushing that will make it easy swap. By changing them with the diff. If the fluid is synthetic fluid ever thing looks good I would change the seals put cover gasket on new u-joints. then it should be good for a few years for the seals & u-joint just add diff gear fluid after your good to go.

[bigtendude]

OOPS, already sold springs for 75.00, so rear axle only cost me 25.00. But mine are still in excellent shape. Also one more question, do I need to change out speedo gears, my speedo was correct with 3.73 in it? I have 350cui. with TH350 tranny, long bed fleetside.