Identifying a 3:91

[jimharvard]

well ... this is probably a dumb question .. but it's been some time since i "verified" a 3:91 rear gear (chunk.. punkin.. third member.. whatever is the popular term these days)

i wanted to install a 3:91 rear gear in my 68 340 S and bought a unit off of ebay. it was advertised as a 3:91. "years ago" we used to just turn the pinion and count the revolutions and if the pinion turned close to 4 times for one revolution of the ring gear - that was a 3:91. can you gear heads confirm this procedure or tell me a better was to identify a 3:91? i suspect that you might be able to confuse a 3:91 with a 3:55 by just "counting."

any tips would be appreciated.

jim coster, pittsburgh, pa

 

[fishy68]

That method works fine. There's quite a bit of difference between 3.55 and 3.91 gears so you should be able to tell the difference. 3.55 gears would be right at 3-1/2 turns and 3.91's would be nearly 4 full turns. A lot of ring gears have the ratio stamped right on them. You might also look the ring gear over good and see

 

[DFX 340 Duster]

If the unit is still a factory one then the part number, gear ratio, and date would be stamped on the outside surface of the ring gear. You might have to rotate the gears to see it. Otherwise it is the usual method that you are already familiar with.

Fishy was faster than I was with the info. LOL

 

[Rocky_JS]

Ring gear x pinion gear. example 41 tooth ring and 10 tooth pinion 41x10= 410 or 4:10.

 

[SGBARRACUDA]

Ring gear x pinion gear. example 41 tooth ring and 10 tooth pinion 41x10= 410 or 4:10.

That will work for coming up with the numbers but the way you figure gear ratio is the amount of teeth on the pinion divided into the amount on the ring gear. 41 divided by 10 equals 4.10.

 

[RustyRatRod]

That method works fine. There's quite a bit of difference between 3.55 and 3.91 gears so you should be able to tell the difference. 3.55 gears would be right at 3-1/2 turns and 3.91's would be nearly 4 full turns. A lot of ring gears have the ratio stamped right on them. You might also look the ring gear over good and see

This^^^^^^ Plain as day. A blind man could see the difference tween a .91 and a .55. You won't miss it.