Fummins

Had some time today and wanted to i.d my rear end ratio. Spinning the right rear tire one revolution showed the driveline rotating approximately 1.5 revolutions....after scratching my head for a while i realized the remembered the rear tires are 18" as installed by the p.o. Looks like i have a 1.5 rear ratio....no wonder the performance is lacking lol. Car is a 1969 Cutlass and came with 2 spd trans and 350 2 bbl carb with a.c. currently has TH350 with shift kit and 2,300 rpm stall. Can you say gear change? Lol.

MDchanic

If only one tire was rotating, the driveshaft will only turn half as far.

Try turning the driveshaft with both wheels off the ground, so that they can both turn.

You've probably got about a 3.08 (or maybe a 2.93).

- Eric

Fummins

Never knew that, thanks. Car was born a 350 2bbl with a/c im thinking its a 2.56

Chesrown 67 OAI

Its really tough to determine a ratio by turning the shaft/wheels on an open rearend. LOL- I think it takes about three people to do it -- one for each wheel and one for the shaft - so theres no movement in the side gears? Removing the cover is really the only "foolproof" method to be positive!!! Even if the tube code is ID checked it could still be wrong due to someone changing the gearset??

joe_padavano

Actually, it's pretty easy. Simply block one wheel so you are sure that only one is turning, then multiply the number by two. Also, since it it difficult to see the difference between, for example, 3.23 and 3.42 turns of the driveshaft, I rotate the tire ten times then divide the driveshaft turns by ten. It's very easy to see the difference between 32.3 and 34.2 turns.

Chesrown 67 OAI

Thanks Joe --- never knew that a x2 multiplier would work! Good to learn! Ive done that manuever but thought it wasnt gonna work --- didnt think of using the multiplier x2!
Love this place --- learn sumptin every day!!!

Magna86

Wow great info because I have the same thing happen to me last night on mine. I was blocking one wheel and the shaft only went around 1.5 times! `

joe_padavano

Yup. Effectively you've created a planetary gearset out of the differential gears, so there's an extra 2:1.

Fummins

Learn something new everday.....thanks fellas.