whats the correct procedure for approximating the rear end ratio by means of rotating the driveline and counting wheel revolutions. I have a 69 Z all original with the BU (3.73) rear end, and would like to verifiy the ratio....seems like its got tons of top-end speed. I would like to change out the ring/pinion to a 4.10 ratio as this seems like a better fit for the high reving 302 it has. any opinions? its a screamer the way it stands now but wonder if the 4.10 would make it easier when town cruising.

Put it on stands and spin the tire (with the tranny in N) 1 full revolution. Count the spins of the driveshaft and this will give you your ratio. I am assuming it is a POSI.

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Steve McCorry
'69 Camaro RS/SS-350 - Daytona Yellow
'95 Camaro Z-28 - Sebring Silver

[This message has been edited by mccorry (edited 01-06-2000).]

Didn't all Z-28's come with a 4:11 rear end?

I thought the standard Z-28 rear was a 3.73, non-posi.

The standard rear end for the 69 Z is coded BUXXXXGX (X's are for date codes) which is a 3.73 posi. However, factory installed several ratio's. Mine has the BU code on the housing, but after 30 years there could be a different ring/pinion, which is why I am checking the ratio. I dont think so, because the performance is real impressive with whats in it....thanks steve for your help.

Allen Carr MO.
1969 R/S Z28 documented original with 44k miles.

if there is posi on the car then spin the tire once and count the drive shaft turns. If there is no posi then you have to turn the tire twice for ratio. Good Luck

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[This message has been edited by 66 bowtie (edited 01-07-2000).]

Whether you have posi or not, (Limited slip was an option on Z/28's) the best method is to raise the car and rotate the driveshaft until the tire has completed one revolution. It will be easy to tell the difference between 3.73's and 4.11's. 4.11's will take almost a quarter more rotation than 3.73's. If you want to see if your gearset is original you will have to remove the differential cover and look on the side of the ring gear. It shows a part number and a date code. The part number for the 3.73 is 3862525. The date code is month and year only and should pre-date the build date of the car and the date code on the axle housing or at least be the same month. Hope this helps.

John

I also have a 69 Z with the BU coded axle. I had suspected that I had the 4.10 gears since the the dang thing revs so high on the Interstate. Anyways I verified the ratio by raising both rear wheels and counted the turns. Sure enough- one revolution of a rear wheel yielded approx. 3 3/4 turns of the drive shaft.
Mark

thanks, coloz28, will check it out soon. Monument hugh, lived there 12 years, back in Mo for good....like the low altitude lots better perf. !!

Which Muncie do you have? With 235-60-15 and 3.73 @ 70 mph your rpm will be 3500 and with 4.10 at 70 your rpm will be close about 3800-3900 rpm.
If you have a M20 with 4.10 your shifts will be sooner through 1-2-3 shifts than compared with 3.73 gears.

Mark