3.08 or 3.73?
Thread Starter
1972 U code Supreme
Joined: Jul 2018
Posts: 4,099
From: Chesapeake, VA
3.08 or 3.73?
Greetings to all ye Oldsmobilers!
I had a long conversation earlier this week with the son of a PO about my '72 Supreme. We shared some stories and he was able to give me some history on the car.
He said that his dad swapped out the rear gears from 3.08 to 3.73, but when I first bought it, I did the old school check of driveshaft turns to tire turns, and came up with 3.08.
I have the original U code 455 backed by the original Turbo 400. The engine has a W-30 cam and is .030 over. Tranny has a 2800 stall converter and a shift kit. Rear tires are 275/60/15.
At 65 on the highway, I'm turning about 2600 rpm and about 2750 at 70 mph. My question is, does that jive with 3.08 or 3.73?
I had a long conversation earlier this week with the son of a PO about my '72 Supreme. We shared some stories and he was able to give me some history on the car.
He said that his dad swapped out the rear gears from 3.08 to 3.73, but when I first bought it, I did the old school check of driveshaft turns to tire turns, and came up with 3.08.
I have the original U code 455 backed by the original Turbo 400. The engine has a W-30 cam and is .030 over. Tranny has a 2800 stall converter and a shift kit. Rear tires are 275/60/15.
At 65 on the highway, I'm turning about 2600 rpm and about 2750 at 70 mph. My question is, does that jive with 3.08 or 3.73?
Registered User
Joined: Feb 2010
Posts: 1,585
From: Modesto CA
https://spicerparts.com/calculators/...rpm-calculator
According to this, 28 inch tires with a 1:1 third gear @ 2750 RPM 3.23 gears will have you at 70. 3.08 would have you closer to 75 and 3.73 would have you around 60. Although if he did swap gears and your non stock tire size would mean your speedo would be off unless he changed the drive and driven gears.
According to this, 28 inch tires with a 1:1 third gear @ 2750 RPM 3.23 gears will have you at 70. 3.08 would have you closer to 75 and 3.73 would have you around 60. Although if he did swap gears and your non stock tire size would mean your speedo would be off unless he changed the drive and driven gears.
Thread Starter
1972 U code Supreme
Joined: Jul 2018
Posts: 4,099
From: Chesapeake, VA
https://spicerparts.com/calculators/...rpm-calculator
According to this, 28 inch tires with a 1:1 third gear @ 2750 RPM 3.23 gears will have you at 70. 3.08 would have you closer to 75 and 3.73 would have you around 60. Although if he did swap gears and your non stock tire size would mean your speedo would be off unless he changed the drive and driven gears.
According to this, 28 inch tires with a 1:1 third gear @ 2750 RPM 3.23 gears will have you at 70. 3.08 would have you closer to 75 and 3.73 would have you around 60. Although if he did swap gears and your non stock tire size would mean your speedo would be off unless he changed the drive and driven gears.
Registered User
Joined: Aug 2019
Posts: 2,088
I'm coming up with a effective gear ratio of 3.45 for your combo. http://www.wallaceracing.com/calc-gear-tire-rpm-mph.php
Factory tire for your exact ride is a G78 14 which is 27.10 inches tall. https://www.performanceplustire.com/...14:ty:Antique/
Your current tire height is 28.10 inches tall so as has been mentioned that is throwing your speedo off. But I will add that my speedo is around 10 mph off at highway speeds just from old age and wear and tear...
Factory tire for your exact ride is a G78 14 which is 27.10 inches tall. https://www.performanceplustire.com/...14:ty:Antique/
Your current tire height is 28.10 inches tall so as has been mentioned that is throwing your speedo off. But I will add that my speedo is around 10 mph off at highway speeds just from old age and wear and tear...
Last edited by 69CSHC; Oct 12, 2019 at 03:16 PM. Reason: spelling
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