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capev86 · 09-30-2011, 10:01 PM

my 72 burb c20 came with the standard 307 with 3 on the tree. i pulled the 2 barrel in favor a Q-jet. being a 3/4 ton, the rear gearing was 4.57. changed it out for 4.10 gears. the speedo would read 54mph when going 60. the original plastic speedo gear in the side of the trans had 25 teeth so i bought a new replacement the same size/style with 22 teeth from classic parts of america.

a trick i used to keep from losing fluid in the trans was to park my truck on a hill with the nose down and the right side lower than the left so the fluid would not seep out the speedo drive hole when i pulled the plug that surrounds the stalk on the speedo gear. after the swap my dad and i took it up on the highway and using a watch and the mile markers we determined that the speedometer was now spot on at 60mph.

my math:

4.10/4.57 = 0.897 or 90%

25 x .90 = 22.4

09-30-2011, 10:01 PM	#26
capev86 Registered User Join Date: May 2009 Location: Eliot, Maine Posts: 1,314	Re: New to site and suburban life. my 72 burb c20 came with the standard 307 with 3 on the tree. i pulled the 2 barrel in favor a Q-jet. being a 3/4 ton, the rear gearing was 4.57. changed it out for 4.10 gears. the speedo would read 54mph when going 60. the original plastic speedo gear in the side of the trans had 25 teeth so i bought a new replacement the same size/style with 22 teeth from classic parts of america. a trick i used to keep from losing fluid in the trans was to park my truck on a hill with the nose down and the right side lower than the left so the fluid would not seep out the speedo drive hole when i pulled the plug that surrounds the stalk on the speedo gear. after the swap my dad and i took it up on the highway and using a watch and the mile markers we determined that the speedometer was now spot on at 60mph. my math: 4.10/4.57 = 0.897 or 90% 25 x .90 = 22.4 Last edited by capev86; 10-10-2011 at 12:06 AM.