Hello, Here is my first concern in replacing the rear end gear in my 66 GMC. The truck is a fully stock 1500 rated at 7500lbs "camper special package" therefore it has the following drivetrain components: Engine: 305E V6 rated at 157hp @ 4000PM Tranny: 4 speed manual SM-420 Diff: Dana 60 with 4.10 gears R. Tires: 7.00 - 15LT 8plys I want to change out the rearend gear to something better for the highway, but I want to calculate the mph/rpm ranges for each gear, therefore I need to know what 1st, 2,nd, and 3rd ratios are. Anyone have any suggestions on where I can find this, or maybe it is in a service manual someone may have? Thanks in advance David
David, The ratios for the SM420 are: 1 - 7.05:1, 2 - 3.57:1, 3- 1.7:1, 4 - 1:1 64K10 http://mywebpage.netscape.com/wck10s/index.html
