Read as much as possible b4 you spend the $$$ to change anything on grand-paw's truck.
Do a search -
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi AS mentioned in this reference is for the T5. But you could go to 'smokemup'
http://www.smokemup.com/auto_math/mph_range2.php and input your 4 speed muncie and see your outcome. I'll bet the 3.08:1 would be just fine. Cause you really only have three choices - 3.08, 3.40, 3.73 besides your 4.11 and your 4 speed starts out in 2nd anyway... and you live in a fairly flat environ (vs. california)
you are better off just rebuilding your 12 bolt. Many of us only have non-rebuildable tube rear ends, always worth replacing them.
http://www.stovebolt.com/bboard/cgi-bin//ultimatebb.cgi?ubb=get_topic;f=2;t=010333;p=1#000000
Borg Warner T5 Transmission and Engine RPM VS. SPEED RATIOS, 3.08~3.43~3.73~4.11:1 REAR END RATIOS.
Speed vs. RPM For Tire and Gear Ratio
A real accurate way to determine your tire diameter is to draw a line on the bottom of your tire, then draw a line on the ground which lines up with the line you just drew on your tire. Then move the car until the car makes one full rotation of the tire and mark the ground where the line on the tire is at the bottom again. Now measure the length between the two lines and divide by pie (3.14) and you'll have your tire diameter to use on the calculator (don't forget to use inches). This is the preffered method to use in determining your actual tire diameter. Tire sized 28.5'' to a typical modern radial Chevy standard size rim. If yours is taller then add MPH.
The columns are a bit off but they really do line up properly. My spreedsheet did not line up as I thought. This is purely for your information. Accurate to +/- 4%.
Enjoy
CHEVROLET T-5 Borg Warner Transmission Ratios:
1st: 2.95 2nd: 1.94 3rd: 1.34 4th: 1 5th: 0.74
Tire Diameter: 28.5 inches
Rear Gear Ratio: 3.08:1
READ MPH ACROSS ROW /READ RPM DOWN the FIRST COLUMN, then LINE UP THE SPEED IN EA. CORRESPONDING CROSS COLUMN.
RPM 1st 2nd 3rd 4th 5th 6th
1000 9 14 20 26 36 0
1500 13 20 30 40 54 0
2000 18 27 39 53 71 0
2500 22 34 49 66 89 0
3000 27 41 59 79 107 0
3500 31 48 69 92 125 0
4000 36 54 79 106 143 0
