selecting a transmission first gear ratio

[grumpyvette]

"HEY GRUMPYVETTE?
I have a Hurst Olds 3.73 rear gear, with a factory manual trans. clutch linkage. What would be the best first gear (say a 3.27 or a 2.87) in a 5-speed overdrive trans. The truck won't see the track, but will see some highway (could see a 100+ mile from time to time) trips. I'm guessing about a 275/60/15 tire.
Two transmissions I'm looking at have gears of:

3.27, 1.97, 1.34, 1.00, and 0.68
2.87, 1.89, 1.28, 1.00, and 0.64

With the 3.73 rear gears, I'm leaning toward the 2.87 first gear.
Thanks!"

http://www.jekylhyderacing.com/HeightofTires.htm

http://vexer.com/automotive-tools/speed-rpm-calculator

most experienced engine builders and transmission shops will tell you that you should always try to get the first gear ratio multiplied by the rear gear ratio to fall in the 10:1-10.5:1 range with tires shorter than about 27" and between 10.5:1-11:1 ratio with tires taller than about 27"
your 275/60/15 tires are about 27.7" tall

3.27 x 3.73=12.2:1 (not going to work well)
2.87 x 3.73=10.7:1 (much closer to ideal)

 

[grumpyvette]

"I have always heard 12-13:1 for final drive in 1st

hell Im @ 2.66*2.41 which is 6.4:1 and a 26" tire

that might work for cruising in a car used mostly on the high way , but Id sure hate to use that gearing in a fairly heavy car in the mountains, or towing a trailer or racing the car because its just going to SUCK compared to the correct gearing range, (10.1-10.25:1 depending on tire diam.)
with a 2.41 rear gear and 26" tires, your now pulling only about 2200rpm at 70 mph, and that may be nice , but the difference between lets say 1150 or so foot pounds of torque and applying a about 1900ft lbs on a steep hill due to gearing in a average 3500 lb car with a mild 350 that probably puts out less than 180 ft lbs of tq at 1800rpms is going to be noticeable, thats why they invented OVER DRIVE ratio transmissions, you get both decent off the mark acceleration AND the low cruise engine rpms

READ THRU THIS THREAD AND SUB LINKED INFO
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