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Gear ratio decision, 4.1 4.3 or 4.56?

1338 Views 24 Replies 12 Participants Last post by  PHWOARchild
Currently a blast to drive but I want to try dial in the whole system even more, wondering of the 373 are just too mellow for what this combo needs? sometimes at part throttle acceleration it seems a little sluggish.. id like more instant snap if that makes sense..

Currently have 373, and its nice, but i wan to push the street factor the the limit - the 383 doesnt always have the brute low end of the big bocks, so some mods are necessary.

Cam is around 2000-6000 236° / 242°, 288° / 294°, 11:1 (installed straight up), 200/64cc alu heads, 5.7 rods. 850 dbl pumper brawler, Vic JR, 1-3/4 LT headers into 3 inch collector and 2.5 true duals. 373 posi, 2004R, 3k stall..

18-20 initial, vac adv disconnected .. Hei stock.
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Here's the formula to calculate engine RPM:

RPM=(MPH x Rear Ratio x 336 x Trans High Gear ratio) / (Tire Dia inches x Torque Conv Efficiency)

Your case:

Assume 70 MPH Speed
Rear Ratio 4.56:1
Trans OD ratio .70:1
Tire dia 28"
Torque converter efficiency 1 because lock up torque converter. Non lock up is about .9 or .95 efficiency.

Plugging in the numbers, your engine will be spinning 2681 RPM at 70 MPH

Obviously if you're not running a lock up torque converter and you're OD ratio is something other than .70:1 the engine RPM number will change.

Plugging in 55_327 numbers and assuming 28" dia tires, his engine was spinning 3830 RPM at 70 MPH. Big difference. He also probably had tires smaller than 28" which would make it scream even more. Let's assume a 25" tire. That would make his engine spin 4290 RPM at 70 MPH. Not a good highway gear at all.
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