Can someone explain to me about automatic gear ratios?

Messages
917
Location
Singapore
code:
Transmission   
Type 4 speed electronically controlled automatic transmission (ECT) with lock-up torque converter 

Gear 1 Ratio 3.943 
Gear 2 Ratio 2.197 
Gear 3 Ratio 1.413 
Gear 4 Ratio 1.020 
Reverse Gear Ratio 3.145 
Final Drive Ratio 2.740 
Drive Type Front wheel drive 

1. Does the Final Drive Ratio mean the differential? Does it mean you turn the propeller shaft 2.74 times, the wheel turns 1 time? Or it is the other way around? 2. Does Gear 1 Ratio mean the engine turns 3.943 revolutions, the transmission output turns 1 time? 3. So if we put the tranmission and differential (Final Drive) together, Gear 1 Ratio of 3.943 and Final Drive Ratio of 2.74 means the Engine has to turn 3.942 x 2.74 = 10.8 times before the wheel turns 1 time? 4. So when the Engine is at 2000rpm in Gear 1 Ratio, the wheels are spinning at 2000/10.8 = 185rpm? So for say a 205/65/15 tire with a circumference of approx 2.03 meters, will be travelling at 185 x 2.03 = 376 meters per minute or 22kph. Is that correct?
 
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36,464
Location
ME
That all sounds correct. "Final drive" came out with transverse (front wheel drive) trannys. This ratio also includes the power transfer chain which might have a different number of teeth on the two sprockets. So you'll get numbers that aren't "old school" based on the number of available gear teeth. Many car mags report the effective ratio in highest gear... helps cut through the clutter of "double overdrive" transmissions in front of 1:4.3 gears. You might also find mph/1000RPM or revs/mile which is RPM at 60MPH.
 

Kestas

Staff member
Messages
13,959
Location
The Motor City
You will have just a bit of slip from the torque converter that may throw the numbers off. This would only be for the gears where the lockup feature doesn't apply. I'm not sure how lockup is configured with different transmissions. I believe some have lockup engaged only in the final gear, and others have lockup engaged in all gears but first. I'd be curious to know exactly how transmissions are set up.
 
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