Originally posted by: exdeath
Originally posted by: fleabag
Yeah but traditionally automatics had shorter gearing than manuals
Originally posted by: Zenmervolt
The majority of automatics will lock the torque converter in the highest gear(s) at cruise, so there's no torque multiplication from the TC in most cases when an automatic is in its highest gear. Even if we assume that lockup is used as an ersatz "gear", the RPM difference at cruise between a fully open TC and a fully locked TC is typically only 200-300 RPM, not nearly enough to fully compensate for the differences the OP is noting.
ZV
Seems to be some confusion with the transmission gear ratios and final drive ratio at the differential.
I am referring to the final drive ratio and the torque multiplication that occurs while accelerating, not cruising.
Torque multiplication allows you to get away with a taller
final drive ratio, which means you still get acceptable acceleration in lower gears, while having a nice tall low RPM/high speed gear while cruising and with the TC locked.
A car with both an AT and MT option might have 4.xx final drive gearing on the MT and 3.xx on the AT. Both will have equal acceleration performance in lower gears due to the torque multiplication from the auto able to overcome the taller final gearing, but the AT will cruise at a lower RPM assuming the highest gear in both transmissions is say, 0.80 (OD for the auto and 5th on the 5 spd).
The torque converter is locked in higher gears when cruising. In all other conditions it is unlocked and multiplying torque, which allows for taller gear ratios while achieving the same performance. The torque converter acts like a gear reduction which temporarily and artificially shortens the taller gears when acceleration is needed. However when the torque converter is locked or otherwise close to 1:1, the taller ratios that the torque multiplication allowed to be acceptable for accelerating now also allow for low RPM cruising as well.
Example for the Gen 3 Camry (have the service manuals handy on flash drive w00t), both used on the 5S-FE 2.2L 130 HP / 145 LB-FT moving the same car:
A140E 4 speed automatic:
1st -- 2.810
2nd -- 1.549
3rd -- 1.000
4th/OD -- .706
Final Drive -- 2.296
S51 5 speed manual:
1st -- 3.538
2nd -- 1.960
3rd -- 1.250
4th -- .945
5th -- .731
Final Drive -- 3.153
We can clearly see that that the automatic will provide a much lower highway cruise RPM at a given speed when both MT and AT are in their highest gear. However it also looks like the price for that is poor acceleration looking at the first gear and final drive ratios... until you factor something I didn't list yet:
A140E 4 speed automatic:
Torque converter clutch stall torque ratio -- 2.0:1 (this is multiplicative with all other ratios)
Now it would appear that the auto both accelerates better AND has better fuel economy. This is where the losses and inefficiency of the torque converter while slipping and multiplying torque factor in and even it back out again.
Example, while
accelerating in 1st:
Auto: 2.0 (from torque multiplication) x 2.810 x 2.296 = 12.904 - (efficiency losses through slipping converter)
MT: 3.538 x 3.153 = 11.155
Accounting for converter losses and the fact that the 2.0:1 ratio is a perfect maximum that is neither constant nor always attained, bringing down the number for the AT a little bit, we can observe that AT is roughly equal to the MT when accelerating, if not slightly less.
Yet when the converter is locked up,
cruising in highest gear:
Auto: .706 x 2.296 = 1.621
MT: .731 x 3.153 = 2.305
Engine RPM is roughly 42% higher with the MT than the AT when cruising at the same speed; meaning the MT will be at 3550 RPM when the auto is doing the same highway speed at 2500 RPM. This is approximately what OP has observed.
Note however that without torque multiplication, a 2.296 final drive gear in the MT would bog the engine and yield poor acceleration in every gear, but is acceptable on the automatic because of the action of the torque converter.
It can also be seen that the AT would have better fuel mileage for someone who does mostly highway driving, while the MT would produce better mileage for someone who does mostly stop and go city driving, where the auto performs at it's worst.
*Edit: numbers are off, I misread/typed reverse gear for diff gear, correction in progress, my manual doesn't have the A140E final drive ratio. My numbers are off but the overall concept/outcome remains similar: the AT will have a taller gear (smaller number) than the MT.
