why are gear ratios in MT lower than AT in same car/engine?

Hobbes

Why is it that the gear ratios in Manual Transmissions are lower than in automatic Transmissions for the same car with same engine? I've noticed this in my 99 Subaru Outback (MT). In sure this is the same case for many other manufacturers.

A lower gear ratio results in higher rpm's, and at highway cruising speeds, the engine roar can be annoying. I've noticed that the Subaru is pulling 3000rpms at 60mph while my friend with the same car but with Automatic is only pulling 2400rpms. The 600rpms make a difference.

Following up, traditionally MT are more fuel efficient than ATs. So if the car is cruising at the same speed but higher rpms, how does the MT car save fuel over its AT counterpart? Don't tell me cruising in Neutral towards stoplights makes THAT much difference.

shipo

You are missing half of the equation; you need to take in to account the gear ratio of your top gear as well, consider the following:

2002 BMW 330i 5-Speed: Final Drive: 2.93:1 – Top Gear: 1:1
2002 BMW 330i Automatic: Final Drive: 3.38:1 – Top Gear: 0.74:1

Fact: The lower the final drive ratio, the LOWER the RPMs will be at any given speed in any given gear.
Fact: The lower the gear ratio in the transmission, the LOWER the RPMs will be at any given speed in that gear.

Combine the two and you get your overall final drive ratio, so, while the 330i 5-Speed has the lower gear ratio in the Differential, it still turns about 3,100 RPMs at 80 MPH in top gear. Meanwhile, the same car with an automatic transmission is only turning about 2,650 RPMs to achieve the same 80 MPH. As you can see, simply having a lower gear ratio in the differential does not necessarily mean a slower turning engine.

I hope this helps.

Best Regards,
Shipo

leadfoot4

of the torque converter in the automatic transmission also figures into the equation. The manual transmission doesn't have the benefit of this.

lancerfixer

I've never considered having a torque converter to be much of a benefit.

:-)

Hobbes

Thanks for the further clarification guys. But its the engine noise that really bothers me. Is there an explanation as to why engineers/manufacturers "do it" so the AT cruises at a lower rpm than the MT version? So I guess my main concern is with the final drive ratio. 1.088:1 is much worse than the 0.694:1 for Subarus. But WHY is it like that? Come to think of it, I'll try e-mailing this to Subaru No. Amer.

I just really miss cruising at 1800rpm @ 60mph in my 5.2l V8 Grand Cherokee.

armtdm

With an auto and going up hills etc., the tranny downshifts without driver intervention. In the MT many drivers may wish to or like to downshift so the gears are set up to permit climbing most hills on turnpikes etc. in the highest gear without the need to downshift.

A thought