Manual Shifting and Gas Mileage Question

[Alessandro]

I have a 2005 Corolla, manual transmission. I was wondering, if at city speeds (around 2k rpm), does it really save gas to use 5th gear instead of 4th? 4th gives much better torque and thus a much more enjoyable ride, and I was wondering if at such low revs whether it makes a significant difference or not. Thanks!

 

[Uhtrinity]

The highest gear without lugging the engine. Lugging can be as bad as over revving on mpg.

 

[overst33r]

Yes it does save gas. How much and if it's worth it compared to the improvement in torque when you keep it in 4th is up to you. In other words, 4th might have better bang for the buck, but 5th is still cheaper.

 

[Zenmervolt]

5th gear at 25-35 mph in a Carolla strikes me as a bad idea because it would almost guarantee that you would be lugging the engine, which may save a little fuel but is incredibly hard on the engine.

ZV

 

[Jahee]

Originally posted by: Zenmervolt
5th gear at 25-35 mph in a Carolla strikes me as a bad idea because it would almost guarantee that you would be lugging the engine, which may save a little fuel but is incredibly hard on the engine.

ZV

Why so?

 

[fstime]

probably because there is not enough torque, its not like a ZR1 or Viper that is still pushing over 300 ft/ lbs at 2,000 rpm.

 

[mooseracing]

Originally posted by: Jahee

Originally posted by: Zenmervolt
5th gear at 25-35 mph in a Carolla strikes me as a bad idea because it would almost guarantee that you would be lugging the engine, which may save a little fuel but is incredibly hard on the engine.

ZV

Why so?

Lugging kills the crank and rod bearings if happening all the time. With a low tq engine you can get away at about 1k rpm but I wouldn't go lower than that. It has to do with the combustion and compression forces and the low rpm doesn't help sustain substantial lube as well.

 

[fuzzybabybunny]

Originally posted by: mooseracing

Originally posted by: Jahee

Originally posted by: Zenmervolt
5th gear at 25-35 mph in a Carolla strikes me as a bad idea because it would almost guarantee that you would be lugging the engine, which may save a little fuel but is incredibly hard on the engine.

ZV

Why so?

Lugging kills the crank and rod bearings if happening all the time. With a low tq engine you can get away at about 1k rpm but I wouldn't go lower than that. It has to do with the combustion and compression forces and the low rpm doesn't help sustain substantial lube as well.

If the Toyota Corolla's gearing and mileage characteristics are anything like my Echo's, 5th at 25-35 is indeed a bad idea. Will definitely decrease mileage. Done it before when I was younger and didn't know any better. Now, if I'm cruising at those speeds I'm actually in 3rd gear. I switch to 4th when I hit 40mph. My engine generally gets the best mileage revving @ 2-3K RPM (higher revs but easier spinning vs. lower revs but harder spinning).

 

[lurk3r]

Dropping your RPM by 500 or so will save you some gas, but those little 4 cylinders don't like to run under 2k, the savings would be minimal, unless you are running for a long stretch at a constant speed (over a mile). The operator's manual will have optimal shift points for mpg, you should not be shifting into a higher gear before what your manual says.

 

[Riverhound777]

In my Mazda 3 at 25-35 cruising on level ground I am in 4th. 40+ on level ground I put it in 5th.

 

[amicold]

I'd say barely touching the gas in fourth is better on mileage than quarter of the way in in fifth gear.

 

[Zenmervolt]

Originally posted by: amicold
I'd say barely touching the gas in fourth is better on mileage than quarter of the way in in fifth gear.

Actually, that's wrong. RPM has much more effect on mileage than throttle opening. An engine is vastly more efficient at low RPM with a heavy load (large throttle opening) than it is at low RPM with a light load (small throttle opening).

The issue with running the engine in 5th at 35 mph is that it will be lugging the engine. Lugging the engine is hard on it because it stresses the crank and rod bearings due to high bearing pressures and lower oil flow (oil pressure increases with RPM, so low RPM plus high load also means lower oil pressure and high load), it also increases blow-by (even with 100% pistons and rings) which dilutes oil faster and requires you to change oil more often. In other words, the small amount you would save in fuel is offset by increased maintenance costs.

ZV