changing gears

Just bought a new stickshift car here in China. I’m having arguements with everyone that there is an efficient rpm/speed range for each gear. Just about every driver in China will shift up the gears as fast as possible. I am not exaggerating that you can be in third gear coming out of the driveway and in 4th doing 30 mph down the street. People tend to shift at about 2000 rpm and 3000 rpm max. They claim it’s the most fuel efficient and easiest on the engine.

I say hooey. What’s the straight dope?

I dunno.

My girlfriend (who is Chinese/Vietnamese - if there is a cultural connection) does this. Well… used to. Maybe not as extreme as the example you gave, but she’d be in 4th gear as soon as it hit 40km/h, and I could definitely hear the car protesting. The tail shaft would make this juddering sound, and we’d take forever to accelerate the last 20 km/h to the posted suburban speed limit.

I don’t consider mydelf an overly fast or aggressive driver, but I do like to keep the engine revs humming away nicely, and I’ve converted my girlfriend to my driving style.

Interestingly, the manuals which come with new cars tend to suggest “changing gear as early as possible” for fuel economy. I disagree with this, and in the average modern stick shift, in average driving conditions, I find an upshift about every 20km/h is a good compromise. It keeps the revs safely towards the middle of the 2000 - 3000rpm range, which is what “feels” comfortable to me.

Just noticed the RPM range in your OP. In my experience, at the road speeds you gave, the engine would be getting down a bit below 2000rpm at times. Thid was how my girlfriend drove. 2000 - 3000 seems easiest on the car. I’ll go above that only in harder driving conditions such as accelerating down a short freeway on-ramp to be able to safely merge at 100km/h (60mph), and on these occasions I’ll wind it out to the 3000-4000rpm range.

By going to higher gears earlier, you reduce some frictional losses in the geartrain. This is not a huge amount, but it is measureable over the long-term.

Every engine has a certain speed or range of speeds that it is designed to be most efficient at. This often corresponds to the typical rpm seen during highway cruising, for obvious reasons. However, like I said, all engines will have a different range, and it’s not easy to make a sweeping generalization about it.

In general, though, if an engine can be tuned properly, it has greater potential to be more efficient at lower engine speeds, due to reduced piston speed, and reduced friction and oil viscocity losses, and (often, but once again, this will vary) increased volumetric efficiency.

But if I had to be pinned down on what was most efficient, in a general sense, then yes, running the car at a lower rpm (so long as you are not lugging it) will give you the best fuel efficiency at any given speed.

I found this on a Nissan site: In general, the engine rpm values that produce maximum combustion efficiency and maximum torque are nearly the same.

That is combustion efficiency but is that really the most efficient on the car as a whole? What about wear and tear on the drive train or clutch? Does someone out there have a guide that suggests what rpm range should be the best time to shift.

Typo in the OP, people tend to shift between 1,500-2,000 rpm, and the cars are not quite bogging down or chugging but the acceleration factor is painfully slow.

There’s all kinds of efficiency:

Volumetric efficiency: how well you can get fuel into the cylinder and how well you can exhaust waste gases.

Combustion efficiency: How effectively you can burn the fuel that gets in.

Torque is strongly dependent on both of these. I would argue that, in modern cars, the greatest differential between cars is likely their volumetric efficiency, but I’m sure someone will disagree.

Thermal efficiency: How well you end up utilizing the heat input into the engine - turning the gasoline into useful work.

Mechanical efficiency: How well you convert the power resulting from all of the above into motive force.

I think you may not be seeing my point - you can only say specifically that one rpm or range is ideal for a specific type or types of engine. Otherwise, you can only make generalizations. Which are:

  • In general, lower engine speeds result in lower friction (increased mechanical efficiency)

  • In general, lower engine speeds result in higher combustion efficiency (more time to burn the fuel completely).

  • In general, lower engine speeds coupled with higher gear ratios results in less gearing loss due to friction.

  • In general, volumetric efficiency is easier to optimize for lower rpms.