When to shift

My husband and I are at odds of when to shift our manual 2009 Chevy Aveo5. He is in 3rd gear by 20 mph, and I think that is going to ruin the engine. He thinks he is saving the life of it by not taxing it too much. Who is right?

Your husband is probably shifting too soon. It’s doing more harm to the clutch than anything. Your husband is wrong. Most driver’s have a “feel” for shifting as every vehicle is different. If he is in high gear by 35 or 40mph he is shifting too soon. Small vehicles have small engines so higher rpm’s are necessary. I think that’s what bothers him.

See the place on the tachometer where the numbers/little lines turn red? Shift before you get there. As for a lower limit, as long as the engine isn’t lugging after you shift, you didn’t shift too soon.

Now, while he’s not going to ruin the engine, he’s also not extending its life, and he’s wasting fuel. You want to shift at a point that keeps the engine in the power band after you shift - - aka the RPM range where you’re producing the most power. From looking at the performance specs on that car, you want to shift somewhere before 6000 rpm such that when you get to the next gear the engine doesn’t drop below 2500 or so.

There’s also the argument that 3rd gear at 20mph is a bad idea from a safety standpoint. If the idiot behind you isn’t going to stop, it’s nice to have some power to accelerate away from the situation. Floor it at 20 in 3rd gear and you’ll be up to 25 by about next month :wink:

Look at your owner’s manual. It will designate shift points for keeping a steady speed and for continued acceleration.

If he’s not creating an unsafe situation, and he’s not lugging the engine, why worry about it? And is he criticizing your shifting?

You don’t state enough information to argue your point or his.
There are different shift points for different reasons.
zero to sixty asap like pulling on to the freeway…
– is vastly different from …
easing up to the 45 on the boulevard with stop lights too.

It’s more about MY worrying about the engine (and annoyance when he shifts in the middle of an small intersection after stopping). He thinks I shift too late. The engine does lug a little when he drives and there is very little accelerating power at all. We plan to drive this car for a long time, and I just want to make sure he is not prematurely ruining the engine (or the clutch as missleman mentioned)
Then there is the shift light. He shifts right when it comes on, if not a little before, and that makes him think he is not causing any damage.
If he is not, then I suppose I just have to go to my happy place when he drives…

He drives the same no matter how fast he needs to go and how quickly he needs to. We have gotten on the xway at 45mph, and he eventually gets to speed. Yes, we are THOSE people…

Yeah, no offense, but I hate your husband :wink:

That’s just plain dangerous. There’s no excuse for it.

I cringe when I see people do this. It is just plain dangerous. There’s a reason that highway entrances/exits have those ramps. What annoys me more is when people give me smug looks about it whenever I am able to get around them. Their smug look says “oh, look at who’s in such in such a hurry.” Meanwhile my annoyed look is actually saying “are you trying to get somebody killed?”

Oh, and as for the shifting - as shadowfax said - anything above lugging and below the red line is fine.

If he’s lugging it, then he should wait a short while after the shift light comes on before shifting. The carmakers put in those shift lights so they can claim extra mpgs on the EPA test, because you’ll get slightly better mpgs following the (very aggressive) instructions of the shift light. Some VW owners would cover the light with a piece of tape to keep from being annoyed.

A study a while ago indicated that shifting around 3000 rpms was a reasonable compromise. Do you know what rpms are when he shifts?

I don’t think either behavior is going to ruin anything. Slow driving can be quite annoying, though, and pose a safety hazard. Most MT vehicles I have driven, the way I drive, have me in third gear by 20 mph, and I am not damaging anything, wasting gas, or creating a hazard. Shifting as early as reasonably possible, as long as you are not trying to accelerate rapidly, will get you the best gas mileage.

I had a friend who had a Ford Ranger five speed, and when I was driving it, he accused me of wasting all of his gas by shifting early. His shifting behavior was to shift to second at 15 mph, third at 35 mph, fourth at 55 mph, and fifth at 75 mph. Sustained speeds were maintained in the gear assigned, including driving through main stretches of town in second or third gear with the engine wailing, and only using fifth for Interstate driving. Normal highways were to be driven on in fourth gear, whether going 2 miles or 200. I told him my “early shifting” habits would save him money on gas if he would adopt them. He eventually did, and admitted that he got much better gas mileage driving my way.

Check the owner’s manual. Sometimes in the section about how to drive the car, it will suggest speeds or engine speeds at which to shift. That will give you two an absolute answer, and my guess is that the instructions will be a compromise between the two of you’s driving habits.

Your husband may be an excellent selling point for automatics. If you use a manual incorrectly, they become more not less expensive to maintain than a comparable car with and automatic.

The optimal rpm or speed to shift at actually depends on the throttle opening. The torque and horsepower curves we see in car and motorcycle magazines were all measured at full throttle and are only relevent to full throttle operation.

Here’a a horsepower graph of a certain engine measured at quarter and half throttle as well as full throttle.

Looking at the 100% open throttle curve, we see that the engine makes its maximum horsepower at around 5000 to 5500 rpm, thus if you want maximum acceleration, you would shift gears in a way to keep this engine turning about 5000 to 5500 rpm.
Also, if you look at the specific fuel consumption graph below the horsepower chart, you will see that the engine’s maximum full throttle efficiency occurs around 3000 to 3500 rpm.

At half throttle, the engine’s horsepower peak occurs at around 4000 rpm and the minimum brake specific fuel consumption occurs at around 3000 rpm.

At quarter throttle, the horsepower peaks at a surpisingly low 2000 to 2500 rpm and the efficiency peaks at around 1500 to 2000 rpm. Notice that the brake specific fuel consumption at quarter throttle goes to the moon at the higher rpms, i.e. the engine becomes extremely inefficient when the driver insists on making the engine buzz at 4000 rpm even if he’s only going a steady 30 mph.

My point is that the ideal shift point depends on the throttle opening. If the driver is just barely stepping on the gas, already being in third gear at 20 mph might well be optimal.
You may notice that automatic transmissions are programed to short shift when the driver is not stepping on the gas very hard, there’s a good reason for that, the car engineers know what they are doing. I would suggest driving a car with an automatic transmission with an engine similar to the one in your car and noticing when the automatic transmission shifts at different throttle openings, and then mimicing the automatic when you drive your manual car. If those shift points are not optimum, they sure won’t be far from it.

doesn’t an engine produce the same amount of power at any given engine speed regardless of half throttle vs. full throttle?

I don’t have an Aveo, but I do drive a 4 cyl Honda Civic with a manual trans. The “sweet spot” on these small 4 cyl motors is right about 3,000 rpm. They can handle a lot more rpm’s when you need power, up to 7,000 rpm.

At below 2,000 you can “lug” the motor which is too few rpm’s for the load on the motor; such as going up a hill. On level ground just maintaining speed you could go down to 1,500 rpm but below that isn’t good for the motor in most cases.

It seems likely your husband is “short shifting” to save gas, but if the motor is turning too slowly for the load it can harm the motor.

From my experience, 20 MPH is awful slow for 3rd gear, but it depends on how your car is geared. This car has a little 1.6L engine that will be somewhat taxed by the weight of a car. Forcing these engines to pull too hard from much below 2000 RPM is REALLY stressful on the rings and bearings.

I call this “senior citizen stick shift syndrome”, and will never understand why some people insist on driving like this. It usually will cause your car to blow blue smoke out of the exhaust after a few years.

In my car, I will generally run 1400-1700 RPM if I am using no power or just very little power to keep the car up to speed on a downhill. If I need any real amount of power I usually like to get it up to at least 1900 RPM. The engine has to be revving a bit to lubricate properly under load, and also to not overstress the piston rings and cylinder walls on the major thrust side.

If that was the case, then there wouldn’t be much point in having a throttle on an engine. The whole purpose of a throttle is to control the power of an engine.

At extremely low rpm’s, i.e. “lugging the engine”, they may be very little difference in full throttle and half throttle power simply because the airflow through the carb is so low that half throttle barely restricts the airflow. In fact you can see that on the dyno chart I posted, at 1500 rpm, where the measurements start, the engine makes only slightly less horsepower at half throttle than it does at full throttle.

My current car doesn’t have a tachometer, so I don’t know my shift points. I don’t shift by engine speed, road speed, or engine sound.

I now mostly shift by memory. I learned my shift points using engine feel, which can be ascertained without watching a gauge or hearing the engine.

Let’s say you put the car in first gear, and don’t shift, and drive with 1/4 throttle. For a while, the car will accelerate rapidly, then more slowly, and eventually reach a steady speed. To go faster you’d either need more throttle or need to upshift.

I try to shift around the range of rapid acceleration, upshifting when leaving the range, and catching my next gear near the bottom of the range. When the rate of acceleration starts to drop, it’s time to upshift. This range changes with throttle position; it’s at higher RPMs with wider throttle opening.

I try to keep the engine in the RPM range that gives rapid acceleration, not for the sake of rapid acceleration, but because it seems to me that rapid acceleration means the engine is running well and efficiently.

It’s possible to lug the engine doing this, if you accelerate slowly. I’ve learned that when I accelerate modestly, as I usually do, my car is unhappy with how early I want to shift to third (based on second-gear acceleration), so I had to adjust my process. I either accelerate a little more briskly in second, to naturally move the RPM range up, or just delay shifting a second or two.

The engine does lug a little when he drives and there is very little accelerating power at all.

This is the key information we needed. If the engine is lugging and it can barely accelerate, this is definitely bad for the engine. He needs to shift later.