Shifting gears: power vs. fuel efficiency?


#1

I’ve been driving a manual (no tachometer) as my commuter city car. One-way is about 10 miles on level roads. Mostly 35 mph streets, but I usually drive at 40 mph with the flow of traffic. I have been driving in 4th gear at 40 mph. Feels right. According to an app that tracks my mpg, I’ve been consistently achieving the car’s published “city mpg” over the past year.

Recently, I gave a lift to a neighbor who told me to shift into 5th at 40 mph. I said nah. He said 5th gear at 40 mph would improve my fuel efficiency. I was doubtful. Then he offered to fill my tank if my mpg drops (but I had to drive in 5th gear everyday on those 35 mph streets for the rest of my tank). Well, he was right. My last full tank of gas in city driving was about 2 mpg higher (and now it’s well above the car’s published city mpg).

The downside was 5th gear at 40 mph feels weak. Like the engine is going to stall (but not even close, apparently).

Do you aim for fuel efficiency over power (or vice versa)?


#2

The reason that your engine feels weak is that 5th gear at 40mph is lugging your engine. That’s not a good thing. You may be damaging your drivetrain. Check your owner’s manual because mine says to shift into 5th gear at cruising speed (55mph or higher).


#3

That was Gen. Doolittle’s idea: drop the RPMs, up the boost…you’ll be home with fuel to roost! (His raid on mainland Japan was made possible by running lower RPMs and more throttle…and boost…than the book called for. Produces the same power at lower throttle, and lower fuel burn.)


“The book” said there wouldn’t be enough fuel to do it…but there was. As a rule, using the lowest RPM setting you can get away with (without lugging) will give the best mileage.


#4

Your engine drives around with a partially closed throttle plate producing vacuum. That vacuum puts a drag on the pistons as the move down in the bore. That takes away efficiency.

When you are in 4th gear, it doesn’t take much throttle plate opening to keep you at 40 mph. As you discovered, it takes more throttle to keep up with traffic in 5th gear at 40 mph. That reduces the vacuum your engine produces and improves efficiency. You are also turning the engine slower so there is less frictional losses. So you get better MPG’s. This is why automatic transmissions are being made with 6,7, 8 or even 9 gear ratios. Some manuals even come with 6 or 7 gears now.

But as you found out, the engine doesn’t feel as peppy or responsive. If you need to accelerate, you will need to downshift to 4th. Its your decision instead of the car’s since you drive a manual as to what you do. But you can anticipate the need for greater acceleration, downshift early so you are ready.


#5

On level ground 35 mph in 5th gear is not going to do any harm to the motor. If you encounter a small hill you might need to downshift to 4th. Staying in 4th is a better power zone for the motor and it will feel quicker if you need to speed up to change lanes or maneuver through traffic. If you want to maximize your mpg staying in 5th as much as possible will get better mpg.

I have a manual 5 speed Honda Civic 2003. On days when I feel in a sportier mood I’d stay in 4th and on days I want to be thrifty and save gas I’d shift up to 5th. As the driver and the guy buying the gas you get to decide.


#6

Agree with the above. That’s what happens with 5th gear, better mpg and less acceleration. If it can accelerate smoothly if slowly in 5th gear you’re not lugging it.


#7

This is hard to answer without being in your car, of course.

The “almost stall” comment makes me wonder if you’re lugging your engine. As already mentioned, you definitely don’t want to be doing that.

If you’re not lugging your engine, then in your situation I myself would probably be in 4th gear in order to have a little immediate power on demand instead of worrying about a small hit to fuel economy.


#8

Using the higher gear (5th) will not only save gas, it will extend the life of your engine. One factor in total engine life is the number of times it rotates, so fewer revolutions per mile means more miles.

You feel the power loss because you are not as high on the HP curve. That is OK unless you actually need the power, then you just downshift. As long as you don’t go so low that it starts bucking on you, you are OK.

Using the highest gear possible as much as possible I believe contributed to the long life of my Saturn. I sold it at 275k miles and it still ran almost as good as new.


#9

Low revs save alternator brushes too.
I have a friend who bought Corolla with the first twin cam engine (1986?).
Had a 7500 rpm redline and he went up there all the time due to the novelty.
At 70K miles the alternator brushes wore out.
What eventually ended the fun was corrosion in the cooling system because the shop he used for maintenance was scamming him on coolant changes.
Also had the timing belt jumped a couple teeth because it stretched (scammed on the change), no automatic tensioner.
Luckily non-interference.


#10
Low revs save alternator brushes too.

I haven’t had to replace tn alternator in any of our vehicles since my 84 GMC Pickup. They seem to last the life of the engine…well past 300k miles.


#11

I stay in 4th at 40 mph w/my Corolla b/c it is too awkward to quickly reposition in traffic if I’m in 5th at that speed. I need a little more ummphh on the ready to be safe in San Jose traffic.


#12

I usually shift to fifth not when I reach a particular speed, but when I’m through accelerating. That might be 40 mph or even 30 mph. It seems not to have harmed any of the car or motorcycle engines I’ve treated this way, having put over a quarter million miles on a particular Geo Metro with a 997 cc three cylinder engine powering it.
Drive a car with an automatic transmission and pay attention to the speeds at which the transmission is programmed to shift. You will notice that the less you open the throttle, the sooner it’s in high gear. They are programmed that way for a reason. Lugging an engine is more than low rpm, it’s low rpm combined with low manifold vacuum.

A vacuum gauge is actually a better indicator of when to shift than a tachometer is. Mark or memorize the no load engine vacuum. Just rev the engine to 2-3000 rpm in neutral and note the manifold vacuum that the engine makes when running at that rpm without a load.
Anytime your vacuum gauge approaches that number, it’s time to upshift. If you are already in 5th and the vacuum = the no load engine vacuum, it means your engine is delivering zero horsepower to the transmission and you could be going the same speed by shifting to neutral and just letting the car freewheel.


#13

General Doolittle may have used it, but it was Lindbergh that actually started this. He was too old to be a fighter pilot in WWII so they had him ferrying planes from the factory to the front. He did some experimenting during these flights and found that increasing the pitch on the prop and using more throttle at lower RPM resulted in having up to 40% more fuel reserves when he arrived at the destination that those flying by the book.


#14

“I haven’t had to replace tn alternator in any of our vehicles since my 84 GMC Pickup.”

Not surprising if all your cars have low-reving V8s.
Brushes gave out in two of my Hondas ('81 & '88 Accords) at about 150k miles.

If I’m puttering along in light traffic on flat ground I’ll shift into 5th ~40mph.
I did a check using an OBD2 scanner instant MPG readout (on a flat empty road) to verify 5th gear uses less gas than 4th at a steady 35 mph.


#15

I’d rather use a little more gas than lug the engine.


#16

If the engine isn’t vibrating excessively or showing other signs of distress then it’s not lugging.
After all, when it’s idling at 700 rpm (or whatever) it’s not lugging.


#17

And don’t forget 5th will result in less engines revs per mile than 4th. Probably about 10% at a guess, so you get 10% less wear in 5th.


#18

@keith, You’re right. I now remember it was Lindbergh, flying missions as a civilian, that demonstrated this practice to be both effective and safe on the engine.


Doolittle was more about lightening non-carrier B25 Mitchells enough to fly from a carrier deck…


#19
Not surprising if all your cars have low-reving V8s.

Not one was a V8…2 were 4-cylinders (320k and 250k - gave this one to niece who put another 100k miles on it…original alternator when she sold it)…two were 6-cylinders (300k+ and 450k+)…and our current vehicles - both are 6 cylinders.


#20

The technology for brushless alternators has existed for decades, why won’t the car makers adapt it?