Best Way to Shift a Manual Trans for Optimal Mileage

manual-transmissions
shifters
fuel-economy

#1

What is the best way to shift through the gears to get the best gas mileage? A guy friend told to wind them out before shifting but that seems counterintuitive compared to shifting quickly into the higher gears.



I drive a 6 speed Mini Cooper S if that makes a difference.


#2

The best way is to accelerate modestly and shift when the engine speeds get into the comfortable midrange of the power curve for that modest gas pedal position. That should put you into a comfortable spot in the lower end of the power curve for the next gear up. “Modest” will vary from driver to driver.

Winding the gears out is definitely not the most efficient way. It’s just the most fun way.


#3

Generally, you want to do pretty much the opposite of what your friend told you: You want to get into the highest gear practical for your final speed as soon as possible. This means shifting as soon as the engine is revving fast enough so that it isn’t lugging when you upshift to the next gear. Many cars with manual transmissions have an upshift light which is pretty close to giving the right advice for upshifting.


#4

If your friend’s advice of “winding it out” means taking each gear close to the red line, he may be punking you. That is certainly the opposite method of extracting the most mpg. But it sure is fun!

Get the best fuel economy out of your car by gentle acceleration and always keeping the rpm’s low. Upshift sooner rather than later. You can judge by the car’s response if you are doing it correctly.


#5

Winding out will not work. Lugging an engine will not save much fuel either and cause excess engine wear. Just drive in a comfortable sweet spot that delivers adequate acceleration and don’t worry about it.


#6

Read your owner’s manual! Most of them include the optimum shift-points for maximum acceleration and maximum fuel economy. There may even be cryptic little hash-marks on the speedometer that indicate them, as there used to be on many European cars. The tachometer is a useless toy-- you should generally make shifting decisions based on vehicle speed.

Your guy friend is sort of right that you want to avoid lugging the engine and, in fact, the optimum shift points for small-engine cars with a bunch of gears may be at a higher engine RPM than you may be used to. But you don’t want to be pushing it to the redline or anything.


#7

Your friend drives an automatic doesn’t he? Winding the engine out means the engine is turning high RPM’s which means the engine is turning faster which means higher fuel consumption which means more money out of your wallet which means you will have to mooch money from your friend who drives the automatic which means after he runs out of money he will stop giving you car advise.


#8

The biggest problem with tachometers it that every different throttle opening has a different ideal shift point. The less you step on the gas, the earlier you should shift, rpm wise. My present car does not have a tach and I pretty much ignore the one on my motorcycle. The factory “redline” is only relevant for full throttle acceleration.
If you test an engine on a dynomometer, you will find that the torque and power peaks happen at lower rpm when the throttle is not completely open. A lot of people though, like your friend for example, seem to dogmatically believe that the engine must always be in it’s full throttle “power band”.
Ignore the tach and shift according to your intuition.

If short shifting is hard on engines, my 288,000 mile '91 Geo Metro sure had a funny way of showing it.


#9

You bought a MINI Cooper S and now you want to drive it to get the best mileage??!!
Drive it like you stole it, that is what the car is for!
If you want mileage, trade it in for a Yaris, or something else that is boring!
However, if you really want mileage out of that car, keep the engine rpm on the low side, and accelerate with a fairly large throttle opening so you do not waste fuel making manifold vacuum. Once you are up to speed, use a light foot on the throttle. The car requires premium fuel and probably will give poor performance and fuel mileage with regular.


#10

Don’t do anything crazy and there will be no difference between that and “best”. There is no best, just common sense. Just drive like you’re not tryna break stuff.


#11

Correct, you can actually accelerate too slow for optimum fuel efficiency, however, once you are at cruising speed, the engine is doing more work making manifold vacuum than it is powering the car, especially at good gas mileage cruising speeds. This is exasperated by the fact that most sporty cars have a pretty low high gear, especially the manual transmissions. Because of that, the most efficient way to cruise is with the engine either on or off using a technique called “pulse and glide”. You accelerate at about 70% full throttle to a speed of 60 or so and then let it coast in neutral for a while until you are down to 50 and then repeat the process. If you open the throttle more than 70%, your fuel air mixture goes rich for maximum power and your engine becomes less efficient.


#12

If you open the throttle more than 70%, your fuel air mixture goes rich for maximum power and your engine becomes less efficient.

Actually, your engine operates more efficiently at full throttle. You will use more fuel at full throttle, but that’s different from engine efficiency. Your engine will be producing lots more power output. The power it produces per unit of fuel improves at full throttle.

The improved efficiency comes primarily from 1: No throttle plate losses at full throttle, and 2: the denser mixture (and hence faster flamefront traveling across the piston).

I remember learning this well in my internal combustion engines course. This concept was a complete surprise to the whole class. Yet after we went through the math, it all made sense.

Joe


#13

"Actually, your engine operates more efficiently at full throttle. You will use more fuel at full throttle, but that’s different from engine efficiency. Your engine will be producing lots more power output. The power it produces per unit of fuel improves at full throttle. "

Joe, that’s only true if your engine’s fuel air mixture stays stoichiometric at full throttle. An overly rich fuel mixture has a higher effective octane rating than a lean mixture does and because of this, the fuel injection goes “open loop” at throttle openings larger than 70 percent or so. Using a rich mixture to quell detonation at full throttle allows the engine designer to optimize the engine’s compression ratio for cruising.
Here’s a real world example. A Continental O-200 airplane engine makes 100 HP at full throttle burning about 8 gallons per hour. At 70 percent power cruise, it burns about 5 gallons per hour. The engine’s fuel mix can be fully lean only at 70% power or less. At high altitudes, the engine makes 70 horsepower at full throttle and can run lean and then the engine really becomes efficient.
There is a device called a scan gauge that plugs into the car’s OBD II port and gives real time readouts of the engines fuel burn and other things like A/F ratio. It can be used to determine if the FI is in open loop mode. Your best efficiency will be at the highest throttle opening that doesn’t have the FI going rich.


#14

On the other hand . . . I just picked up my RX-8 from a warranty appointment and the older, more experienced mechanic whom I speak with about the rotary told me . . . you’re “hurting” the engine by shifting at 3,000 . . . wind it up to 5 or 6 all the time . . . once in a while 7 or 8000. Believe me . . that’s hard to do, since I’ve been driving piston engine cars with a stick shift since 1973 . . . hard to get used to shifting so high. I’ll let you folks know if this hurts the mpg . . . as I’m just starting this new shifting pattern today. Rocketman BTW . . . I’m doing about 62 mph in 6th at 3000 rpm . . . yikes!


#15

Many years ago we had a VW Bus that kept having engine trouble (cracked block). We eventually found that we were shifting at too low a speed and stressing the engine - smaller engines like to operate at higher RPM. Changing shifting habbits cured the issue.

All that said… just how much gas will you save by changing your shifting “spot”? I bet you will hardly notice a difference unless your are doing something extreme or measuring very closely. I’d suspect driving conditions and terrain have a larger effect on fuel economy than shifting habbits (unless you’re being extreme). Does anyone have data on this?


#16

I took a road trip yesterday to Independence Hall in Philadelphia . . . about 75% highway, the rest city and in-between. I ran normal on the highway, (turning about 3000-3200 at 65mph), but wound it up a lot in my city driving and on/off ramps and stuff . . . guess what? Same mpg as before. I filled-up on the AM before we left 'till it would not take any more . . drove all day, repeated the fill-up at the same gas station back home. Go figure. Dosen’t make much sense to me. Still better than EPA, BTW. My mpg was 27 combined. Think I’ll go talk to the rotary mechanic again. Rocketman


#17

Hi,
I actually have a ScanGauge II and it has made a huge difference on my driving. I used to drive my Scion xA like it was stolen, and on 100% city driving, I would get 26mpg. Then I bought the ScanGauge, which, in addition to being an engine monitor (gives you all the current stats such as voltage, temp, etc. and mileage by the trip and the tank), it also can give you the “check engine” codes and reset them. I love it. But, I’m still learning how to use it. I have managed to improve my mileage from 25-26mpg in 100% city driving, to 32-33mpg 100% city. I get to cruising speed as soon as possible, whether it is 25mph or 40mph or 65mph, and put it in 5th gear. I found it to be really important to keep a very light foot on the throttle, since a small displacement on the accelerator can make a 10mpg difference on mileage.

Here’s my question to you: the scangauge has a reading for whatever open/closed loop is. Is it more efficient to run at open loop or at close loop?
Thanks!
elcuc0


#18

“Closed loop” is when the computer uses the O2 sensor in the exhaust to correct the amount of fuel injected to maintain a “perfect” air fuel ratio of one pound of gasoline for each 14.7 pounds of air that goes into the engine.

Open loop is when the computer “guesses” at how much fuel it needs to deliver based on barometric pressure, temperature, engine rpm, throttle opening, etc. Fuel injection operates open loop at full throttle because this condition requires a rich mixture, usually about 12:1, to prevent detonation and make maximum power. It may also be open loop during engine warm up and it reverts to open loop “limp home” mode if the O2 sensor in the exhaust fails. If that happens, you will get a check engine light.

Older fuel injection systems and a lot of motorcycle fuel injection is open loop only, as are most carburettors.

You really want to avoid open loop when you drive, the exception being deceleration fuel cut off at zero throttle.


#19

Thank you. That’s a very informative response!
elcuc0


#20

The efficiency of reciprocating piston engines decreases RAPIDLY as RPM increases. The reciprocating mass increases with the cube of the RPM, something like that. Yes, horsepower increases even MORE rapidly with RPM, up to a certain point when power begins to fall off because pumping losses are consuming more power than the engine is gaining…You can feel this by “using the engine to brake descending steep hills”. It’s not compression that’s slowing the car, (there is nothing to compress) it’s internal engine pumping (inertia) loses.

So gentle acceleration and shift as soon as you can to keep the RPM per mile as low as possible…