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Top gear engine rpm - manual vs. automatic


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8 replies to this topic

#1 Stelcom66

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Posted 17 November 2012 - 10:41 AM

Before my '02 Subaru Forester 5MT I had an '02 Outback 4EAT -at 65mph the Outback ran around 2750 rpm. My Forester is about 2650 - an '02 Outback 5MT I drive runs around 3200 rpm. The '98 Outback I had also ran around 3200 at 65 mph. A 92 Legacy with an automatic I had ran only around 2250. I do wonder why Subaru chose the gear ratio for higher rpm with the manual. The engine is certainly capable of moving the car just fine at the lower rpm. I also wonder is the gas mileage penalty (I assume there's at least some) is significant if running for a few hours at 65 mph on level road.

I've read where lower rpm doesn't necessarily mean less fuel consumption. In the case when still in top gear when going up an incline I'll open the throttle to keep the speed up. When doing this I assumed I'm not using any more fuel since the engine rpm is remaining constant, say 2100 rpm. Since the rpms are staying constant I thought this will not actually use more fuel even though the throttle may be 3/4 open. Similar to the idea when braking in gear - the enging is turning a certain rpm going downhill but the gas pedal is not pressed. To try to summarize my question - is fuel consumption based on engine rpm (I thought yes except going downhill) or how much the throttle is open. I know it's not good to lug the engine - at the same time I try to keep the rpms to at least a medium level.

#2 Fairtax4me

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Posted 17 November 2012 - 12:56 PM

Engine load has more effect on fuel consumption than RPM. Giving more throttle to maintain constant speed going up hill has nearly the same effect as accelerating quickly on flat ground. Even if the engine speed does not increase, the load on the engine does, so it has to use more fuel to provide the power necessary to climb the hill.

While the engine does operate more efficiently at highway mid range rpm, at some point wind resistance becomes a major factor. Very similar to climbing the hill, as the vehicle speed increases more wind pushes against the car, and this turns into extra load on the engine to keep pushing through the wind. While the engine may be at its most fuel efficient operating speed at 3000 rpm, at 65mph the wind resistance causes the engine to have to work harder to maintain that speed. Bring vehicle speed down to around 55 mph, engine speed is closer to 2500 rpm, but the load on the engine is reduced because there is less wind resistance.

Having driven significant distance in both situations, I can say with absolute certainty that you will get better fuel mileage at 55 mph than will will at 65-70 mph, at least with a manual transmission vehicle. My car will do over 30mpg on my daily commute to school if I drive the 55mph road, rather than taking the interstate at 70mph, where I get only about 26mpg.

#3 porcupine73

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Posted 17 November 2012 - 02:55 PM

Right it's not entirely based on rpm. It's really based primarily on airflow, which the ECU then injects the right amount of fuel to match the air flowing into the engine. With the throttle closed and coasting down hill very little air enters the engine. Whereas you might be full throttle at the same rpm and it's drawing in as much air as it can under normal aspiration.

#4 forester2002s

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Posted 17 November 2012 - 03:40 PM

Torque also comes into the equation.
Your 2002 5MT Forester puts out peak torque at about 4,000 rpm.
And the 5 speeds are chosen to allow adequate torque in each range.
The automatic 4EAT has 4 gears, and these would also be chosen to give adequate torque, but with different speed ranges than the 5MT.

With the old carburetted engines, one could achieve lowest fuel consumption by using a vacuum-gauge connected to the inlet manifold, and using the throttle to control vacuum level.

I don't know if this still works with the new-fangled computerized fuel-injected engines. Does anyone know?

#5 Fairtax4me

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Posted 17 November 2012 - 04:07 PM

It does. The same principle still applies, but I'm not sure of specifics.

#6 Idasho

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Posted 17 November 2012 - 04:48 PM

With the old carburetted engines, one could achieve lowest fuel consumption by using a vacuum-gauge connected to the inlet manifold, and using the throttle to control vacuum level.

I don't know if this still works with the new-fangled computerized fuel-injected engines. Does anyone know?


As complicated as the new rigs are, a simple vac. gauge WILL go a LONG way in obtaining the best MPG you can get.

New rigs use a pile of sensors and preset tables to determine how the engine runs and how hard it works, but the basics are still there, and when you break it down most of it comes down to one thing.... LOAD As it is a simple (yeah right) calculation for the computer to determine how much fuel and timing to feed the motor to match the load it is being subject to. And it uses all those funny sensors under the good to do it. mass air, throttle position, knock, absolute pressure, O2s, etc...

#7 Stelcom66

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Posted 17 November 2012 - 07:39 PM

I do try not to exceed 65mph. Thanks for the information on other factors to consider. While I prefer manual transmissions - I'd consider the CVT in the newer Outbacks. My company vehicle, a small SUV, has a CVT. If the Outback's is anything like the Jeep's CVT it does keep engine rpms down and gets decent fuel economy. Although it is just FWD, the Jeep averages between 27 - 29mpg in warmer weather. With the 2.4L 4 it's running at only 2k rpm at 65mph. I've heard the Outback is that or maybe even lower.

A few Subarus around this house - along with my Forester my son has a '95 Legacy LS with the 2.2L. His girfriend has an '02 or '03 Outback, Last month my other half just got an '02 Outback with the 5MT with 121k. Check engine light just came on - other than that runs like it's new. It has the nice wheels with the goldish tint. I thought those were only available on the Limited or H6 models, which hers is not.

#8 WoodsWagon

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Posted 17 November 2012 - 10:39 PM

I think subaru used the .87 overdrive on the outbacks because they assumed people would think the car was gutless if they had to downshift for hills on the highway. The car was heavier and had bigger tires than before, so that was the compromise they made. Make 5th nearly the same as 4th and people will never have reason to downshift to 4th.

I know the older 2.2l's can pull a legacy L along just fine with the 205/70r15 outback tires and a 3.9 final drive with the .78 overdrive. My mom's 98 outback with the auto has a .70 overdrive, which works out to almost the exact same RPM's with the 4.44 final that the L's manual has with the same tires. It's a 3.04 total reduction ratio instead of a 3.06.

The 98 outback's best MPG was 29.7, which was fully loaded, 3 adults plus luggage, doing 80mph across nebraska. Tracking the MPG the whole trip across from the east coast, the faster we went, the better the MPG was. It was also better with the front windows down AC off vs up with the AC on. So pretty much the opposite of everything that goes as common knowledge. The aerodynamics on that generation of outback were good enough that they didn't seriously hurt the efficiency as the speed went up. I would hope that the next generation of outback didn't take a big hit in that area.

The extra weight of the next gen outback is offset by the higher power and torque of the phase 2 2.5l. So there's no reason it shouldn't be able to move the car along just fine at the same engine RPM's. Which we know it does with the 4eat trans cars because they have the same ratios as the earlier ones. The 5mt's though have a 3.57 total reduction instead of the identical automatic with a 3.06. Same tire size, same weight, same aero, different top gear running speed. The only conclusion is that they knew the auto would downshift as needed and that the manual wouldn't because the drivers are too lazy.

I've got a busted WRX transmission in the shed. 2nd gear is smoked, we used the center diff and rear transfer gears on another transmission, so the .74 5th gears are sitting there staring at me. I'm going to swap them into my mom's new 2003 Outback because the engine is screaming on the highway. With those gears, it drops the total gear reduction down to 3.04 just like the legacy L had. Hopefully that will reduce the engine noise on the highway and bump up the MPG's a bit.

#9 Stelcom66

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Posted 18 November 2012 - 09:38 PM

I thought the Outback's higher weight and tire size may have been the reason for the 5th gear choice. As you said - the 2.2L does just fine with those lower ratios. Comparing my '92 Legacy auto and '98 Outback with the 5MT - there was about a 1,000 rpm difference at 65mph.

Getting 29mpg in an Outback is great - surprising given the conditions. Maybe Subaru would think the perception would be the car was underpowered with the manual if it had the same reduction of the automatic. We know that isn't the case. Up a significant incline (as long as I'm not behind someone doing 40 - 45mph) I can hold the Forester in 5th. At the crest of the hill I'm down to 45mph, the speed limit and the shift point recommended in the manual from 4th to 5th gear. I don't do that all the time - usually don't, it's just nice knowing the car can.

That's cool you'll be able to get the WRX 5th gear into the '03 Outback - I would probably but that car! I assume that's currently turning around 3200 rpm at 65mph like significant other's new '02 Outback.

On a different note - changed the oil in my son's '95 Legacy today . Was pleased the owner's manual was in the glove compartment - along with all the service records. The car has 165k miles - I saw the water pump and timing belt were changed at 112k. The manual transmission recommended shift point speeds in '92 are identical to the 2002's.




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