Official BRZ MPG Numbers. Big Gear Ratio Differences?
 

Hi guys, I'm new here, so please keep that in mind.

I was on Subaru's website the other day, and I saw they had the official specs brochure up. link

Manual: 22/30 mpg
Auto: 25/34 mpg

Almost everything on it is old stuff, except for the MPG numbers. I know Scion hasn't released their official EPA numbers, but this seems just about right.

I'm planning on buying one of these(still have to decide what brand I want) and as soon as I saw 34 mpg highway, I got really excited, mainly because that's a great achievement for a NA 200hp flat four. It is an automatic, but if AutoGuide is correct, it's just a 6 speed version of the 8 speed "Sport Direct Shift" that's in the IS F, which means shift times of about 0.1s. Manual on the other hand, is 22/30, which is by all means respectable.

Then I started thinking, I am definitely getting the manual, simply because the only reason I'm buying this car is to drive. Real driving. Either way, the other point I would like to stress is this: again, if the rumors are correct, the auto from IS F is already a pretty sporty transmission with amazing gear ratios. To me that simply says that the manual gear ratios are tweaked for that extra "oomph", which brings even more joy to the heart.

What do you guys think? Is my reasoning correct? Which one would you choose knowing that there is a significant difference in the MPG numbers between the two options. More importantly, I will try to do the math, but what kind of gear ratios would the manual Aisin transmission have given that the Torsen in the back is identical.

I'm excited about buying one these and I hope I can contribute to this forum.

Thanks, Alek.

i think you have a fundamental misunderstanding of the differences between an automatic and manual transmission

manual does not have more "oompf", manual has a 3rd pedal and requires you to do weird shit with your feet.

In return for this dexterous activity you receive the ability to dictate which gear you are in, and how long you are in it for. (there are other issues, such as strength and heat capacities, but we'll ignore those for now)

since no one can read your mind, automatics are discarded for race use because you don't want the car to downshift or upshift when you don't want it to.

However modern automatics are bridging that gap, engineers are finding ways to program into the gearboxes the behaviour a driver on a track would expect to perform as needed.

so in that regard, we're slowly on our way into an all automatic world.

if anything, these new MPG findings are only proof of that. The machine simply shifts gears at more optimal points than the human does. Skynet is online, hide yo kids.


and also, regarding "oompfs" in general... you don't want oompfs, because they are stress points, you want smooth power delivery during corner entrance, exit, and so forth.

Agreed but...
 

I agree with you 100%, but manual or automatic, gear ratios is what dictates how the power is being put to the wheels. You can have a v6 300hp engine, behave like a 400hp V8, with the right kind of gearing. The engine isn't designed for that kind of behavior, but you can do it.

Typically you get the "gear ratio" of the car, by multiplying transmition gear ratio by final drive ratio(diff ratio). That is then used to calculate how much torque is being applied to each wheel.(Fast Car Physics)

And that was the point I was trying to drive, given that the final ratio is exactly the same(unless they use a different configuration of a Torsen, which wouldn't make any business sense), there must be a big difference in the transmission ratio.

Once the car is the last gear, at the same rpm's, the only difference between an auto and a manual is that gear ratio. Otherwise, you would get the same exact mpg.

the only way you would get the exact same MPG with exact same gear ratios is if you shifted gears in the manual the same you would in an automatic

which you wouldn't, unless you have OCD or drive with an AFR gauge right in front of you and follow it religiously

You make a very good point. This does apply to city driving. I agree. You can't compete with computer.

Maybe I'm too unexperienced to know this, but math just leads me to think that if in last gear and same engine load(rpm's) and on the highway, manual and automatic should be identical(again, given that the gear ratio and weight are the same).

No, the mpg numbers do not prove the machine knows how to shift better than you do. It just shows that Toyota/Subaru are clever and programmed it to behave in a way that gets good numbers in a controlled test ;)

You could get better mpg with the manual if you know what you're doing. Computers are dumb :P

Also the gearbox is not remotely related to the ISF, it's just got sporty programming. Same philosophy though. The manual gearbox has slightly closer gear ratios than the auto, and 6th gear ends up much shorter, which is why there's a highway mpg difference.

By the way, everyone knew this already, and we've had a lot of threads on it.

I apologize for reposting.

Official EPA mpgs have been out for almost a month now, so nothing new.

Pretty much what he said.

Look at a company that makes great DCT systems. VW and all their auto's get worse mileage than the comparable manual. I think we'll see that in the real world. The manual will get better gas mileage. It certainly wouldn't be the first time its happend.

well manual has traditionally been better on gas than auto, i'm saying the opposite that perhaps we are moving towards the other spectrum.

Around the city it's hard to beat the low parasitic loss manual, as long as you're not dragging the engine along at high rpm (although some people like a "responsive" cruise, and this kills their mpg somewhat), as there's no hydraulic system to power, and idle consumption is lower (although if you shift into neutral it's almost even and if you turn off ignition when waiting at lights and stuff then it's the same).

On the highway you're stuck in 1 gear more or less, so it comes down to the gearing, and an automatic transmission usually has a longer gear for a variety of reasons related to fuel economy. I'm guessing manufacturers make the highest gear shorter on a manual because they assume 1. manual drivers want a "sporty" feel 2. no one can be bothered to downshift 3. no hydraulic pump gives them slightly more room in terms of fuel economy.

There's a lot more to it than just comparing the gear ratios on paper.

I have contemplated making a dedicated thread that explains what's different about this style of transmission than what most of you guys are probably used to. I have studied several of the popular 6 speed (and 7 or 8 speed) transmissions.

Most people think of a modern 6 speed auto like it's an old 4 speed + 2 more gears. That's not how they work. The gearsets are different, the torque converter control is different, the holding elements (clutches etc) are different, the shift logic is different, and in the case of Toyota the valve body hydraulics are different.

The increased fuel economy rating here is for a lot of reasons. You have to understand that fuel economy is performed through 5 different test cycles averaged together according to an EPA formula. There can also be differences in the ENGINE calibration between the manual and automatic--the throttle mapping, torque reduction requests during shifting.

Don't "under-think" it.

That would be welcomed, thanks!

the thing that I'm concerned about is that very few people have more than a fuzzy idea of how a basic automatic transmission works. An old 3 speed is actually very complicated. If you don't know how that works, it's hard to appreciate the differences between that and a 6 speed. The thread could easily go over everybody's head... if you don't know what simpson gearset is, how a valve body works, how shifting works, what holding elements are and why you need them, what torque converter lock up is, then a lot would go over people's heads. So I would have to figure out what level of detail to provide, and what is a length that people would actually read.