FR-S/BRZ MPG

[miata]

MT must be geared for pure joy/performance while AT is geared for max mpg. Makes sense.

[serialk11r]

Originally Posted by miata

MT must be geared for pure joy/performance while AT is geared for max mpg. Makes sense.

Why does it make sense? The only major difference between the transmissions is the last gear...how does a short highway cruise gear contribute to "pure joy/performance"? "oh man I can floor it in 6th and I can sorta feel a little bit of acceleration wow so cool!"

My hope is that Toyota/Subaru notice the distress that the poor manual highway fuel economy is causing, and that they adjust the gear ratios in an update.

[tranzformer]

Originally Posted by serialk11r

Why does it make sense? The only major difference between the transmissions is the last gear...how does a short highway cruise gear contribute to "pure joy/performance"? "oh man I can floor it in 6th and I can sorta feel a little bit of acceleration wow so cool!"

My hope is that Toyota/Subaru notice the distress that the poor manual highway fuel economy is causing, and that they adjust the gear ratios in an update.

You are going to have to downshift to pass a car on the highway or ascend a hill pass anyways. Not sure what the point is either. Isn't like this car is putting out 350-400 torques.

[Quentin]

With taller gearing, you might have to downshift to pull long hills on an interstate, though.

[tranzformer]

Originally Posted by Quentin

With taller gearing, you might have to downshift to pull long hills on an interstate, though.

Depends on your definition of "long hills." If your definition is Rockies/Sierra Nevadas/Cascades...etc., you will have to downshift to 5th or 4th anyways. Taller gearing for the 6th gear would make more sense for every day driving as it would allow lower RPM at highway speeds w/ better MPGs. Plus the car/engine is meant to be shift, hence the AZ6 transmission. No problem going down a gear to pass a car for 10 sec and back into 6th.

[serialk11r]

According to my calculations if you have 2000rpm 60mph cruising, you can still go up a 10% grade at 60 without using 100% of the available torque, although you'll be close. I think people exaggerate the need for downshifts, you only need it when passing.

[tranzformer]

Originally Posted by serialk11r

According to my calculations if you have 2000rpm 60mph cruising, you can still go up a 10% grade at 60 without using 100% of the available torque, although you'll be close. I think people exaggerate the need for downshifts, you only need it when passing.

Most of the time going up mountain passes you have to pass slow moving semis and other drivers that are in the right lane. Might as well downshift and finish the pass properly. Probably more efficient instead of leaving it in 6th and bogging the engine down.

[miata]

Originally Posted by serialk11r

Why does it make sense? The only major difference between the transmissions is the last gear...how does a short highway cruise gear contribute to "pure joy/performance"? "oh man I can floor it in 6th and I can sorta feel a little bit of acceleration wow so cool!"

My hope is that Toyota/Subaru notice the distress that the poor manual highway fuel economy is causing, and that they adjust the gear ratios in an update.

You are not buying an economic fuel sipper. I did. You are not.

[Quentin]

Originally Posted by tranzformer

Depends on your definition of "long hills." If your definition is Rockies/Sierra Nevadas/Cascades...etc., you will have to downshift to 5th or 4th anyways. Taller gearing for the 6th gear would make more sense for every day driving as it would allow lower RPM at highway speeds w/ better MPGs. Plus the car/engine is meant to be shift, hence the AZ6 transmission. No problem going down a gear to pass a car for 10 sec and back into 6th.

My definition is interstates running through the Appalachian mountains. My 6MT GTI reved around 3k at 70mph and had plenty of immediate power if I just punched it without a downshift. It was very pleasant to drive on the interstate because it constantly felt on boil. It felt engaging like it was ready to leap into action like a sporty car should feel.

Shifting to make minor speed changes on the interstate is for the birds, IMO. This is a sports car, not an economy car. Going 70mph everywhere instead of 80mph will save you more money in gas than a taller final drive will.

[tranzformer]

Originally Posted by Quentin

My definition is interstates running through the Appalachian mountains. My 6MT GTI reved around 3k at 70mph and had plenty of immediate power if I just punched it without a downshift. It was very pleasant to drive on the interstate because it constantly felt on boil. It felt engaging like it was ready to leap into action like a sporty car should feel.

Shifting to make minor speed changes on the interstate is for the birds, IMO. This is a sports car, not an economy car. Going 70mph everywhere instead of 80mph will save you more money in gas than a taller final drive will.

How about a taller gear going 70mph? Or a taller gear going 65mph? I don't mind the mpgs. It suits my requirements. Price is all that is left.

[serialk11r]

This is a sports car, the whole point of having close ratios is so you have more choices for the optimal gear, and you have to put work into shifting to make that happen. Tell me why you need lots of passing power in 6th gear when your 5th gear can do it much better? Fuel efficiency doesn't matter in a sports car? Tell me, would you rather be cruising at 50mpg, engine very quiet, or 32mpg, engine roaring? Fuel efficiency is a byproduct of proper, intelligent design.

And no, a short gear going 70 vs long gear going 80 is a massive difference in load. A long gear going 80 could be putting the engine at 90% peak efficiency while the short gear could be putting the engine at as low as 50% peak efficiency on some cars, while the actual power requirement difference between 70 and 80 is something like 40%.

[tranzformer]

Originally Posted by serialk11r

This is a sports car, the whole point of having close ratios is so you have more choices for the optimal gear, and you have to put work into shifting to make that happen. Tell me why you need lots of passing power in 6th gear when your 5th gear can do it much better? Fuel efficiency doesn't matter in a sports car? Tell me, would you rather be cruising at 50mpg, engine very quiet, or 32mpg, engine roaring?

And no, a short gear going 70 vs long gear going 80 is a massive difference in load. A long gear going 80 could be putting the engine at 90% peak efficiency while the short gear could be putting the engine at as low as 50% peak efficiency on some cars, while the actual power requirement difference between 70 and 80 is something like 40%.

Thank you. :happy0180:

[Quentin]

Holy hyperbole, Batman. 50mpg vs 32mpg? Until we have the specific fuel consumption maps, we don't know how much of a real benefit you'll get by slightly dropping the revs at interstate speeds. I'm going to put my faith in the engineers at Toyota and Subaru and their mantra of fun to drive and them actually having all the data. With this car making relatively little torque, I'm not shocked at all that they are having it rev a little higher at cruising speed. It isn't like this thing is doing 20mpg like an RX8.

Gas isn't going to be close to the deal breaker as far as cost per mile in this car, anyway. It will be insurance. I can drive my 22 mpg 4Runner for the same cost/mi as the MKV GTI I had previously because of the high insurance rates of the GTI and the super low rates of my 4Runner.

[serialk11r]

Sorry that was perhaps a slight exaggeration. You don't really need a BSFC map though, it's very reasonable to expect that the low load efficiency is going to be similar to say a 2ZR-FXE, which has similar cam timing, although direct injection changes things a bit.

A Prius can cruise at over 95% efficiency, while a Honda S2000 cruises at around 60% efficiency. That's the kind of spread you see even among small engines. That's the difference between good gearing and bad gearing really.

The AT will require about 52Nm from the engine at 60mph, while the manual requires about 40Nm. The 2ZR is 1.8L instead of 2L so we can multiply these by 0.9, which will give the approximate corresponding load on the 2ZR.

The AT will be able to run the engine at around 270g/kWh efficiency. The MT runs the engine at around 300g/kWh efficiency. Peak efficiency is slightly better than 230g/kWh.
Sorry for all the edits by the way.
EDIT: I'll add in the numbers at 75mph for reference, in a moment.

Okay 75mph, AT needs about 77Nm from the engine, the MT needs about 60Nm. AT is running at 240g/kWh, MT is running at a bit over 260g/kWh. And at 80 (do 80mph speed limits even exist? I haven't seen one before...highest I've seen is 75, so that's technically the fastest you should drive :P) MT is a little over 250g/kWh, while AT is at 230g/kWh. So consistent ~10% difference or so.

Also note that at 80 your engine is running pretty efficiently, although the power requirement is much higher. With a longer rear diff or slightly longer 6th gear we can close the gap pretty much all the way, especially since an AT packs hydraulic pump losses, which eat up part of the advantage.

Moral of the story is, if you care about mpg, in the city, go manual. Lots of highway miles, AT will have probably somewhere between 5-10% better fuel economy on average. EPA test is done at lower speeds, at higher speeds the gap will close up a bit. The reason being, a modern engine has a huge range in which it is within 10% or so of peak efficiency from 100% load down to the high 30s, and across several thousand rpm. Efficiency drops off rapidly in the area where the EPA test happens, cruising at low speed, and the MT will get to the low efficiency zone faster.