[MENTION=118321]Jetboy[/MENTION]
Appreciate the explanations and graphs, but I'm struggling with my "real world" experiences to see the gains in fuel efficiency by running a 6AT and 4.56 gearing.
Here's what's clouding my thoughts; my 2015 Sierra ext cab (granted, probably lighter than my Tundra crew cab) had a 5.3 V8 and 3.73 gears if I remember correctly. It would return 16.5 in city driving, and 21 hwy if I was nice to it and conditions were right. But it liked to rev and I always felt like I had to whip it to get it going.
My Tundra with 5.7 and 6AT with 4.30 gears is returning at best 14.5 if I drive like Grandpa in city, and the best hwy I've returned is 18. Granted, it's still "green" at 2100 miles... however, I can lope along with traffic at lower rpms and use the torque to keep up (most likely due to where hp/tq is made in the rpm band of the 5.7 vs 5.3). No doubt this will haul/tow better on the low end than the Sierra. While I do enjoy the driving experience MUCH more with the Tundra, I don't see fuel efficiency being a gain. Or was efficiency you mentioned energy transfer in general? But still... wouldn't more efficient energy transfer = better fuel economy too?
I'm trying to imagine 4.56 gearing in a 4R on a factory set up intended to appease the masses... and after the Tundra at 4.30 with a 6AT, I can't fathom that being livable for the daily driver behind the 4.0 (on stock tires it seems like waay too much gear to me). But I could be wrong, wouldn't be the first time.
There's a few answers here.
The first - just to be clear - is that the 4.56/6AT combo would likely not improve mileage in a significant way. The engine rpms at highway speeds would be within 0.1% of what they are now - so less than 10rpm difference from the current transmission/axle combo we have now. It would effectively be exactly the same. In fact all of gears 2-6 on the 6AT/4.56 combo would line up nearly identically to gears 1-5 in the 5AT/3.73. The difference would simply be an extra lower gear in 1st gear. Driving it in city would literally be identical to driving a current 4Runner with 4.56 gears. Gears 1-5 are the same either way. If you ever get a chance to drive a current 4Runner with 4.56 - it's fantastic in the city. Initial launch is so much better. It's how it should be. But you run out of gearing on the highway. Going to the 6AT would add one extra gear for highway use that would put you back right to where it is now. The result is you get the improved city driving and keep the exact same highway cruising gearing. As it is now - you only get one or the other. You can't have both.
The result of that would probably be around a half second or better improvement in 0-60 (who cares about the time - but it would feel a lot quicker), better, easier towing.
With the 8AT - then we could see some real mpg benefits. The biggest one being that the drop from 8th to 7th gear is much less rpm difference from 5th to 4th gear in the 5AT. And often towing I end up dropping down into 3rd gear up most mountain passes. The torque converter won't lock up in any gears other than 4 and 5. So it's both creating a lot of internal heat, but also that's wasted energy. The 8AT in the same condition has more gearing options and will also lock the torque converter - less heat, better efficiency, and probably more power output by choosing the correct gear. So when you're going up a big hill, or you've added a bigger tire, or whatever you've done to increase drag or load by say 5%, then you dont' have to drop down a gear that's ideal for a 25% increase. You can change the gearing a small increment to keep the engine in it's best condition for the load.
The second problem we have is not gearing - its the engine. The 1GR simply isn't that fuel efficient and it appears to have a small window of high efficiency that does not, under any scenario, work efficiently at high output. So - for example - if all of the mods to the 4Runner, or a trailer, or ?? result in a requirement of say 100hp continuous output to maintain steady state cruising - it's possible, and I think is probable in our case, that the 1GR simply cannot produce 100hp at high thermal efficiency (good mpg). In that case if you still want 100hp for cruising, there's nothing you can do with gears to get great mileage. But - what you could do is maybe move from 14mpg up to 17mpg.
With an engine designed around a wider range of power needs - atkinson cycle, cylinder deactivation v8, turbo 6 all of those do the same thing, they change effective engine displacement, or potentially in our case just a larger engine, then you have the range of efficient output to handle things like higher loads well. The big failure IMO of the atkinson engines from Toyota is that they're just too small. They are a better way solve the problem of widely variable power needs than cylinder deactivation. But the 3.5L for example can either run as a 3.5v6 or it can run effectively at around 2.0L displacement. Functionally it's about the same as a 2.0l turbo. The two do the same thing, but in different ways. But what a Tacoma or a next gen 4Runner needs is not a turbo 2.0 that can also act like a 3.5L when needed. It needs is an engine that can be a 3.5L when needed or a 5.0L when needed. So it's a good engine. But it's the wrong engine for the Tacoma. You never need it to be a 2.0L. It's completely wasted other than at idle.
As a comparison with the Bronco - the 2.7 ecoboost can run anywhere from around 2.7 liters of displacement all the way up to probably 6 or 7 liters depending on boost. So, on demand it can act just like a 6 liter v8. And it'll be a very stark difference side by side when you compare one engine that has an effective range of displacement form 2.7-6.0 vs a Tacoma or new 3.5L 4Runner that is 2.0-3.5. Night and day. The comparable engine would be Toyota's 2UR. And frankly - that's what should be in the 4Runner and GX by now. That's the correct engine for efficiency and power. Even if you de-tune it to 300hp. Or a smaller version of the V35A turbo 6. Both engines have huge ranges of high efficiency in the power levels we want for lifted, loaded, towing etc.
So with respect to your tundra. The 5.7 is not very fuel efficient. And it really struggles with thermal efficiency at low output. So cruising empty - it get terrible mpg. Where it's most efficient is under higher load. My experience is that the Tundra actually does okay when towing heavy loads. It's just really out-dated. It's an engine from 2007 that has had no updates. It's a fantastic engine for durability. It's basically a big block (4.6" bore spacing) in GM terms and has lots of great features. But it's tuned very rich - presumably to keep cylinder temps down - and that's also probably why the supercharger actually increases mpg. Magnusson leaned out the fuel mix a bit. A re-tune could probably get 2-3mpg just by leaning it out a bit. But also it is way overdue for an update and apparently all reports suggest it's just going away. Supposedly they're building a stockpile right now of engines to finish out the Tundra models and then all v8's are going away and they'll switch to production of turbo 6 engines in the next few months.
So the tundra isn't so much a gearing issue. It's just an inefficient engine that's also combined in a truck that uses bigger heavier components than any other 1/2 ton, so it has a lot more parasitic losses throughout. The plus side is that you get huge running gear. The wheel bearings are bigger than an F250. The transmission internals are bigger and stronger than a GM Allison 1000. The rear diff is extremely strong. It has giant brakes. All of those things lead to a heavy truck that's not very efficient. Especially when combined with about a 20 year old engine. If the tundra had a 5AT and 3.73 diffs - it would probably be even worse.
The other consideration between say GM and Toyota is that you need to consider the total gearing. So transmission, transfer case, and differentials. I'm assuming everyone is currently using a 1:1 transfer case in high range (keep in mind that's not always true), so we'll ignore the transfer case for now.
If the transmission has a 4.7:1 first gear - and the axle has a 3.5:1 first gear, that's no different in effect from the differential having a 4.7 ratio and the transmission have a 3.5:1 first gear. The final gearing is the same in both. And incidentially it actually fits for this comparison with the GM truck. Both GMs 8AT and 10AT truck transmissions have a first gear of 4.7 or 4.56. And they are usually paired with a axle ratio of 3.55 IIRC. So that's actually exactly the same result as what I'm suggesting would work well in the 4Runner. The 1st gear is 3.5 in the 6AT and you'd match that with 4.56 diffs. Net - you'd get the same first gear ratio as GM's 8AT and 3.55 diff. If you compare to the Tundra - first gear is only 3.33 in the tundra 6AT and with 4.30 diffs, it's actually still a decent amount higher than GM. - in short the 2015 GM truck actually has lower net gears than the Tundra because the transmission is so much lower geared.
Now - just for fun, if you compared the Bronco 1st gear, it's 4.7:1 and the diff option is also 4.7:1. With our transmission - we'd need 6.3:1 differentials to match 1st gear in the Bronco. It's almost as low as the 4Runner in 4lo - while still in high range. And, still because the Bronco has a wide ratio 10 speed - it also has almost exactly the same highway cruising net gearing. In low range - the Bronco is soo much lower it's not really even comparable - it's better than my FJ40 would be with dual transfer cases and 5.29 diffs. Holy shit.
