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Old 09-11-2008, 03:33 PM   #1
SpeedShift
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Default not subaru related, but how can you increase mpg with gear ratios?

this article pertains to chrysler minivans and their 09 models. they are stating that they achieved an 8% mpg increase just by reprogramming the ecu and tweaking the gear ratios. what i don't understand is that they have lowered the gear ratio of first gear for quicker launches and spaced the the rest of the gears closer together. how can a lower gear ratio help mpg? my thinking is that it would actually hurt mpg because it puts the vehicle into a higher rpm band quicker and keeps it there longer under normal driving...


Quote:
General Motors has proven that it's not all that difficult to eke a bit more efficiency out of already-finished models with its XFE line. By waving a magic wand over the engine's computer, subtle reprogramming tweaks can make a big difference. Other tricks involve altering transmission gear ratios for improved efficiency. Chrysler has gone ahead and made changes such as this to its minivans equipped with the top-level 4.0-liter SOHC V6 engine and six speed automatic transmission combo. The biggest changes seem to be to that auto gearbox, which sees a lower first gear for better take-off and more closely-spaced ratios the rest of the way up. The newly-optimized engine features 251-horsepower and 259 lb.-ft. of torque. The end result is an improvement in fuel mileage of up to eight percent. For the number crunchers out there, that puts the Dodge Caravan and Chrysler Town and Country at 17 mpg city and 25 mpg highway.
the 08 and 09 models have the exact same drivetrain and engine with the exceptoin of the ecu reflash and difference gear ratios. i am hoping that the ecu reflash was behing the 8% mpg increase and therefore might be offered in the 08 models for existing customers through chrysler....everyone is in search for better fuel economy these days.
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Old 09-11-2008, 05:21 PM   #2
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Gears make an amazing impact on acceleration and fuel economy. Higher gears = better acceleration yet worse fuel economy, Lower gears = the opposite. Most likely they changed the top most gears to achieve those numbers. The lower gears don't matter as much as you aren't in them that long. Look at the Corvette for example....it has an astronomically low sixth gear and gets great MPG when you look at the highway sticker.
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Old 09-11-2008, 05:30 PM   #3
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My buddy installed an 4.44 RA tranny with an R160 rear end in his GC which absolutely RAGED on everything when he was tooling around L.A. The whine from that tranny sounded so sick and it gripped like crazy. The only downside to that incredibly saucy transmission was L.A. freeway mileage. He got screwed because he was running at about 4000 RPM in 5th at freeway speeds and it guzzled gas as a result.

Starting a bit of a bull session, if you want to go the built transmission route (and I'm just kicking this idea around, not sure if it's feasible), taking the 5 gears from an RA tranny and using the 6th gear from a USDM STi as a cruise/overdrive gear, wrapped up in the STi tranny casing might give you the best of both worlds. Or you could steal an STi 6th and get rid of the useless RA 5th gear and put it in the RA casing.
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Old 09-11-2008, 05:32 PM   #4
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ok maybe i have the low gear ratio vs. high gear ratio backwards. isn't low gear ratio a higher number, thus revs quicker and achieves faster acceleration?
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Old 09-11-2008, 10:25 PM   #5
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Ummm, swapping 5MT with 6MT gears won't work.

Low vs. high is sucky and I thought I would explain in my first post but didn't. This is how it is as I understand it:

(Fictional example)
Old 1st gear 3.50
New 1st gear 2.50

2.50 is higher
3.50 is lower

These are the TECHNICAL terms, but we common folks usually mentally equate the opposite since the lower number in our minds mean lower gearing.

It's a double edged sword that bites me every time I use the "right" terms.
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Old 09-11-2008, 10:37 PM   #6
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Same, I just call them "long" and "short" gears. I think of what it would be like to watch someone with long legs race someone with short legs. Someone with short legs can get going quickly but it's hard to keep running at high speed (lots of energy to move short legs back and forth in order to make the same pace). Vice versa for long legs. Just bear with me, it makes sense to me.

Like my friend's RA tranny could get going ridiculously fast but there wasn't much headroom in the top end because the gears spooled up so quickly. Very short legs.

http://www.5speeds.com/ratios.html

Last edited by oldhat; 09-11-2008 at 11:00 PM.
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Old 09-11-2008, 10:46 PM   #7
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Quote:
Originally Posted by Unabomber View Post
Ummm, swapping 5MT with 6MT gears won't work.
That sounds like a challenge, Kaczynski.
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Old 09-11-2008, 11:25 PM   #8
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Actually shorter gears and different shift points can be beneficial to gas milage while accelerating... There is less load on the engine and higher manifold vaccum while accelerating under normal conditions (not racing away from a stop)... So given the same rate of acceleration, your engine is operating at a higher RPM, but at a lower volumetric efficiency. Shorter gearing in your final gear however is hard on cruise mileage because the engine sits at a higher rpm and on average will be sucking in a larger volume of air per unit of time.

Theoretical example. Let's use an average naturally aspirated engine tuned for good around the town driving torque so there is no artificial amplification of volumetric efficiency under boost. Accelerating from a stop to 30 miles per hour in 2nd gear @ full throttle is probably going to eat more gas and reach 30 mph slower than accelerating from a stop to 30 miles per hour in 1st gear at 25-50% throttle in 1st gear.

This is also why a lot of CVT transmission designs keep a short gear ratio initially to reach the engine's peak torque rpm fairly quickly and then start tapering the gear ratios down to "motorboat" the car ahead while maintaining that peak torque rpm where the engine is at peak efficiency therefor getting the best possible gas mileage that the engine capable of.

I love my 4.44:1 final drive... And since I live 2.6 miles away from work and rarely cruise the freeway, if anything my gas milage has improved (when I have the self control to drive like a normal human being anyways ).

Last edited by jhargis; 09-11-2008 at 11:35 PM.
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