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Discussion Starter · #1 ·
I read earlier about owners saying they got better gas mileage by using the manual shift in 8th gear compared to leaving it in drive. I tested mine this weekend and saw a reduction in my RPM's on the open road. Can anyone explain why when I shift from drive to manual it goes into 7th gear and the RPM's stay the same. However when I manually shift up, it goes into 8th gear and the RPM's go down? Why does it not go into 8th gear when it's in drive? Is there any harm caused by manually shifting and using 8th gear?

Hope somebody can give me some insight into this situation.

Thanks
 

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I read earlier about owners saying they got better gas mileage by using the manual shift in 8th gear compared to leaving it in drive. I tested mine this weekend and saw a reduction in my RPM's on the open road. Can anyone explain why when I shift from drive to manual it goes into 7th gear and the RPM's stay the same. However when I manually shift up, it goes into 8th gear and the RPM's go down? Why does it not go into 8th gear when it's in drive? Is there any harm caused by manually shifting and using 8th gear?

Hope somebody can give me some insight into this situation.

Thanks
I've done similar tests and my car will go into "8th gear" when in Drive. I can also reproduce what you've experienced. It all depends on the driving conditions. When I saw 8th in Drive, it was at highway speed (60-65) on a very slight decline. I responded in one of the other threads that I think there might be more than one overdrive in the 8 speeds. I think 8th is too tall to be used in many situations. I *just* looked up the ratios and here they are attached here.

If I'm reading the chart correctly (corrections are very welcome), then 5, 6, 7, and 8 are all technically overdrive. It just takes a situation with very little power demand to use 8th gear. If you manually upshift to 8th while in Drive, then I suspect it will go back to 7th or lower as soon as you make a slight adjustment to the throttle, or the road starts to incline. I have not tested that however.

On the many threads where anything is discussed, there is a lot of conjecture. That's totally normal when random people have a discussion. It is rare when an actual engineer can shed light and give facts, or somebody actually approaches a hypothesis with scientific methods. It's fun to theorize, but nothing is fact until it's proven.
 

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yes anything above 1:1 is "overdrive"
a 0.467 gear is going to require a tall final drive (which 4.444 is) to even think about being used.

1000 rpm / 0.467 / 4.444 = 481 rpm wheel

For the touring 245 50 r20 @ 93.13 in circumference means 42 mph.

1000 rpm looks like 225Nm = 165 ft-lb
HP = tq * rpm / 5252 = 31hp vs about 260hp at peak 5600rpm

Long story, 8th gear isn't going to be very effective except for fighting headwind on flat or slight decline.

 

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IMO, given the state of computing power and controls in today's cars, I think it's probably faulty to "assume" that being in a higher gear ratio (lower engine rpm) is the optimum for fuel mileage, especially where you've got a turbo charger involved that can effectively alter torque (and therefore horsepower) at various RPM's.

Not saying that it does, but conceivable that using less boost at a higher rpm for a given load yields better gas mileage that lower rpm at higher boost.
 

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Hypothetically...

yes anything above 1:1 is "overdrive"
a 0.467 gear is going to require a tall final drive (which 4.444 is) to even think about being used.

1000 rpm / 0.467 / 4.444 = 481 rpm wheel

For the touring 245 50 r20 @ 93.13 in circumference means 42 mph.

1000 rpm looks like 225Nm = 165 ft-lb
HP = tq * rpm / 5252 = 31hp vs about 260hp at peak 5600rpm

Long story, 8th gear isn't going to be very effective except for fighting headwind on flat or slight decline.
Unless you're doing 110mph. I've never ever done that. More than a handful of times. Ever. Never. Not once.

It's amazing how much slower the beast actually seemed to be going. Hypothetically. :tango_face_wink:
 

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Unless you're doing 110mph. I've never ever done that. More than a handful of times. Ever. Never. Not once.

It's amazing how much slower the beast actually seemed to be going. Hypothetically. :tango_face_wink:
True, it becomes more effective when the rpm's get in the engine's powerband, hypothetically of course :)
 
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This sort of thread will become very common across all makes as auto makers add transmissions with a much wider range of gearing options. Places where people spend hours at 80+mph you won’t see those folks on those threads but you’ll find lots of 65-55mph tops people thinking those bigger gears are some magical myth they can turn on by going manual. LOL

One thing is for sure modern DIT engines don’t do well being run on the low end of the rpm range with loads. Ie lugging it, You’ll likely have some costly engine issues later.
 

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Discussion Starter · #8 ·
So if I'm on the turnpike (for hours) doing 65-75 mph is it a good idea to use manual shift and go into 8th gear or just leave it in drive?
 

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I'd say just leave it in drive so the CVT can find the optimum ratio for the given conditions. My opinion.

If there are any fuel savings to be had by forcing 8th, it will translate to very little money saved.
If there are any negative consequences to forcing 8th, it might translate to many many moneys. :smile_big:

You can conduct your own tests in a controlled manner, otherwise we're all just spitballing.
 
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