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Old 08-21-2008, 11:37 AM   #1
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CVT & Mpg

I have a 2007 Nissan Maxima V6 with a CVT (Contiuously Variable Transmission). I have noticed my gas mileage is not on par with my 2000 Maxima with a regualr automatic transmission ...even though EPA estimates are similar. Has any one else with CVT (in any make and model) noticed lower than expected gas mileage?

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Old 08-21-2008, 12:00 PM   #2
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The Nissan Maxima CVT is not tuned for efficiency, but for performance.

A CVT's primary advantage is that you can make it do whatever you want. It has an infinite (or just extremely large, depending on the technology used) number of possible gear ratios.

Some CVTs allow the user to tell it whether they want performance or economy (i.e. the manufacturer put in a switch which tells the computer controlling the CVT which mode to operate in, and gave it two (or more) different profiles of gear ratios to use.

At peak efficiency, your CVT would keep the engine at the highest efficiency spot on the BSFC map (probably near the point of peak torque, but maybe not) all the time. In other words, when you step on the gas, the engine would rev up to that RPM (I'll use 2200 just as an example), and just stay there as long as you are moving. The engine would stay at 2200 RPM and when you need to accelerate (pressing on the pedal), the gear ratio of the transmission would change (taller) and you would go faster, if you let up on the pedal, the gearing would get shorter, and you would slow down. But your engine would stay at 2200RPM all the time. This is kind of weird and a bit disconcerting to many drivers, so most manufacturers of CVTs have programmed them to behind kind of like a normal automatic transmission (with shift points, so the RPMs go up and down as you drive), but that makes them less fuel efficient. Also, for performance (i.e. acceleration), you don't want the peak BSFC RPM, you want peak power whenever you are on the accelerator pedal, neutral when you not on either pedal, and a low gear when you are braking.*

*I'm kinda making up the performance part as I go along, take it with a grain of salt. I'm pretty confident of the efficiency part, as I spent quite a bit of time researching CVTs.

EDIT: The peak efficiency part is ONLY about CVT technology, and ignores the possibility of DFCO or neutral coasting.
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