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Old 11-02-2013, 06:48 PM   #1 (permalink)
sheepdog 44
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Transmission Efficiencies. MT, Auto, CVT.

This question has come up a lot: How efficient is a CVT/MT? Why do some CVT cars have better EPA mpg ratings than their MT counterparts?
Since i've looked into this in the past, it's better to link direct sources than to respond from memory. I'm not an expert, these are my general observations...

Transmission type Efficiency
Manual neutral engine off.100% @MPG <----- Fun Fact.
Manual 1:1 gear ratio .......98%
Manual other gear ratios .96-97%
CVT toroidal ......................93%
CVT belt ............................88%
Automatic .........................86%

None of the CVTs have efficiencies as high as manual transmissions: about 96% in all but the 1:1 gear which is about 98%...... -Note that manual transmissions in standard automobiles do not require water cooling. If a particular CVT requires water cooling, that is proof that it is less efficient than a manual transmission.
Read more: Physics Help and Math Help - Physics Forums
If a CVT can be up to 8% less efficient than a MT, why doesn't a car with a CVT get 8% worse fuel economy?

From a mathematical viewpoint, a transmission is only one component of many that determines overall efficiency. BSFC Engine thermal efficiency% X Trans efficiency% X Rolling resistance% X Friction etc etc. All of these eat at the fraction of how efficient the engine is. For example MT vs CVT at 35% engine efficiency: (.96 x .35) - (.88 x .35) = 0.028 , Or you lose 2.8% overall efficiency MT vs CVT in this instance.

On page 34 of this link, you can see a CVT can cherry pick the best BSFC curve for efficiency. Sometimes at the expense of torque or power.

Many Manual Transmissions are geared for low end acceleration, NOT efficiency. Many don't have a tall enough top gear for decent highway cruising; going a couple hundred rpm over what you would want it to. Not to mention a proper overdrive gear for efficiency.

People are stupid! A CVT will seamlessly lower an engines rpm at any road speed. Giving better mpg and lower emmisions. And there have been vast improvements in CVT efficiency lately which go unreported as proprietary information from automakers.

All this leads to more MPG's from a less efficient transmission. A couple less horsepower lost, a second less to 60mph, and 50 more lbs.

I try to be helpful. I'm not an expert.

Last edited by sheepdog 44; 11-16-2013 at 08:00 PM..
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