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I've heard that ic engines are more efficient at wide open throttle and low rpm, i.e fifth gear and gas pedal fully depressed. Is it true? Doesn't this weaken the engine over time?

 
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No, going WOT all the time will kill your fuel economy. Running in the highest gear possible, so long as you can accelerate comfortably, will boost fuel economy.

The lowest mileage I've seen on my car was 30mpg after a few days of racing around. Generally, if I granny shift and cruise in 5th with the accelerator hardly pressed I get 40mpg.

 
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<< No, going WOT all the time will kill your fuel economy. Running in the highest gear possible, so long as you can accelerate comfortably, will boost fuel economy.

The lowest mileage I've seen on my car was 30mpg after a few days of racing around. Generally, if I granny shift and cruise in 5th with the accelerator hardly pressed I get 40mpg. >>



i usually get 39.1-39.6mpg and i usually shift into 3rd at or before 20mph, 4th at 25 and 5th at 35, unless im going up a hill or need more power for some other reason. ive never gone WOT, and not counting the first 3 tanks, ive never gotten below 34mpg


what you heard was probably that they are more effiecient than either older models or other manufacturers at WOT

 
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engines are more efficient at WOT that is why diesels are more efficient than gasoline engines (diesels have no throttle)
at wide open throttle pumping losses (pumping air into the motor) are minimized. However gasoline cars need to maintain a specific air/fuel ratio whereas diesels do not. Therefore with gasoline, more throttle = more gas. Optimally gasoline motors would deactivate cylinders on the highway to be able to run at WOT cruising at highway speeds (where only 30-40 hp is needed). But only benz and a few others offer this high tech soln currently.

-m

 
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Yeah, what i meant was that if im going at highway speed in 5th gear and i encounter a slope, should i downshift to 4th or go WOT for better fuel economy?

 
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for your given example, you should down shift to 4th to lessen the load on the motor when the load is too high for 5th. I don't know what it does for gas milage but it is better for your car.

 
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on Gas motors WOT will NOT improve MPG, if you come on a hill downshift to 4th and just cruise up the hill then shift to 5th, every downshift makes it easier for your engine to pull the car, but the higher the rpm the more gas you use.