Gear Ratio (rant)

[16LT4]

 

I know you have watched your fuel mileage a lot and think it is heavily dependent on temperature. Well I live in Wisconsin and have driven in over 100 degrees F of temp fluctuations from below 0 to the upper 90's. I can tell you your fuel mileage depends way more on SPEED than air temp.

 

I can specifically remember my best ever trip mpg. I was driving exactly 60 mph in September- around 60-70 degrees outside.

 

no vehicle that drives on the ground is most efficient at 75 mph. It is a fact that vehicles are more fuel efficient at or below 50 mph because any speed greater than that creates excessive wind drag. I challenge you to put the cruise control on 60 mph for 1 hour and tell me what mpgs you get.

 

http://www.caranddriver.com/columns/driving-for-fuel-economy

 

"Aerodynamic drag increases with the square of vehicle speed, and the engine power required to overcome aerodynamic drag increases with the cube of vehicle speed, so going 18 percent fasterfrom 55 mph to 65increases drag by nearly 40 percent and requires 65 percent more engine power."

Thanks for the lessons in aero. However, my statements regarding temperature's affect on mpg were made holding speed as a constant. You can't compare speed AND temperature together and obtain a meaningful result. But since you asked, I use cruise control every weekend and well over a few hundred miles at a pop, and I consistently get better mileage 70-72 than at 60 because the engine isn't lugging, ESPECIALLY when the ambient air temperature is above 60. As an aero aficionado, you must surely know that warm air offers less resistance and drag than does cool air, just like humid air is less resistant than dry air (given the same temperature)? So, depending on temp, 70 mph may actually have less wind resistance than 65 mph. But, engine peak efficiency comes into play as wel, as you don't want that engine lugging around sucking fuel. Miles per gallon is not a fuel flow rate, but a fuel flow through time coupled with distance traveled in time comparison. If a 1% increase in fuel flow per hour to travel, say, 60 miles (fuel consumed in that hour) results in a 2% reduction in time to travel that 60 miles, it no longer takes an hour to travel and you've saved fuel. Fuel mpg and speed is not a linear relationship.

[aseibel]

I consistently get better mileage 70-72 than at 60 because the engine isn't lugging, ESPECIALLY when the ambient air temperature is above 60.

 

OK, your statement above is littered with facts, such as: hot, dry air is less dense than cold, moist air. I'm not disputing any of that. I appreciate your thought out response.

 

I just don't believe you live in a world where your truck is more efficient at higher speeds than everyone else's. Air drag always increases with the square of speed.

Any decrease you get in drag due to lower air density only means that your engine is taking in that same hot air- thus lowering compression.

 

And even if you can optimize your engine speed, Your slightly higher engine efficiency will never overcome the much higher drag of your truck going 70-75mpg.

 

I'm not a physics professor and I can't prove it to you if you don't believe me. So feel free to drive whatever speed you want.