16 MPG City | 12 MPG Highway ???
this is strange, i get a constant mpg of 16 in the city, i took it on a road trip where i did 80-90mph the whole way and filled my tank up and did the math and it comes to 12 mpg on the highway..... why would my mpg drop for highway driving, i dont get it

 

That's not highway driving, that's speeding.

The drop in economy with increase in speed is not linear.

 

The power required to move a mass is proportional to velocity to the 3rd power, so it takes alot more power to go a little faster.

 

so for the weight of the car, it would be more efficient at a lower speed, lighter cars would be more effecient at a higher speed, that makes sense, so what would be a good speed to keep for the best effeceincy?

 

speed limit...


Your lucky, you more than double my gas milage in town and i would really hate to see what mine would do cruising highways.

 

The mass of the car does not come into play when you are cruising at a constant velocity, it is all about aerodynamics. Assuming a constant drag coefficient, lets pretend your car takes 25 horsepower to cruise at 50 mph, it would then take 84 horsepower to go 75 mph, and it would take 200 hp to go 100 mph. You see the idea, it is not at all linear, it is a cubic relationship.

 

That is why the EPA tests highway gas mileage for cars using aerodynamic drag model assuming the car is only going 65 mph, that is why no one gets the gas mileage that the epa says their car is supposed to get, cause everyone speeds, even a little bit.

 

not only weight but aerodynamic drag
you go faster your make for a lot more drag

 

but there are times that you get better then what the EPA says you get also


I have quite a few times have gotten more then what the EPA says and generally in my mazda it was those times I was traveling at speeds faster then 65mph

my honda which is a higher mileage car was rated at 25/35 and yet I have gotten an AVG of 38mpg quite a few times doing a mix of about 40%city and the rest hwy

 

That is true, but generally consumer reports don't report what the epa does, but that is besides the point, the point is the slower you go, it takes relatively much less power. By relatively i mean if you are driving a semi or a ferrari.

P~v^3

 

agreed :-)


but will say the difference between a between speeds using a ferrarri vs a semi the difference is going to be much larger with that semi

 

after 65mph you're pretty much hurting fuel economy.

 

Plus, the way you drive in general helps. If you stay at a steady. consistent speed, instead of slowing down, then speeding back up or generally letting your MPH change by more than a couple either way, you'll get much better fuel economy.

 

Weight certainly is a factor in cruise economy if you're going up & down hills.

 

You need 4X the power to go 2X faster.