if i'm driving down the highway...what is the perfect speed for best MPG?

km/h or mph

if i'm driving down the highway...what is the perfect speed for best MPG?

km/h or mph

The common claim has always been 80kph (50 mph).

In today's pressurized lifestyles, doing 50 on a highway is likely to cause a collision.

I average 110 kph (68 mph) with my '04 Toyota Matrix 1.8L and get 33.5 mpg.

At 100 kph (62 mph) I get 35 mpg.

Theoretically it is the speed when the engine is turning the slowest (top gear and torque converter is locked up). This is where the wind drag is lowest. It depends on the car, but you should be able to figure it out while driving. Accelerate slowly and watch your tach needle...you'll see it drop when it shifts into top gear and then a little more when TCC locks up, typically around 45 mph.

Technically, the drag force increases as the square of the car's speed, which is why it doesn't really become a factor until you are travelling upwards of 55 mph. Here's a nice wiki on automobile drag coefficients...some stunningly unexpected numbers for some of the cars listed:

http://en.wikipedia.org/wiki/Automobile_drag_coefficient

Hope this helps!