Most efficient gear for Interstate Cruising?

Well, I do most of my country road crusing at about 65-70 and highway cruising at about 75-80 which means I'm taching around 2900-3000 rpm. I measured the past 3 tanks for mileage, and I average 30.2 mpg. On one tank full, where I know I was a bit more of a lead-foot than usual (ok, I was really putting my foot into it), I dropped it down to 28.5 mpg. Not bad at all, really.

[Jump]

Thanks for the replies and discussion. To back up PorscheDoc, things are inexpensive here. Also, the speed limit is 75 so the HP doesn't even bat an eye at 80 mph. There can also be some long, straight, boring roads when commuting from one community to another by Interstate, hence the original question on gas efficiency.

[mtch]

Originally Posted by denverpete

It's difficult to say what kind of penalty you'd have with the top down. That would be dependant upon the car's drag coefficient up versus down. But, my experience is that the top down significantly alters the aerodynamics of the car - and not in a good way. So, I would also expect to see a corresponding decrease in fuel efficiency.

A note on drag coefficient differential between top up and down. I went cruising top up a couple months ago and pulled from 70-148 using 4th and 5th gears. The car pulled hard and felt like it had plenty left when I let off. A couple of weeks ago, I tried the same pull and could not get above about 128 top down. I pushed to redline and could not climb any more. Not only would it not climb as high, it took much longer to get there.

At one time, I knew the equations for relationships between hp, vehicle speed and drag coefficient. I don't know anymore, and I'm to lazy to look them up, but experiencing the difference was astonishing to me

[blue2000s]

Originally Posted by mtch

A note on drag coefficient differential between top up and down. I went cruising top up a couple months ago and pulled from 70-148 using 4th and 5th gears. The car pulled hard and felt like it had plenty left when I let off. A couple of weeks ago, I tried the same pull and could not get above about 128 top down. I pushed to redline and could not climb any more. Not only would it not climb as high, it took much longer to get there.

At one time, I knew the equations for relationships between hp, vehicle speed and drag coefficient. I don't know anymore, and I'm to lazy to look them up, but experiencing the difference was astonishing to me

It's not a really simple formula. It also has to include the rolling resistance of the tires, which varies from tire to tire and you'd have to know what the wind was doing.

[Rail26]

That provided my humor for the day...Thanks! So let me get this straight...do the tumbleweeds and ex-wives go in the wood chipper in South Dakota?

Originally Posted by denverpete

'Round these parts we like to stick with MPG n'all. Not much use 'round here for GPM 'cept if'n you're talkin bout how much water it takes to get a horse from the farm to the big city.

Gotta go and clear out some tumbleweeds. Them critters just be getting ever where!

[Brucelee]

Pete is correct.

6th gear for 80 MPH.