Rule of 7's

93Cobra2771 wrote:
I wager you'll pick up at least 1mpg, perhaps closer to 2mpg...

I wager that he will pick up .1 to .3 mpg at max if he does the same trip. That 8.1 is really consistent towing.

Now if he slows down to 55 mph, he might get closer to 2 mpg.

BurbMan wrote:
Thanks guys, will try 65 on the next trip out and report back....I recall seeing that wind resistance increases at the square of the speed or something like that so that is likely the killer here.

The aerodynamic resistance increases approximately as the square of the relative air speed,
but the power required to overcome aerodynamic resistance increases approximately as the cube of the relative air speed.

The power required to overcome drag force at 70 mph is approximately 25% greater than at 65 mph.
(In keeping with the title of this thread, the power required at 65 mph is about 77% of that required at 70 mph.)

The difference in fuel consumption would depend on how much of the engine power is being used to overcome aerodynamic drag, along with any difference in engine efficiency at 70 mph versus 60 mph.

Ron

That list was from 2006, I assume a lot has changed in 7 years..

jtobbe wrote:
Your speed data is out of date. Kentucky does not have a separate towing/not towing speed. The rural interstate speed limit is 70 for all.

Ohio changed to this also as of July first.

Your speed data is out of date. Kentucky does not have a separate towing/not towing speed. The rural interstate speed limit is 70 for all.

I wager you'll pick up at least 1mpg, perhaps closer to 2mpg...

Yes, have seen that link. Just curious why they wouldn't post it if it was lower than the regular limit. Not unusual to see speed limit signs that say "speed limit 65, trucks 55", or over a certain GVWR or etc.

I upgraded to LT tires after last year's blowout and these are rated at 75 mph.

BurbMan wrote:
Thanks guys, will try 65 on the next trip out and report back....I recall seeing that wind resistance increases at the square of the speed or something like that so that is likely the killer here.

Didn't know that KY has a max towing speed of 65, you would think that would be posted on the speed limit signs, most of the rural highways are 70.

Yes KY max towing speed is 65 mph........
Towing Speeds etc. by State

Just curious, have you checked what the speed rating is for your trailer tires is??
Most are rated for 65 mph max.

Thanks guys, will try 65 on the next trip out and report back....I recall seeing that wind resistance increases at the square of the speed or something like that so that is likely the killer here.

Didn't know that KY has a max towing speed of 65, you would think that would be posted on the speed limit signs, most of the rural highways are 70.

M GO BLUE wrote:
Might be amazed what happens to your MPG if you slow down...

I would have to agree. 5 mph would likely give you another mpg, maybe more.

My rig is different than yours but the difference between 60 and 65 is around 1 1/2 mpg.