Rule of 7's

[BurbMan]

Just got the burb back on the road and took her down to the lake over the hills and dales in SW KY. Ran better than it ever has, smooth as glass and plenty of power. It occurred to me that I can tow that trailer all day long at 70 mph, 7 days/week, and get 7mpg consistently. Hence the Rule of 7's. Actually 3 tanks computed at 6.92 with startling consistency....I need to consider if 65 produces any savings.

Fresh water was full, TT was about 8800 lbs FWIW.

[Dadoffourgirls]

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.

[Ron_Gratz]

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

[christopherglen]

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

[dclaarjr]

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Your speed data is out of date.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.

[jtobbe]

Your speed data is out of date.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.

[93Cobra2771]

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

[BurbMan]

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.

[Old-Biscuit]

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.

[BurbMan]

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.

[TubaPete]

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.

[Campfire_Time]

I know what I'll get at 70. Just a little better than what you get. I wonder what you would get if you ran 62 mph?...

[M_GO_BLUE1]

Might be amazed what happens to your MPG if you slow down...

[the_happiestcam]

Legally, Kentucky's max tow speed limit is 65.

[BurbMan]

So happens the speed limit is 70 on this stretch of highway called the Western KY Parkway. Very smooth road surface, extremely light traffic to the point where passing or being passed is rare.