The tires are 10 inches wide on both vehicles. The distance between
the tires on the truck is 56 inches on the truck and 80 inches on the
trailer. This staggers them enough to make a frontal area for the
tires of 8 sq ft or 4 sq ft per side. The low point on the truck is the
front air dam which is only 7 inches off the ground. The Truck can
be treated as 72 inches wide for air dam purposes.
This means the low point on the truck adds 6.6 sq ft to the frontal
area of the rig in total. 8 sq ft for the tires totals to 14.6 sq ft. The
frontal area of the trailer is 72.75 sq ft and adding 14.6 sq ft to that
yields 87.35 sq ft. rounding up for skylights could bring this total up
to 88 sq ft and up to 90 sq ft.
If I can reduce the wake area of the trailer to 36 sq ft adding the tires
adds 8 sq ft to that for a total of 44 sq ft. So using the low figure I am
seeing.
88/44 = 0.5 which means the ratio is 50%. Half of that is 25%
1.25 x 8.2 mpg = 10.25 mpg.
There is some plumbing pipe under the trailer and the axle. Using a
higher figure of 90 sq ft yields
90/46 = 51.1% / 2 = 25.55 %
1.255 x 8.2 mpg = 10.29 mpg.
Basically these rough estimates are looking more and more like a
2 mpg improvement. That amounts to 122 gallons of fuel v 98 gallons
of fuel over 1,000 miles. At $2 a gallon that would save $48 of gas.
John Gilkison