Thread resurrection.
Been looking into this again. Part of the reason tire width makes such a big difference in the real world is that wheels have such a bad CD that a small change in their frontal area has a big impact on the total CD of a car.
Tires and wheels count for around 50% of the drag in a streamlined car like the Prius but are only a small part of the frontal area. Seemingly small changes in wheels should make a proportionally larger difference in total drag.
The air under a car flows outward from the centre and hits the tires at a yaw angle. Because of this all four tires are, for practical purposes, part of the frontal area. If we assume that's the case this may then double the frontal area change from the usual assumption when estimating changes in FE from wider tires. (I didn't do this in the following calculation)
At highway speeds these changes would far outweigh changes in RR.
This would seem to back up the assertions of many Prius owners who have seen a big reduction in their MPG from a increase in tire width.
Plugging some numbers into the
calculator also backed up my thoughts on the subject.
I got about a 4% reduction in mpg from wider tires using the following:
Data from
http://www.energy.ca.gov/transportat...%20Testing.pdf(thanks to Capriracer for finding that.)
In that study the CRR dropped .002 going from a 175 to a 235 which closely matches the change in width in the Hucho study Darin posted.
CD + .026 (Hucho)
Frontal area + .4 square feet. (calculated)
Results at 60 mph Narrow tire: 45.67 mpg wide: 43.74 mpg: ~4% difference
@ 75 mph 5% difference
At 40 mph it's a wash.
There are no hard numbers showing a strong correlation between tire width and RR. There does seem to be a slight reduction in CRR for wider tires of the same dia. but there would need to be a huge reduction for it to wash out the hard CD numbers we have from Hucho.
That's my 2 cents based on the limited data and using the calculator.