If you can get 20 miles in 'perfect' conditions per day...
THen you can probably get 5-10 miles per day in imperfect conditions.
I do 20 miles per day, so this would be a 25-50% improvement in fuel economy, even in 'not good' conditions
I think it is definitely worth it.
These would be even better in a lighter, more efficient electric car, with a larger surface area of solar panels. Like that 3-wheeler that I posted in the 'electric cars' section
__________________