I think it would help to a point of around 30 % rather then 60% to the fact of the sq. ft coverage of the roof as compared to the sides of the car which would be a higher temperature on at at least one of the darker sides at a time. Convection heat will be cooking the sides. I think a light color or shade is all that is needed, Without being to much of a stickler.
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Save gas
Ride a Mtn bike for errands exercise entertainment and outright fun
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