Tires 215R65 or 205R75, which has lower rolling resistance
 

Am I thinking correct that 205R75 has a lower rolling resistance?

Most cars built for extreme fuel economy use tall, narrow wheels. A larger diameter wheel should have less angular deflection (?) than one with a smaller diameter, so going up in diameter should in theory reduce rolling resistance.

A wider and a narrower tire with the same pressure should favor the wider tire, also in theory. The wider tire can carry more load, so at the same pressure it will deform less to the road. However, adding air to a narrower tire will net the same result, without adding all of the extra mass a wider tire has.

Narrower tires give a measurable improvement in aerodynamics.

Lighter tires eat less fuel in stop and go traffic and improve both handling and braking. Narrower tires have the huge benefit of being lighter.

Most important of all is rubber compound.

My vote, go narrow, add a little bit more air, pick a tire with a good LRR compound, like the Michelin Defender.

Long version: Barry's Tire Tech

Short version: Make and model of tire is way, way, way more important than tire size that this question is virtually irrelevant. But to answer your question: All other things being equal a 215/65R15 is 1.7% worse for RR than a 205/75R15. Don't forget, you can get up 60% better depending on what tire make and model you choose.

Wow! Great info on your site Barry! I have an old sienna van and it needs tires. So I want to do it as cheap as possible and maybe get better mpg. In the $60 buck tire range, I figure I could try 200. Or 190 tires. Would that help any? They would be cheaper too, I bet.