I replaced my back tire last night.
The old tire - a Vittoria Randonneur, 28 width, in white rubber. (For summer, you know.) It had 2400-odd miles on it and I knew it was time to replace because I could see the red underneath the white. (I assume it's the puncture-resistant layer. In those 2400 miles I only had one flat.)
I was a little disappointed in the mileage. The previous tire - a 32 width with black rubber - accumulated 4000+ miles. Both tires are about the same price. I'm not sure if the mileage difference is due to the different rubber compound, or the width. I suppose the 32 has somewhat more rolling resistance than the 28, but not appreciably more. I'll take the increased mileage, if I have to choose one or the other.
I put another "32 black" on the rear - we'll see how she does.
In another month or so - when wet or icy roads become more common - I'll switch over to tires with more tread. Or just break down and ride the fat-tire bike.