23c vs 25c tires

The differance is not so much the rolling resistance between 23-25's it's more to do with the added weight on the outside of the rim adding a significant amount of rotational mass therefore slower acceleration and harder work climbing.

If your worried about rolling ressistance try Specialized Roubaix's they have a 23c tread on a 25c body.

+1 it's weight and a slight increase in air resistance. Personally I run 28 on the back for durability + easy ride and a 23 or 25 on the front to save a bit of rotating mass.

Some studies have actually shown a wider tyre has less rolling resistance (up to a point we are not talking 2" here). It's quite a complicated relationship.

I can't find it at the moment but there was a study that was done as MIT to find the best wheel diameter. The result for pure rolling resistance was near 16", the only problem being going over bumps takes more energy. This is why a Moulton rolls so well, small amount of suspension to help with bumps and 16" wheels. Hence Moulton bike hold several cycling records and are band from road races.