Friend who is a great BSer, claims that changing from 700x30 to 700x25 or 700x23 will increase average speed by about 2 mph.
[ ] fact
[ ] fiction

I'm guessing that changing from a touring/mtb with standard wheels to a road bike is really what he is talking about. That change passes my smell test, but just changing tire width does not make sense to me. However, if it is true, I would like to be pleasantly surprised.

the tread patterns are both smooth? then your friend is wrong. It'll be easier to get a 25 through your brakes, though.

Kruezotter would suggest about 1mph or thereabouts improvement from 30mm to 23mm at typical cruising speeds.

Who's Kruezotter? And yes both tires have smooth center lines, no knobbies. these were x30 tires not x35

https://www.noping.net/english/

Play with the tire type and see how the speed changes. You have to make some assumptions. I ran a comparison of "narrow racing tire" and wide high pressure slick, and got around 1mph.

Depending on wattage, weight, which tires you select etc, it will move the results around.

do so at your own peril.

his next conversation with you will probably be about how he can't get through a turn with a little debris without wiping.

Going from a trek 7600 fx (hybrid with wider tires) to a Gary Fisher ar super (road with 23), I gained a little over 1 MPH.

Since converting my commuter to 23C tires for the summer, I wipe on a little debris pretty much every day.

You'd think I'd just learn to steer around it by the fifth wipe or so.

Sure, if you "forget" to reprogram your bike computer. Narrower tires are also smaller in diameter.

Besides the width per se, there are some very good (low rolling resistance) 23 and 25mm tires out there. The 30mm+ stuff is all pretty stiff and heavily built.

2 mph seems hard to swallow, though.

The increase in average speed will be less than the increase in cruising speed because the thinner tires will not be that much faster going up hills

If he's talking about the big picture like a typical hybrid equipped with 30c tires compared to a typical road racer equipped with 23c tires, he might be pretty close. If he's talking about the tires alone, he's full of she-at.

When the weather is sucking for a big charity ride, I'll leave the road racer at home and replace the 32c commuting tires on the cross bike with some 25c training tires and it gets me a hair more than 0.5 mph. Leaving the aerodynamic penalty associated with the mini-panniers filled with clothing at home gets me almost as much.

Once the panniers have been dropped, removing the rack hardly makes a difference. The bike is still 0.5 mph slower than the road racer but that's due to it's weighing another 5-6 lbs.

However, we are likely talking about different brands of tire (probably the OP's friend meant a cheap wide tire and a excellent narrow racing slick) and thus the gains could be more than kruezotter predicts because of dramatically reduced rolling resistance (e.g. the rr of an evo corsa CX is half of that predicted by kruezotter).

Sean

Earlier misstatement re kruezotters program edited out - looked again at the site and the rolling resistance does indeed drop for the wider tire.