Originally Posted by epnnf
I assume the wider the tire, the slower?
Not as a general rule, no. People often think that wide tires are slow for a few reasons:
Lots of wide tires aren't very speedy. This is true. Mountain bike tires, for instance, are very poor on pavement. But really, this is primarily because of the knobs. A lot of wide urban/touring road tires are also somewhat sluggish, but this is primarily because they're often built as thick kevlar-belted slabs of rubber, lacking the suppleness required for low rolling resistance.
Beach cruisers use wide tires, and tend to not be great at high speeds. But there are a lot of things that often slow them down relative to racing bikes that have nothing to do with tires. Like upright posture, high weight going well beyond tire choices, and often a low number of gears. The fact that the tires are wide isn't significant.
In tests performed by rolling wheels against steel drums, higher PSI shows less rolling resistance. Wider tires can't accept high PSI, therefore people sometimes assume they're slow. However, in reality, wider tires don't need to be pumped as high as narrower tires to achieve comparable rolling resistance in such tests.
Furthermore, in the real world, you can actually lose speed by pumping tires too stiff due to transmission of shocks to the bike and rider. The rougher the surface, the lower the PSI sweet spot will be for a given tire+bike+rider combination. On extremely rough surfaces, wide tires can have an advantage, since they can be run softer without damaging their sidewalls or rolling off the rim than narrower tires. This is easily seen in the mountain biking world, with people doing blazing fast performances over rough ground on very wide tires that are being run ultra-squishy.
Wider tires have a bigger aerodynamic profile. This is true, although it's worth keeping in mind that even a very
wide tire will be responsible for only a small fraction of the total aerodynamic drag of bike+rider. Also, the lower your speeds, the less significant aerodynamics is compared with other things holding you back. At 15mph I wouldn't even think about tire aerodynamics when making choices.
Wider tires are heavier. This is also true. Is it significant? A couple pounds added to the outsides of your wheels might slow you down by 1-2% on extremely steep climbs, and will make acceleration feel a little bit less snappy, but otherwise won't really affect speeds at all.
Additional benefits you get from wide tires are more confident cornering grip, and greater versatility for riding on bumpy and loose surfaces.
W/2", 70psi tires, how much slower?
70PSI is extremely
high for 2" tires, and you quite possibly wouldn't gain much or any comfort over what you're doing right now.
I have a bike with Compass Rat Trap Pass Extralight tires, which inflate to 53mm on my rims. At 165lbs, I inflate them to around 35-40PSI. That bike seems to do about the same on level pavement as my Emonda ALR 5 with 23mm Bontrager R3 tires that I pump to 90-100PSI, when I'm riding in the 20mph ballpark.
The Big Apple tires on that Kona? They're pretty thick and heavy, and the tread pattern looks like a compromise to improve sand performance. It'll be alright on roads but I'd guess somewhat behind a good pure road tire. If you try it and want to be speedier without giving up much width, you could look into something like a Compass Snoqualmie Pass.