Tire Pressure on gravel
A Cushion of Air
When you ride a bike you float on a cushion of air, but how big a cushion can make all the difference. When riding on the road compared to gravel, it makes sense to have more air pressure in your tire to reduce rolling friction and put more of your energy into going forward. Because most roads are smoother than gravel, you don't pay a price in terms of comfort or control. On gravel, though, reduced tire pressure will not only increase comfort and control, you'll also go faster. And over-inflated tire on gravel will slip more, since less of the tread is on the gravel to increase grip. Also, when you hit a bump on a gravel road the tire will bounce, causing the forward motion to go up in the air instead of straight ahead; that's why a lower tire pressure can actually increase speed even while it increases rolling resistance.
Adjust for Gravel
When riding partly on the road and partly on gravel, I suggest keeping your tire pressure where it should be for gravel riding; uncomfortable gravel riding is much worse than going a little slower on the road (if you can even tell the difference in speed).
What’s Right?
So what's the 'right' tire pressure? This has to do with how wide your tires are, how high your tire is off the rim (measured by what's called the 'aspect'), the weight of the rider, and riding style. That's a lot to factor in. But in general you can do this: on the side of most bike tires you will see a recommended range of air pressure to put into the tire. When gravel riding, try inflating your tires to meet the minimum recommendations. See how that works, and then you can decide whether or not to increase the pressure.
How Low Can You Go?
What about trying going below the minimum? There are riders who do this, but for this blog I'm going to stick with the pressure range recommended by the tire's manufacturer. If you do decide to try lower pressures, you'll know you've gone too low if you start feeling the rim bottom out through the tire, and especially if that causes the tire to go flat. Silca has a handy online tool for recommended tire pressure that takes everything I mentioned into account: https://silca.cc/pages/sppc-form
A Really Deep Dive
For really deep dives into the physics of how tires work on a bicycle, there is no better writer right now than Lennard Zinn. Here are two recent FAQs he's posted that include discussions of tire function:
https://www.velonews.com/gear/technical-faq-tire-grip-wet-conditions/