PSI and better gas mileage

So I have been driving the old Honda lately and I've noticed my fuel economy tank. I hadn't changed my driving habits, I just had a oil change, they even 'filled up my tires'. Couldn't figure it out. However, my tires still looked low, so I checked them out and they were all at 30 but the max for my tires is 40 psi. I put another 5 back in my tires and Bam!-the gas mileage is back to normal.

here's a little pearl
"Gas mileage can be improved by 3.3 percent by keeping tires inflated to the proper pressure. Under-inflated tires can lower gas mileage by 0.4 percent for every 1 Pounds per Square Inch (psi) drop in pressure of all four tires."

I must have changed something else too, but either way, we're back on track. You can all stop worrying now...