To be honest, I don't think the tires by themselves will make a vast amount of difference. I suspect it's a combination of tire construction, tire pressure, bicycle style, weight, and fitness.
If you're considering replacing the tires, you might try running them at 45 or so and see if there's really any difference.
Also, double-check your gauge. I use a digital gauge from the auto parts store, and it's several PSI different from the built-in gauge on my Walmart floor pump.
Also check your seat height, which can make a big difference in ease of pedaling for any distance. You want your leg to be almost straight on the downstroke.
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