That decrease isn't completely out of the ordinary for a few reasons
Heavier tires obviously require more power to turn
If you went wider a larger contact patch creates more friction, requires more power
A more aggressive tread pattern with squared off blocks are harder to roll. Look at " fuel saving tires" they all have rounded off tread blocks and grooves
New tires in general see a decrease in gas mileage. They are heavier than their worn counterparts. Plus the deeper tread flexes more than a shallow tread. This means energy that could be going to moving you forward is going to flex those tread blocks and then to move you
In addition to that, you are also now less aerodynamic because of the height increase
Check your tire pressure too if that's too low it will hurt your mileage. With oversized tires the best way to determine air pressure is to draw a chalk or grease line across your tires and drive a few feet then see if it's worn even across the tread. If it's worn on the sides your pressure is too low if it's worn in the center your pressure is too high.
After your lift did you have an alignment done? Camber issues from an alignment can have a small effect on mileage because it causes the tires to not contact the road squarely
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