there are a lot of factors here, and you provided very little information;
If the Only change was an increase in wheel/tire weight,
Then you will see a insignificant -> slight 0.1-0.2 mpg decrease in City FE,
with negligible/no impact on Highway FE.
If you change tire height (outside diameter or revs/mile) then yuo change the gearing, and height (which impacts aero).
If you change the tire width, then you change the frontal area (aero).
If you change tire model, brand, 'tread style', or even load rating (P -> LT), you change the tire's rolling resistance.
Even changing the wheel 'style' will change aero...
These are all small, nearly insignificant changes, but they can add up..
And typically when you switch from OE to 'larger' you Worsen every one of these -
(except for the gearing) ...
Then you get posts like "lost 1.5 mpg with tire change ! WTF?"
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