Hi... I've recently switched from stock 17" to aftermarket 20". First thing to consider is the tire profile you'll put on those 20s. If the overall diameter remains unchanged (i.e. new rubber should have lower sidewall) no problem will arise in terms of speedo indicator nor gas mileage. So for a 20 rim tire profile should perhaps be a 50 or 60 instead of stock 75, just to yield a total 32 or 33 in. Let's do some maths here...
265*0.75*2+17*25.4 = 843,9 mm overall diameter => 33 in
275*0.65*2+20*25,4 = 865,5 mm overall dia. => 34 in
1 inch difference will make little in gas mileage, I think...
Radical changes tho, you'll notice in handling. Can't tell a word on riding on snow, since I live in the Caribbean (perhaps half a century later, climate crisis, y'know
) anyway... ridin' is harsher, lo-profile tires copy better any surface unevenness, theres a noticeable change in steering tact also... many people improves steering famping due to this.
Good luck... Hope you'll be back home soon.