Changing the tires will bring a mpg worst than stock trx, I really like the 37's but their 15-20lbs per tire heavier than stock. Unless we can find a 37" tire 60-70 lbs we lose a significant amount of mpg. FYI stock tire weight is 62lbs
Stock weight of the Goodyear Territory AT LT325/65R18 is 64.19 lbs. Jump that up to a 37", Goodyear has either 17 or 20 inch tires in a 12.5 configuration to support 37, that equals 79 lbs.
Tires run various weights in 37's, here's a list I found, most are accurate, some slight 1-2 lbs variances on few, but gives you an idea of weight to make a decision. Personally right now, I'm running 37x12.50R20 Nitto Ridge Grappler (weight 81.5 lbs) with Fuel Clash Wheels at 39 lbs, so my next tire will go most like to BFgoodrich All Terrain TA KO2 37X12.50R17 (weighing in at 73 lbs). It's not a huge difference, but nearly 10 lbs of inertia removed per wheel, so I'll take it. If you want a wheel under 30 lbs, you'll have to go Forged, but you will absolutely pay for it.
Discoverer AT3 XLT 37X12.50R17 68 lbs
Extreme Country 37X12.50R17 69 lbs
Courser MXT 37X12.50R17 69 lbs
Grabber AT X 37X12.50R17 72 lbs
All Terrain TA KO2 37X12.50R17 73 lbs
Wrangler MTR with Kevlar 37X12.50R17 73 lbs
Deegan 38 37X12.50R17 73 lbs
Discoverer STT PRO 37X12.50R17 75 lbs
Terra Grappler G2 37X12.50R17 75.1 lbs
Geolandar MT G003 37X12.50R17 76.1 lbs
Geolandar X AT 37X12.50R17 76.9 lbs
Couragia MT 37X12.50R17 77 lbsMud Terrain TA KM3 37X12.50R17 77.4 lbs
Discoverer ST MAXX 37X12.50R17 78 lbs
SSR 37X12.50R17 78 lbs
TrXus MT 37X12.50R17 78 lbs
VorTrac 37X12.50R17 78 lbs
Open Country RT 37X12.50R17 78 lbs
Destination MT2 37X12.50R17 79 lbs
Baja Boss 37X12.50R17 79 lbs
Grabber X3 37X12.50R17 79.7 lbs
All Country MT 37X12.50R17 81.4 lbs
Ridge Grappler 37X12.50R17 81.5 lbs
Wildpeak MT 37X12.50R17 81.8 lbs
Wildpeak AT3W 37X12.50R17 82.5 lbs
Dynapro MT 37X12.50R17 83 lbs
IROK 37X12.50R17 83 lbs
Baja MTZ P3 37X12.50R17 83 lbs
Krawler TA KX 37X12.50R17 83.6 lbs
Open Country MT 37X12.50R17 84 lbs
Trail Grappler MT 37X12.50R17 84.8 lbs
Baja ATZ P3 37X12.50R17 85 lbs