In my experience, swapping tires on an EV like the R1T is a real balancing act because that instant torque and heavy curb weight can really punish the wrong tread. I've looked at the data on a few setups over the years, and here’s what I recommend for your specific mountain-to-commute lifestyle:

* The Pirelli Scorpion All Terrain Plus 275/65R20 (OEM version) is actually highly tuned for the truck’s cabin harmonics, but I totally get why you want more bite. They can feel a bit "slick" in actual mud.
* If you're considering the BFGoodrich All-Terrain T/A KO2 LT275/65R20, just be warned—they are heavy. In my testing, the extra unsprung mass and rolling resistance can tank your efficiency by 12-15%. Plus, they get pretty hummy on the highway after a few thousand miles.
* Honestly, I think the Nitto Recon Grappler A/T 275/65R20 is the sweet spot. They have a more aggressive lug pattern than the Pirellis but use a tread compound that stays surprisingly quiet at 70mph.
* Well, actually... if you want something really tough, look at the Nokian Tyres Outpost nAT 275/65R20. They use Aramid fibers in the sidewalls (the same stuff in bulletproof vests), which is great for rocky trails, yet they seem to preserve the range better than the BFGs.

That said, you're definitely gonna see some range loss regardless of which AT you pick... it's just the nature of the beast when you move away from the low-rolling-resistance rubber. Hope this helps! Let me know if you want the specific Wh/mi breakdowns I've seen.

I'd actually suggest a different approach. While those others are popular, they often compromise on wet braking and sidewall stability for a 7,000lb EV. Honestly, I’d look at the Yokohama Geolandar A/T G015 275/65R20. It's a safer bet with better hydroplaning resistance than the KO2s. Make sure you check the load rating—with that instant torque, safety-first is a must for your suspension’s longevity. Hope this helps!

Seconding the recommendation above. KO2s are heavy... Try Toyo Open Country A/T III 275/65R20 or General Grabber A/TX 275/65R20. Theyre cheaper and handle trails fine without crazy noise. Hope this helps!

Jumping in here because I've struggled with this too! Honestly, the BFGoodrich All-Terrain T/A KO2 275/65R20 was a huge letdown for me—too much noise and a massive hit to efficiency. Check out the Rivian Forums tire megathread or the "Tire Size Calculator" on TacomaWorld to compare diameters. I'd definitely look at the Falken Wildpeak A/T4W 275/65R20 instead; it balances that trail bite with a much better highway feel than the KO2s. Hope this helps!