What's your VO2 max? This ride will test it for sure. The exact elevation gain isn't known but it's a lot!
Old Red River Pass is an out and back ride with an option for the more aerobically fit riders. Follow Forest Service Road 488 up to the top. At this point you can either turn around and head back, or continue on into the 4th of July Canyon for some serious climbing.
How to get there:
From Red River, head South on HWY 578 to Forest Service Road 488 on your left. Park here.