A contrarian method, which I have used to good effect when cooking for folks I know like less tender meat (which I know sounds odd, but, actually, many of the farm and ranch folks I know do not like 'pink' meat).
Prepare a marinade of 1/2 cup Italian salad dressing and 1/4 cup BBQ sauce. I like Sweet Baby Ray's. You could also just add some ketchup, molasses and Worcsestershire sauce to make 1/4 cup total. Place tri-tip(s) and sauce into a vacuum bag or ziploc (vacuum sucker is better) and get all of the marinade in touch with the meat. Let sit for 24 to 72 hours.
Get a direct grill hot, to medium, and grill direct until lightly charred on outside and inside is done to temperature. I still like 130F internal, but, when cooking for others, 140F is pulling temperature. Then rest, slice and bask in the glory.
the meat thermometer was so far past the top reading, it read Taylor
"perhaps...but then again...maybe not..."