This route became a favorite a few years ago when Cottonwood road got paved to Gallatin Gateway. It's about 30 miles, with a fun, fast consistent downhill run on Cottonwood. For the return trip Gooch Hill road climbs much more steeply than Cottonwood, so a lot of your climbing is over fairly quickly.
This is a fun, fast ride without a lot of traffic. I've tried to limit the amount of busy roads you need to ride on.
Here's a Google Map.
From town, head south on Willson
At the light on Kagy, go straight onto South 3rd
At the stop sign, go straight onto Wagon Wheel Rd.
Next stop, make a left back onto 3rd.
Right on Goldenstein
Quick left onto 3rd
Stay on 3rd for a few miles until you get to a 4-way stop at Nash
Straight through the intersection, the road will make two 90 degree turns: a left and a right.
Make a right on Bristol
At the T, left on Nez Perce
You'll come to a stop at 19th. Make a left carefully, cars can be traveling up to 75 mph.
Stay on 19th for a few miles until you get to a T.
Left on Cottonwood.
Stay on Cottonwood, after about 3.5 miles (13.5 total) you'll come to an intersection. Straight is a dirt road, right Cottonwood continues. Make the right.
Another 90 degree turn, go left to stay on Cottonwood
Keep going until you get to a T at Rt. 191
Go right, being mindful of the traffic.
After about .5 mile make a right on Gooch Hill Rd.
Stay on Gooch Hill. It's a gradual climb until about mile 20, then it gets a bit steeper.
Keep going until about mile 24 and make a right on Stucky.
Stay on Stucky, crossing Cottonwood.
At the T, make a left on South 19th. Watch for traffic. The light at 19th and Stucky does not respond to bicycles, so you'll have to go through the red when the road is clear.
Head north on 19th for about a mile until you get to the light on College.
Right on College.
At the 4-way stop sign go straight. Be careful, this intersection is a nexus of stupidity.
At the next 4-way stop, make a left on South 8th.
Take 8th back to main.