The very long road from Salmon to Mud Lake/Terreton (28) ~125 mi. map car used/pic1 offers some of the most scenic and plausible ways of reaching top speed of your car anywhere in America. I absolutely adore this drive - very highly recommended! Another favorite is I15 north of Dubois going into Montana. map Much of it is ruler-straight and well kept so you know what that means... wheeee! The mountain views in the distance are phenomenal in case you'd like to look to the sides every now and then in the middle of your speed fix. Also nice is a curvy (but flat) stretch from Lolo, Montana to Kooskia (12); map this road follows the outline of a pretty stream and there's no towns between the endpoints to slow you down. Rt. 55 from Boise to Grangeville is another similarly scenic and curvy drive.