best route from New Mexico to South Dakota

It depends on what you want. The flattest and shortest may be through Kansas.

If there is lots of time then taking a more northerly route may be more interesting, but more challenging too. I love Mesa Verde.

Driving through Denver where the minimum speed is the double nickle may be stressful.

I25 N !40 E to I35 N to US81 to I90 E to Sioux Falls. We took I35 US81 last year to Mitchell SD fron South Texas We found some nice campgrounds along that route with Freecampsites.net Not all of them free but $5 -10-15 .

I-25 north will take you over Raton Pass. Not a bad pass as far as mountain passes go, but still a mountain pass. To avoid any mountains at all, you will need to drift over into Texas and then head north.

You have a big decision to make. You can head east and drive through the boring and flat Great Plains states or go through the mountains. A compromise might be going north on the interstate to Denver. Then you can see the mountains and make some short trips into them on the way. Eastern Wyoming is easy to pull. Then you can visit western SoDak if you want. I had a job in eastern SD and drove from Laramie. Since it was for work I the drove direct way through Nebraska which is boring as Hell except for the buffalo ranches.

Agree with the route stickdog suggested. We snowbird in Las Cruces and will be heading to Sioux Falls and points north next month. We generally use I-25 between Denver and Las Cruces and will be using I-80 and US 81 next month.

However, we took a slightly different route in February due to recurring snowstorms. We spent 3 nights in a Caballo Lake SP FHU campsite to wait for the storm to pass. Then we took I-25 to Albuquerque and I-40 to Santa Rosa, where we spent another night. Then US 84 and back onto I-25 to Denver to avoid icy roads in Santa Fe. You shouldn't have worry about snow, however.

Another flat route is US 287 from I-40 at Amarillo to Lamar, CO, then US 385 north to either I-80 or I-90. Not scenic until northwestern NE, but easy driving and little traffic. It can get you to the Black Hills, if you are interested in visiting that area. From there it is a relatively short day's drive to Sioux Falls via I-90.

If you want to stay off interstates and avoid big cities, your easiest route is US 70 to Clovis and US 60 to Amarillo. We used that route recently and nearly all of it--NM & TX--was 4-lane, so no problem with speeding vehicles getting around our RV.

Then you use any route going north from I-40 and it will be relatively flat.

Thanks everyone! I will head to Amarillo and then north.
Cool beans, thanks for the input.