State comparison · Updated October 2026
Montana vs. South Dakota Driving Laws (2026)
Montana and South Dakota differ on 5 of 9 topics we track: phone law, car insurance, car sales tax, new resident deadline and lane splitting.
A drive from Billings to Sioux Falls takes you across the border, and 5 of these rules switch at the line. Main routes include I-90 on the Montana side and I-90 in South Dakota; the capitals, Helena and Pierre, set the rules for each side.
What changes most: car tax on a $30,000 purchase differs by $1,200. Residents of Missoula heading to Rapid City, or the other way around, follow both sets of laws: Montana's on I-15 and South Dakota's after the state line on I-29. Use the Montana vs South Dakota guide before driving from Billings to Sioux Falls.
For a weekend trip from Helena to Pierre, keep your documents if you ride, plan the route before leaving, and expect the same 80 mph top limit at the line.
| Topic | Montana | South Dakota |
|---|---|---|
| Max speed limit | 80 mph | 80 mph |
| Phone law | No texting ban | Texting ban only |
| Learner's permit age | Permit at 14 | Permit at 14 |
| Car insurance | 25/50/20 | 25/50/25 |
| Car sales tax | None | 4% |
| Car inspection | No regular inspection | No regular inspection |
| New resident deadline | 60 days | 90 days |
| Helmet law | Under 18 | Under 18 |
| Lane splitting | Filtering only | Not allowed |
Driving laws compared: Montana vs South Dakota
Montana vs South Dakota: South Dakota and Montana agree on max speed limit: 80 mph and learner's permit age (Permit at 14).
Phone law: Montana vs South Dakota
The law is stricter in South Dakota on phone law, with Montana with no texting ban and South Dakota with texting ban only. A call that is legal in Billings can be illegal around Sioux Falls once you cross.
Owning a car compared: Montana vs South Dakota
Montana vs South Dakota: Montana and South Dakota match on car inspection: No regular inspection.
Car insurance: Montana vs South Dakota
South Dakota is higher on car insurance: Montana with 25/50/20 and South Dakota with 25/50/25. Montana is at-fault and South Dakota is at-fault. An insurance policy from Billings must be switched when you settle in Sioux Falls.
Car sales tax: Montana vs South Dakota
The rule is higher in South Dakota on car sales tax: Montana at none and South Dakota with 4%. For a $30,000 purchase, state tax comes to roughly $0 in Montana versus $1,200 in South Dakota. Car hunters in Missoula or Rapid City pay car tax based on their registration address, not the dealer's.
New resident deadlines: Montana vs South Dakota
South Dakota is longer on new resident deadline: Montana at 60 days and South Dakota with 90 days. Anyone moving from Missoula to Rapid City ought to book DMV visits in advance.
Motorcycle: Montana vs South Dakota
Montana vs South Dakota: Montana and South Dakota match on helmet law (Under 18).
Lane splitting laws: Montana vs South Dakota
The law is more permissive in Montana on lane splitting: Montana with filtering only and South Dakota at not allowed. Motorcyclists traveling between Missoula and Rapid City must change habits at the line.
Moving or buying across the border
- Montana vs South Dakota: moving from Montana to South Dakota means 90 days to register the car and 90 days for a license.
- Going from South Dakota to Montana: 60 days for registration and 60 days to switch licenses.
Montana vs South Dakota: common questions
Where may you drive faster, Montana or South Dakota?
South Dakota and Montana each allow up to 80 mph.
Where is the car tax smaller — Montana or South Dakota?
Montana charges less at the state level (None vs. 4%).
Do I need a helmet in both Montana and South Dakota?
Montana: Under 18; South Dakota: Under 18.