State comparison · Updated October 2026
Idaho vs. Montana Driving Laws (2026)
Idaho and Montana differ on 5 of 9 topics we track: phone law, car insurance, car sales tax, new resident deadline and lane splitting.
Heading from Boise to Billings crosses the state line, and 5 of these rules differ once you cross. Main routes are I-84 on the Idaho side and I-90 in Montana; the capitals, Boise and Helena, set the rules for each side.
What changes most: the state tax on a $30,000 car differs by $1,800. Drivers based in Meridian heading to Missoula, or vice versa, live under both sets of rules: Idaho's on I-15 and Montana's across the border on I-15. Read the Idaho vs Montana guide when moving from Boise and Billings.
For a quick visit from Boise to Helena, keep your documents if you ride, set navigation ahead of time, and expect the same 80 mph top limit at the border.
| Topic | Idaho | Montana |
|---|---|---|
| Max speed limit | 80 mph | 80 mph |
| Phone law | Hands-free law | No texting ban |
| Learner's permit age | Permit at 14 | Permit at 14 |
| Car insurance | 25/50/15 | 25/50/20 |
| Car sales tax | 6% | None |
| Car inspection | No regular inspection | No regular inspection |
| New resident deadline | 30 days | 60 days |
| Helmet law | Under 18 | Under 18 |
| Lane splitting | Not allowed | Filtering only |
Driving laws side by side: Idaho vs Montana
Idaho vs Montana: Montana and Idaho agree on 80 mph for max speed limit and learner's permit age: Permit at 14.
Cell phone and texting laws: Idaho vs Montana
Idaho is stricter on phone law, with Idaho at hands-free law and Montana at no texting ban. Phone use that is fine around Boise can be illegal close to Billings on the other side.
Owning a car: Idaho vs Montana
Idaho vs Montana: Idaho and Montana agree on No regular inspection for car inspection.
Minimum car insurance requirements: Idaho vs Montana
The rule is higher in Montana on car insurance: Idaho at 25/50/15 and Montana with 25/50/20. Idaho is at-fault while Montana is at-fault. An insurance policy from Boise needs to be rewritten following a move to Billings.
Car sales tax: Idaho vs Montana
Montana is lower on car sales tax: Idaho at 6% and Montana with none. On a $30,000 car, state tax is roughly $1,800 in Idaho and $0 in Montana. Car hunters in Meridian and Missoula owe car tax of the state where they register.
New resident deadlines: Idaho vs Montana
Idaho is shorter on new resident deadline, with Idaho at 30 days and Montana at 60 days. Households relocating from Meridian into Missoula ought to book DMV visits in advance.
Motorcycle: Idaho vs Montana
Idaho vs Montana: Montana and Idaho agree on helmet law: Under 18.
Lane splitting: Idaho vs Montana
Montana is more permissive on lane splitting, with Idaho at not allowed and Montana with filtering only. Riders splitting time between Meridian and Missoula get no exception.
Crossing the line for good
- Idaho vs Montana: relocating from Idaho to Montana means 60 days for the car registration and 60 days for a license.
- Moving from Montana to Idaho: 30 days for registration and 30 days to switch licenses.
Idaho vs Montana: common questions
Which has the higher speed limit, Idaho or Montana?
Both Idaho and Montana top out at 80 mph.
Which state taxes cars less, Idaho or Montana?
Montana charges less at the state level (6% vs. None).
Is a motorcycle helmet mandatory in both Idaho and Montana?
Idaho: Under 18; Montana: Under 18.