A 120-hour fuel permit is available separate from or in conjunction with the 120-hour Registration Permit for carriers who are operating interstate (crossing state lines) and do not have a current IFTA license in any IFTA jurisdiction. There is no limit to the number of temporary fuel permits that may be purchased. If you will not be crossing state lines or if you have a current IFTA license, you do not need a temporary fuel permit.
Fees are: $60 per permit.
A fuel permit is required if you are operating a vehicle which is:
Duration - 5 days (120 hours)
If the Power Unit is registered for Idaho and the trailer is registered with Idaho or another state: No Permit fee is required
If the Power Unit is not registered for Idaho and the trailer(s) is registered in Idaho or another state: the Single Permit Fee is $60.00
If the Power Unit is registered for Idaho, but the trailer is not registered in Idaho or another state or if hauling piggy-back: the Single Permit Fee is $60.00
If the Power Unit is registered for Idaho and two or more following trailers are not registered at all with any state: the Combination Permit Fee is $120.00 (no matter how many trailers are, the maximum is $120.00).
There is a limit of three (3) permits per vehicle, per calendar year. After the third permit, the vehicle must be registered.
This is for vehicles which are already registered for Idaho, but wish to temporarily operate at a higher gross weight than currently registered for. You may purchase a temporary permit to operate a vehicle or combination of vehicles in excess of the current registered gross vehicle weight up to 105,500 lbs (divisible loads only and not to exceed the legal axle and/or gross weights allowed by Federal Formula B). Temporary permits to increase gross registered weight must be purchased prior to movement on the highway. Registration permits purchased for vehicles hauling reducible loads in excess of 80,000 lbs must also purchase an annual excess weight permit. Extra length permits may also be required.
Source of information: https://itd.idaho.gov/wp-content/uploads/2016/06/MotorCarrierServicesManual.pdf