Can I use an indoor cell phone signal booster in a vehicle?
In a word, no. The reason for this is that it is illegal because FCC regulations limit the gain on boosters installed in vehicles to 50 decibels (dB). The smallest indoor booster models have a 60dB gain. Now you see the reason why you cannot use an in-building, in-home, in-apartment, or any indoor cell phone signal booster inside a vehicle such as a car, truck, or SUV.
Secondly, it is also virtually impossible to achieve the Minimum Required Separation Distance (MRSD) between outside and inside antennas in most vehicles. This distance is required for an indoor booster to work properly.
If both antennas are too close together, they pick up each other's signals, thus creating a feedback loop. Signal boosters automatically detect a feedback loop and will shut down to prevent it from damaging the cellular network.
If oscillation is detected, the booster will not operate until the problem is resolved by increasing the separation distance between antennas.
The only exception is to install an indoor booster in a recreational vehicle (RV), provided the booster is only used when the RV is parked. Although the MRSD requirements still apply, the size of an RV and its rugged roof material often allows MRSD to be achieved.