The strange thing my phone does when im in a car

So you guys know the mobile internet thing called 4G right? For some reason mine switches to H+ or just H, it is still able to connect to the internet but H+ seems to have the power of 3G and H the power of 2G

Now im wondering, is it the car radio doing this or the speed im traveling at? If its the speed then im confused because it still had problems connecting when we stood still. If its the radio, can someone please explain why? Im very confused why my phone acts so weird with wireless internet roaming in a car