Always drive responsibly and abide by the laws and guidelines that Japan and/or your command has in place.
I have been asked a few times about cars here in Okinawa and there is no doubt that people coming to Japan want to know about what they can get their hands on which is usually followed by what type of modifications are legal to perform on your car here. Now before we go any further there are a few things I want to get out of the way so you know the direction of this post. 1. This information goes for anyone who is planning on modifying anything on their car. That's right. Whether you want to make your car look like something out of Fast and Furious, want to make your SUV roll through the mud and up mountains with the best of them or simply replace your headlight bulbs this applies to you. 2. This post offers information on the areas you will need to look out for but does NOT offer regulation or actual standards. If you want to know actual standards please conduct your own research and contact authorities on this matter. The reason for this? I'm not into giving out bad or outdated information. I am very aware of the basics but when it gets to the nitty gritty (which is what you are going to have to be aware of) like decibels and lumens I honestly don't know the exact requirements as of right now and therefore it's best if when you read this and when you choose to make your modifications you do your own research (or suffer any potential consequences but we will get more into this later).
Let's start from the beginning:
Chances are if you are reading this you have not yet come to Okinawa yet and you have preconceived notions about what Okinawa will be like. If you are like many of the people who come through here (whether you want to admit it or not) you have this image of or similar to Tokyo Drift embedded in your brain and yes you've got that itch to get into the bucket seat of some car that you are willing to make your masterpiece. That's completely natural I mean come on you're about to depart for the affordable sports car capitol of the world right? Before we slip her into gear and take off down the strip there are some things that you need to keep in mind right from the get go. Okinawa does not have the hustling bustling sports car culture that some people might imagine because of what they have read, seen on YouTube or even imagined based on movies about Japan. As far as getting around here in Okinawa goes (and I speak from experience) sports cars are expensive, difficult to get around in and inefficient. It is for this reason that you won't see many Japanese people driving their sports cars on the road, they usually have them for track use only. Now this is not to say that you never see them on the road, they are out there but it's not like you might imagine.
Now I realize that this is about car modification here in Okinawa but I am sure that you are going to want me to explain why I just said that sports cars here are expensive, difficult to get around in and inefficient. I will go through these quickly and briefly but if you have more questions please feel free to leave a comment or email me at firstname.lastname@example.org and I will answer your promptly.
To say sports cars are expensive is much more than to say they will cost a pretty penny to purchase. In fact you may find here that it doesn't cost much at all to purchase a car outright. This, however, doesn't mean that the car will be affordable throughout the entire time you own it. Insurance and road tax are significantly higher on a lot of the more desirable sports cars out there. They also require a higher octane gas (if you are one of the people who is going to modify your car or has a high powered combustion engine and care about how it runs) than you can get from the on base gas stations (information based on current available gas octane os of June 30 2012.) You will also find that repairing and finding good (and I stress good here) parts for your car can also cost you a pretty penny. A lot of these cars are older and in some cases need parts special order. In other cases they can have parts replaces with newer models which are again expensive. There are also some cars out there, such as the Nissan 300zx, which requires a special mechanic to work on the engine because of it's complexity and therefore more money is involved. Another point which we will get into later is the amount of money it takes to actually modify and maintain these vehicles.
Difficult to get around:
If you are serious into getting a sports car it's going to be a stick shift. I do apologize to all of those people out there who feel that an automatic is a perfectly good sports car mostly because they don't know how to drive stick. For your own benefit learn and I promise you that you will never look at an "automatic sports car" the same way again. Trust me when I say that there is nothing like trying to drive back from Naha to Okinawa City on a Friday afternoon in bumper to bumper traffic with a stick shift let alone a stick shift with some sort of high performance clutch. I've been here. . . . I don't care how much you like sports cars or what your passion is for them this is not enjoyable. This same concept goes for trying to get back home for many of us who live in areas where the size of a sports car (and to be fair a large van or SUV) is difficult to maneuver.
Aside from cost usually sports cars don't offer the same ability to get your family (which many sports car people have) from point A to point B so you're going to need a second car and for many families here in Okinawa you might find yourself with only one parking space if any at all.
Of course these things are solely based on my opinion and for some these things may not be a problem but I am just explaining what I had said before.
Back to modifications
I think that it is safe to say that things you couldn't modify in your home state are the same things that you are not supposed to modify here. Ride height (both too high or too low), window tint, headlights/tail lights, tires, how your wheels sit (under the car, camber) and exhaust (decibel based). It is also important that in some rare cases you may run into a problem with adding things to your car like a roof rack or brush guard. I can't speak too much about this because I do not off road and therefore I do not have a full knowledge but I have seen people being ticketed (by military police) for having a brush guard on their SUV even through the brush guard was stock. Again just reporting what I have been made aware of which in this case I saw first hand. With all of this said this does tnot mean that you will never see cars with these types of "illegal" modifications here on the road. So if these modifications are illegal why do people have them? It's pretty much like anywhere else I suppose. You do what you want either until you get caught or until it's time for an inspection.
Enforcing the rules
So how are these rules enforced? To be completely honest how these rules are enforced are greatly dependent on how you compose yourself while you are driving. For example if you are drying a heavily modified car down the road like you would drive any normal car minding your own business, obeying the traffic signals/signs the odds are that you're going to be left alone. The only time that this is not necessarily the case is if you happen to be passing what many refer to as a safety check. To be completely honest I have never experienced one of these safety checks. I have heard about them from other people but I have not been pulled into one or even witnessed one. That being said I really can't speak about this.
Unfortunately the chances are if you are reading this you are going to be affiliated with the military bases. Your affiliation will mean that you are going through the gates at least once each day and therefore have a much better chance of being flagged for your car not meeting the standards required to pass an inspection. It is also much more common to be ticketed for modifications on a military installation.
Overall there are a number of details as to what can and can't be done to a car. My personal suggestion is to know what it is that you want to do to your car and then from there find out what your limitations are. If you are like me and prefer to stay within the limits while still doing as much modification as possible this is not out of reach. Good luck and please remember to drive responsibly.