car
Actually i think its awesome that you are taking the tape to your car and listening to it. Thats a classic listening environment. Not only that, but its so unorthodox and minimalist and honest compared to some bastard that doesnt even like music anymore who owns multiple sets of 3 thousand dollar monitors that were scientifically engineered to impress people who barely have decent taste in music anyways. So i applaud you.
BUT
Whats fun and naive is rarely the best method. And that goes for everything, not just mastering. So i think you should try to do some critical listening practice for now, until you save up for your monitors. This way, when they arrive youll be ten times more thankful and aware of what you are really doing with the equipment.
Take an old Beatles album like Meet the Beatles etc and play it over your home stereo with the EQ on your receiver set to "flat". No bass boost, no treble boost. Now turn it up to decent listening level and try to concentrate on the sound itself, not the song riffs. Ask yourself: is the bass a bit boomy? Or is it nonexistent? How are the mids? Do they seem sharp or piercing? Is the high end more dull or too splashy and artificial? If your speakers are decent and your stereo is decent you should be able to get that record to sound where the bass can be felt, but its not booming and groaning. The mids should let the sound have character and body, but not be harsh on the ears or completely sucked out like its coming from a tin can. The highs shouldnt be boosted too brightly that a nasty hiss is present, but if you go too low it starts to sound like a blanket over the speaker. Basically, you are using a well recorded album to tune your hi-fi to where it sounds nice and even. No too much of any 1 part of the spectrum etc. You will be tempted to boost bass to give your music impact and power. But resist for now and just let the bass be present and punchy enough to be felt a little bit but not too boomy. Do this for a few records and find the sweet spots that allow all the different albums to sound decent.
Now take your song and master it using those speakers and EQ balance. Try to get your recording not necessarily like the Beatles, but to listen to it critically the same way. Is it too boomy? Is it too harsh? etc etc Adjust your mixer eq for different instruments to get it balanced. It will be very hard to get it to sound as balanced as your favorite records. But just bear with it for now and do the best you can.
Once you have it mastered to where it sounded great on your home speakers, try listening to that same version in your car. Or with your headphones on. Or a friends house. You will be probably hearing entire instruments faded into the mix. Or suddenly the shaker is loud as balls and doesnt even sound like a shaker anymore. You might also hear details like conflicting reverb types that will make the recording instantly sound amateurish in a non-chic, shitty manner. Your first results will most likely be less than stellar. But you arent failing, you are succeeding by actually doing all this and learning to critically listen and starting to realize how bad things can sound on accident despite perfect intentions.
Through practice, you will learn what types of listening environments affect what types of frequencies in what ways. For example, i happen to know that for my cheap cpu speakers to sound the most honest and flat, i have to make them less bassy. Also, they dont lend the music any flattery in the realm of power or impact sonically. They make everything sound more distant and thinner. But, since i know this, i can actually do a decent job of mixing over them if i absolutely have to. Simply because i know not to try too hard to boost frequencies in the track EQ to over correct something that my speakers are doing to the sound, rather than what the sound really is and will be like on other stereos and setups. Its possible for me because i know these speakers and how good albums sound over them until i adjust them. Of course though, this is the hack method, but it also can teach you alot about how to listen honestly. If only we didnt have to spend all our time trying to fight with the inaccuracy of our speakers eh? EH?!?
This is where the concept of a "studio monitor" comes in. No matter what the level of design, money, cheapness etc etc that goes into making a studio monitor, the ultimate goal would be to represent the frequencies of your recording fairly flat and unbiased so that you dont have to do all that adjusting and guesswork that you were doing with cheap speakers or a car stereo. You will still end up adjusting your monitors further after they come out of the box. But, you will have a MUCH MUCH easier time mixing your tunes over them and getting decent playback on other peoples stereos.