I hope I don't miss anything out.. Here's my understanding.
The rule of 78 is a loan that skews the repayment towards the interest rather than principal in the earlier parts of repayment. In other words, your principal will decrease much slower in the early parts of the loan.
Regardless of loan duration (1yr OR 10yrs), by the time you hit about 30% of your loan duration, you would have effectively paid close to 50% of the interest owed to the bank.
So if the total interest incurred in your loan is $20,000 for example in a 7yr loan.
By the end of the 2nd year, aprx $10,000 would have been paid to the bank as interest earnings.
If you service the entire loan without selling your car in the loan duration, you are not really affected. However, if you intend to sell your car in the earlier parts of the loan (e.g. 1-2 year), you'll notice that the principal has hardly gone down despite you paying alot more than that.
For a more detailed explanation, you can check out this url:
http://www.tiac.net/~mabaker/rule_of_78.html