My Own Method:
Balance=(Loan Balance)
- [(Down Payment)
+(Trade In Amount, Or Scrap Value)
+(Interest Rebate)
+(COE Rebate, if any)+(Some Rebate)]
- Loan Principle=(List Price + Interest)
- Loan Balance=(Loan Principle - Amount Paid)
- Trade In Amount=The amount you can fetch from 1st car
- Interest Rebate {(Year till mature)*(Interest Rate)*0.01}
- COE Rebate (for example, signed COE was 12,000 but bid COE is 9,000, then you may get 3,000 rebate)
- Other Rebate (e.g., 3 month free installment etc.)
Examaple:
- List Price of 1st Car: 100,000
- Interest Rate: 2.5% per Year (3 year ago for example)
- 7 years bank loan=84Months
- Downpayment: 20,000
- Loan Principle: (100,000-20,000)+80,000*7*0.025=94,000
- Interest to Pay: 14,000
- Monthly Amount to Pay=1,120
Now I want to change current car after 3 years driving:
- Total Loan Paid=1120*36=40,320
- Bank Loan Balance=94,000-40,320=53,680
- Interest for 4 left years=14,000*4/7=8,000=Interest Rebate
- Trade In Amount=65,000 (Depreciated 35,000 the 1st 3 years)
- COE Rebate=3,000
Target=53,680-(65,000+8,000+3,000+2,000)=-24,320
For this example 24,320 can be cach on hand which can be used for downpayment for new car purchase.
* Other consideration
- COE Price
- Interest Rate Change
- Gas Price
Please note that I did not calculate this conservative manner, as I need to justify my changing car every three years to my family members.
The amount I've lost by owing a car is too big but I could not resist the charm of the latest cars.
CONCLUSION: 3 year seems acceptible for me.