This decision is far from a no-brainer.
1st make sure that you compare apples to apples:
The 12.5% nominal interest rate offered by the bank for financing the vehicle equates to an effective rate of 13.2%. This is the rate you need to use to compare to expected (annual compounded) investment returns.
Inflation is currently 4.4%, so 13.2% represents a real after inflation return of 8.8%.
This is the return you will earn on your money guaranteed after tax if you use your cash to buy the car, because this is the interest payments saved in that scenario.
8.8% real, after tax, guaranteed is a big number. The best guaranteed after inflation rate out there currently is the 1.75% offered on SA retail bonds. The bank will be very happy to earn that return off you.
I assume when you say you are earning 20% returns on your investment for the last few years, that you refer to the compounded annual growth rate (cagr- this is the return shown in the fund’s fact sheet). If not, you are not comparing apples to apples.
I had a look at the last 5 and 10 years SA equity fund returns and found that over 5 years only 5 out of 118 equity funds (6.8%) achieved a cagr of more than 20%, over 10 years 4 of 75 (5.3%) achieved this return. Index funds tracking the Alsi achieved about 17.5% over 5 years and 17% over 10 years.
I also looked at the last 40 years returns over 5 year rolling periods for the Alsi and the MSCI World index. On average the JSE returned 6.7% and the MSCI World 5.9% after inflation over the 36 rolling 5 year periods. It was also very risky with the worst 5 year period losing 30% and 54% of real starting capital for the JSE and MSCI investments respectively. Compare this to the guaranteed 8.8%pa ( ie. 53% real return after 5 years) you can secure by paying cash for the car.
Only 9 of the 36 (25%) rolling 5 year periods achieved a real return greater than the 8.8%pa real return you will get by paying cash.
In short I don’t think it is realistic to expect 20% returns going forward on your investment. At best you should expect to earn about 11% (5% inflation and 6% real return) over the next 5 years, with a significant probability of a negative real return.