Question: I just bought a new car, which will be used half for business, half for personal use. How do I go about figuring depreciation under the new tax rules? And what about the investment tax credit?

Answer: The new depreciation rules enacted as part of the Economic Recovery Tax Act of 1981 (ERTA) are known as the Accelerated Cost Recovery System (ACRS). They were intended to encourage capital investment by providing a rapid tax write-off, or recovery, of money spent on capital assets.

The hoped-for surge in investment hasn't materialized, perhaps because it doesn't make economic sense to upgrade or expand production facilities when demand is soft and the projected recovery--a weak one at that--is still in the future.

Meanwhile, the benefits of ACRS are available to anyone who, for whatever reason, makes a capital investment in business property.

You have a choice of two ways to go on writing off half the cost of the new car. Since the other half is for nonbusiness use, it provides no tax relief except that you can claim the personal half of sales tax paid if you itemize on Schedule A.

You can depreciate the car over a span of three tax years, starting with 25 percent of the cost in the year of purchase regardless of the month it was bought.

Then in the following year you take an additional 38 percent, and finally in the third year of ownership, you claim the remaining 37 percent. This gives you a recovery of 100 percent of the cost, with no requirement any longer to consider salvage value.

The alternative method is straight-line depreciation extended over 36 months but running through four tax years. The odd timing results from something called the "half-year convention."

You may also disregard salvage value when you use the straight-line method. In the year of purchase, you take credit for a half-year of depreciation, again regardless of what month you bought the car.

In practical arithmetic that comes to one-sixth of the cost. In your case, it's actually one-sixth of half the cost, since you claim only 50 percent business use.

Then in each of the two years following the year of purchase you take one-third of the applicable cost, leaving a final one-sixth to be claimed in the fourth tax year.

Note: Under the straight-line method, you have a choice of life spans. Since a car is three-year property under ACRS, you may elect a life of three, five or 12 years. I talk here of three years because normally you want to recover your investment cost as quickly as possible.

If you sell or otherwise dispose of the car before it is fully depreciated, you get no depreciation deduction in the year of sale regardless of the method used.

Regarding your second question: You may claim the investment tax credit (ITC) in the year of purchase against half the cost of the car. If you dispose of the car within 12 months after purchase, you must recapture the entire ITC by adding it to your tax in the year of disposition, not by amending the prior return.

Disposition during the second year after purchase requires recapture of 66 percent. If the car is sold after the three-year period, no recapture is required.

There is another option available that you should consider. Under ERTA, you may write off as an immediate expense up to $5,000 each year for expenditures to purchase capital assets that are normally depreciated.

So if you paid, say, $9,000 for the car, $4,500 of the purchase price applies to business use. You may write off the entire $4,500 as an expense on your 1982 tax return.

Of course, you will then have no depreciation for the coming years, and you forfeit the investment tax credit too.