SINCE PRESIDENT FORD'S politically posthumous tax-cut proposals aren't going anywhere but into the file, it's hardly necessary to dissect them in detail. But Mr. Ford's parting guidance on tax matters has unavoidably raised one pressing issue that Presidents and Congresses have been uneasily stepping around for years: Social Security is financed by the most ruthlessly regressive tax in the country, and - inequities and all - that tax is growing at great speed.

In fact, the average family now pays more in Social Security payroll taxes than it pays in federal income taxes. For every dollar deducted directly from your paycheck for Social Security, there is another dollar that your employer pays for you. The economists who have studied the matter report that your employer considers the Social Security tax to be part of your wages, like any other fringe benefit, so that in effect you - the employee - are actually paying both halves. The Treasury Department says that a family will pay $1,756 this year in Social Security taxes - half through direct payroll deductions, the other half indirectly through the employer's contribution.

The difference between these two taxes is that, for all of its loopholes and defects, the income-tax law puts a higher rate on higher incomes; and it provides a great variety of deductions and exemptions for taxpayers with special burdens and necessities. By contrast, the Social Security tax has no exemptions or deductions. It is a flat rate levied on the first $16,500 that you earn, whether you're living in poverty or Palm Springs. But it's not levied on investment income, or capital gains or anything you might win at the race track.

Social Security is not like a commercial pension, in which the insurance company invests your contributions and returns them to you when you retire. Under Social Security, this year's contributions from one person are paid out in this year's benefits to another. That's why the system, over the next decade, is likely to run a little short. The last recession, unfortunately, hit the country harder than Congress (or anyone else) expected. Unemployment has been much higher, retirements more numerous, and wages somewhat lower than the financial technicians assumed when they worked out the present rates. The result is that, even with scheduled increases in the Social Security tax rate over the next several years, the tax won't raise quite enough money to meet the system's obligations and maintain a safe level of reserves through the early 1980s.

Here, we come to the choice. Congress can either just keep on raising the payroll tax rate - it is scheduled to go to 12.6 per cent in 1981 - aggravating all of its inequities. Or it can begin to shift part of the burden to general revenues - which means the income tax. President Ford firmly favors raising the payroll-tax rate again. He also favors cutting personal income taxes. Taken together those two policies would shift the burden of federal taxation away from the progressive income tax and increasingly onto the regressive payroll tax.

There's a better way to do it. The time has come to begin using income taxes to support the Social Security system. Fierce resistance will come from the traditionalists who fear that the whole system would be jeopardized if taxpayers ceased to think of Social Security as an earned right. But an elegant compromise was suggested by the most recent Advisory Council on Social Security: The current tax includes 1.8 per cent of Medicare hospital benefits. Why not let general revenues pay for that Medicare insurance? The income from the present 1.8 per cent Medicare payroll tax could then go to the Social Security pensions. That increase in the pension fund would be more than enough to carry it through the next decade.

President-elect Carter has talked a good deal about tax reform. That term is generally understood to refer to the correction of the inequities in the income tax law. But whatever its shortcomings, the income tax makes thousands of concessions to the ideal of fair treatment. The payroll tax makes none.