Virginia will almost surely switch to once-a-year automobile safety inspections after July 1--but if your car becomes due for the currently required simiannual check before then, you still must meet the earlier deadline.

At its recently adjourned session, the General Assembly approved two similar bills repealing the twice-yearly inspection requirement and switching to an annual check. The change will take effect July 1 if, as expected, Gov. Charles S. Robb signs one of the measures into law.

The legislation also raises the cost of each auto inspection from $4 to $7 which, for most Virginia motorists, would be a slight annual reduction. But in Northern Virginia that saving is largely illusory, since owners of 1975-or-newer cars in all the suburbs except Loudoun County must now get separate annual exhaust-emission tests costing $3.50.

It will be up to the Virginia State Police to figure how to shift from the semiannual to annual safety inspections. Capt. Roy Terry, the department's safety officer, has proposed that future inspections be required in the same month a motorist's annual license tags are issued or renewed.

However, Terry has proposed a transitional plan--not yet adopted--for synchronizing the two sets of procedures. Motorists are likely to be confused during the first year of the transition. Details have not been announced, but one thing is clear: Since no parts of the new law would take effect before July, inspections made this month or in April, May or June would be valid only for six months regardless of when the car owner's tags expire.