The number of fatal car crashes in Fairfax County jumped 17.2 percent during the first six months of 1986, according to statistics released yesterday by the county's police department.

At the same time, the county experienced a 14 percent decrease in robberies and a slight drop in other crimes against property, according to a report released by Col. John E. Granfield, police chief. The statistics for this year were compared with the same period in 1985.

The numbers of most other crimes handled by the department fluctuated only slightly from last year to this. At the same time, the department experienced an increase of nearly 4 percent in calls for service.

"We feel that we're holding our own in virtually every area, and in some, we're doing better than that," said department spokesman Warren Carmichael. "There are no real surprises."

Carmichael said the increase in fatal traffic crashes, from 29 during the first six months of 1985, in which 29 people were killed, to 34 this year, in which 36 people were killed, could be attributed to increased vehicular traffic on county roads.

The number of auto-related deaths involving alcohol decreased from 12 in 1985 to 10 this year.

The report indicated that county police also investigated more of all types of traffic crashes during the first six months of this year -- 8,878, as opposed to 8,543 last year.

Burglaries decreased slightly from 1,561 in 1985 to 1,508 so far this year as did auto theft, down to 704 from 715.

Larcenies dropped from 8,355 in the first half of last year to 8,022 in the same period this year. Robberies also declined nearly 14 percent, from 240 to 207.

The three categories of indicator crimes that increased were homicides, rape and aggravated assault.

While no homicides were recorded in Fairfax County during the first half of last year, there were eight from January through June of this year.

Spokesman Carmichael said the county usually averages 10 to 20 murders a year. "Obviously, 1985 was an aberration," he said.

Reported rapes also increased from 30 during the first six months of last year to 35 this year, and aggravated assaults from 140 to 180.

The report also indicated that police have been able to solve 74 percent of the rapes reported this year and 52 percent of the robberies -- rates that please them, Carmichael said. Of this year's eight homicides, only one remains unsolved, he added.

Fairfax County was able to solve 24.4 percent of all its crimes in 1984 as opposed to a national average for suburban counties of 20.9 in 1984.