TAMPA ó MacDill Air Force Base has long been a key driver of the Tampa Bay economy, spending billions on payroll, construction and retiree pensions.But just how much of a powerhouse the base is depends on how you look at the numbers presented Wednesday by MacDill officials.Thanks largely to a jump in the number of local jobs the base creates, officials say MacDill has increased its total economic impact on the region by 8 percent over the last three years, to $3.18 billion in 2017. Base officials got that number by combining total payroll, local expenditures and the value of local job creation. But when you factor in the pensions of military retirees and surviving spouses living within 50 miles of MacDill, there is an overall drop of about $600 million between the fiscal year ending Sept. 30, 2014, and the one that ended last September.The reason: The ranks of those retirees and surviving spouses has dropped by nearly half, from 73,000 in 2014 to nearly 38,000 last year. That cut the baseís overall economic impact from $4.7 billion in 2014 to $4.1 billion last year.The base doesnít include the retirees and surviving spouses in its economic impact methodology, but included the figures in its economic report to "highlight the tremendous impact they have on the local economy."And none of the figures presented by MacDill include the billions spent every year on items like weapons, vehicles, drones and other military procurements by U.S. Special Operations Command and, to a lesser extent, U.S. Central Command, two unified combatant commands on base. Overall, defense spending brings about $7.5 billion in total direct spending to the seven-county Tampa Bay area, according to a study released in December by Enterprise Florida.The report "reinforces the significance of MacDill Air Force Base to our community," said Tampa Mayor Bob Buckhorn. "Not only is it a critical linchpin to our war-fighting capabilities, it is also an economic engine that is unrivaled in our region."The biggest change in MacDillís economic impact between 2014 and 2017 is an increase of $240 million in the value of job creation, a statistic that multiplies the number of jobs the base claims to have created by their average annual salary.In 2017, the number of jobs MacDill said it created rose to 30,000 from 24,400, while the average annual salary decreased 1.5 percent to $46,080.Part of that job creation can be attributed to an increase in base personnel, said Air Force 2nd Lt. Allison Mills, a base spokeswoman. The base saw a 20 percent increase in personnel between 2014 and 2017.Last year, there were nearly 23,000 military and civilian workers on base, compared to nearly 19,000 in 2014. Drilling deeper, the number of military personnel increased by almost 4,800 while the number of civilians dropped by about 900.Mills attributed the overall increase to eight additional KC-135 aerial refueling jets at the base, bringing the total to 24, a new refueling squadron and a move by the Air Force to increase personnel across the flying branch.The base has also reduced the amount of money it spends directly in the community by about 3.3 percent, to $705.7 million. Thatís mostly due to a fall off in construction.Contact Howard Altman at [email protected] or (813) 225-3112. Follow @haltman.