The town of Tisbury is considering withdrawing from an arrangement with the Martha’s Vineyard Hospital under which it provides an ambulance for off-Island transports.

The reason is that the agreement, which originally was hoped to provide funds sufficient for the town to upgrade its ambulance service, is not paying for itself.

A letter has been drafted to the hospital, seeking talks on possible ways to amend the agreement, so the town will be able to continue operating without placing an extra tax burden on residents.

Tisbury finance director Tim McLean said yesterday the town is “barely breaking even” on the direct costs of running the service, but not on the indirect costs such as wear and tear on equipment.

“It’s paying for the people who are doing the runs, but . . . we’re wearing out an ambulance. If we have to buy a new ambulance it has to go on the tax base. That wasn’t the plan,” he said.

As things now stand, Oak Bluffs provides the great bulk of off-Island ambulance transports. Only when Oak Bluffs cannot meet a request does the hospital come to Tisbury with requests.

“Probably five years ago, we set up a revolving fund for off-Island nonemergency transports, so if somebody needed to go up to Boston or over to the Falmouth Hospital, for example, we would take those ambulance requests and bill the insurance company or the patient,” Mr. McLean said.

“We decided we would do this if it was a self-sustaining fund, not adding any cost to the taxpayers. We thought we would make $10, $15, $20,000 a year, with which we could buy a new ambulance every five years or so.

“But we have barely broken even. There is no money there when we need a new ambulance. So the view is maybe we shouldn’t be providing this service,” he said.

The essence of Tisbury’s concern, he said, is the lack of volume. Oak Bluffs, which operates in the same way, but does many more runs, has earned significant sums over the years.

“Oak Bluffs is doing the same thing we are — billing the insurance company or patient — but they get the bulk of the calls and we get maybe a call or two a week. That’s not enough calls for us to keep up the service without adding to the tax base,” said Mr. McLean.

“That wasn’t the intent, so we’re going to send a letter to the hospital saying it doesn’t look like we’ll be able to continue, unless they have some other ideas, like giving us more runs.

“If they really need us to be in the service, then we have to reach some new arrangement. Maybe they can throw us a few more runs or suggest some other way for us to make it pay.

“If we can’t do that we have to re-look at the service and decide whether it’s the right thing for us to be doing, because to date it hasn’t produced revenue sufficient to sustain the service,” Mr. McLean said.