Chain Rubbing on Tire in Lowest gear

I have a lwb Recumbent with 26 x 1.75 rear tire, with a 9 sp cassette.
When in the lowest gear, the chain rubs against the rear tire. This occurs all the way arrond the tire, so the rim doesn't appear bent.
Is this because the tire is too wide, or the wheel is dished wrong, I have a disc brake on the rear, so I cannot use the brake pads as a reference point.

Thanks for any help
Jeff