Jeff Caplan makes excellent reproduction gears. http://www.odometergears.com/subpages/cable.html. This link goes directly to the page for Volvo mechanical speedometer gears.
The one that usually goes bad is the one in the picture. Examine the one from your speedometer. It may look fine but it is probably split along the axis. Count the number of teeth and order accordingly.

Or see http://www.homestead.com/volvo2/Odometer.html for an alternate method of repair. The instrument cluster pictured is the '81 and later mechanical type but the mechanism is pretty much the same.

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'80 DL 2 dr