Speed issue

I have a 1996 Gem Trans2 and it goes 16mph and wont goe faster on flat pavment.So I took the batteries to autozone to test them the said 3 were good and one the tester couldnt test but them were sure it was bad so I took them to advance auto and they said they were all good.Whats the issue?

do have the switch on TURF or on Road. turf oes more than 18mph

also the speedomete is not accurate mine showing 22 but GPS says 29