Voltmeter isn't showing proper reading on 1999 Chevrolet Suburban?

I have a 1999 Chevy Suburban LS 1500 2wd and I recently replaced the battery and alternator, and the voltmeter is still reading between 7 and 8, I went to the local parts store and had both the battery and alternator checked. Alternator was charging correctly and the battery passed. I'm thinking maybe now it might be a bad ground cable from the battery, or is there anything else I should be looking for?