FWIW, the Ecopia 422+ tires got back all the miles I was hoping to in about 1000 miles, and at least part of that was probably just waiting for spring to thaw. I'd been getting 42-44 on the estimator in fall on the shoulder-worn Goodyears, bought the Ecopias in January, and by April the low 40s estimates were happening again. (Still have never seen higher than 45 at unplug, but I'm not exactly TRYING hard.)The OEM Goodyear tires have been a sore point for many Volt owners. They are noisy, prone to punctures ( at least they appear to be that way ) and wear on the tread shoulders too fast and many have had cracking issues. The only plus seems to be that they did provide low rolling resistance thus good battery mileage.
Everyone who has replaced tires has noticed a small hit to mileage for the first few thousand miles but then it seems to largely return to normal mileage. Of course, with all of the other variables it is hard to be exact.
Also, everyone who has replaced the OEM's with something different have noticed better handling and a quieter ride.
For the cost of new tires, you will certainly have some peace of mind (unless of course the roads you travel are littered with nails).