I've heard a lot of people over the years say they changed their tire to "correct" their factory speedometer reading. I've never owned a bike that didn't come stock almost perfect (maybe one to three MPH at most off) which wasn't enough for me to care so I never messed with it. Usually it was sport bikes. I think every cruiser I've had was dead on.
If you changed your aspect ratio but kept the same width of the tire then you will have an off reading. If you increase outer diameter then you should be going faster than the speedometer reads. I'm not 100% sure but likely Victory uses a Hall effect/pulse generator which gets a reading off the transmission.
The speedometer on your bike is setup to read X amount of revolutions with X amount of stock outside diameter tire = x distance/speed. Now you have a higher outside diameter so you're traveling farther per revolution but the bike doesn't know that.
2013 Victory Hardball - VM1DR cams, Lloydz filter, timing wheel, PCV-AT, 1/4 turn throttle, modified pipes
2008 Yamaha Warrior -Churchkey intake, V&H Bigshots
2007 Scion TC 5spd
1997 F150 -used to pull my car hauler
1989 Nissan Hardbody -absolutely indestructible
1968 Coupe Deville -all black, so nice