Exactly. Larger wheels/tires rotate slower than stock, therefore your speedometer will read slower than you’re actually driving (& your odometer will read lower than actual). What’s more, your speedo will read variably-slower at different speeds. If you’re driving 80 with a speedo reading of 70mph, your speedo would likely read 35mph while driving 40mph..They'll fit fine on the back. I had 31"x10.5" on the rear for about a year and had no problems with clearance. Don't know about the front.
Bigger tires than stock will make the speedo read lower than actual speed. You're covering more ground per rotation than the speedo is geared for. With those 31s my speedo read 71 but my GPS showed 80.