Help me get new tires!

[JRANGER]

Well my tires are worn out and since im getting my Tax Return I figure I might as well get rubber for the truck in case the roads get icy...That is what i bought her for in the first place anyway...


So that being said....

I have 255/70R15's on the truck....Can I go bigger without messing up the speedo? I have a 21 gear speedometer gear and ell all I know about the rear end is a couple #s on the axel.... which are 1249578 R which was on the bottom.. On the 0 FI680...all though the 0 look like a flat head almost haha


Im wanting a good tire for winter as this truck isnt going to be driven on the regular