Join the Car Talk Community!  I have a GMC Suburban The original
tires were -...  Dear Tom and Ray:  I have a 1996 GMC Suburban.
The original tires were 235-75-16s. I replaced them with
255-70-16s. Will the size difference change my speedometer
reading? -- Lew  TOM: Well, GMC doesn't recommend 255-70-16s for
this truck, Lew. And throwing off the speedometer reading is
probably the least of its concerns.  RAY: The main concern when
you change to an unauthorized tire size is how it will affect
the vehicle's handling and safety. When cars and trucks are
tested for their handling and load-carrying abilities -- and
things like their propensity to roll over -- they're tested only
with their original-equipment tires.  TOM: Other sizes of tires
might turn out to be OK, but the manufacturer simply hasn't
tested them. So there's no way to know exactly how the new tires
you bought will affect your Suburban's behavior on the road.
RAY: Although GMC hasn't tested 255s on this truck, it does
caution that you might have tr