Are there any issues with putting stock 18's on the back instead of 19's? Will this effect the speedometer or odometer at all?

Was thinking of getting two stock 18's to put blizzaks on or just in general to have a solution to get tires rotated so that they last longer than 18k miles, which my last 2 sets of Continentals lasted. I just basically commute 50 miles a day highway. They should last longer than that IMO.