Ignoring brand differences, a 235/50/18 and 235/45/19 have the nearly the exact same diameter (27.3") so the speedometer would reflect the same speed and odometer would reflect the same mileage.
Going to a 240/40/19 would have a diameter of 26.6" (-2.6%) and your speedometer/odometer would read slightly fast (and odometer would accumulate slightly more miles than you actually traveled). When your speedometer indicated 70mph you'd actually be traveling 68.2 mph.
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