Sorry, cruise control has nothing to do with speedometer accuracy (or tire size). If cruise control can maintain a steady speed on the highway (even if it is not what you set it at) and if the cruise set speed will increase and hold when you press and hold the SET button for a few moments (this is important) then the problem may not have anything to do with vacuum leaks or adjustments. As much as cruise control problems are more often associated with vacuum leaks, this may not be so in your case. If the speedometer is working normally and if absolutely everything else about cruise control works other than the behaviour you describe then the control module is suspect.

When you press SET, the control unit stores the current speed sensor signal (from the back of the speedometer) as a reference. When you are in cruise control mode (either after the SET or after a RESUME), the control unit operates the throttle using the vacuum pump so that the signal from the speedometer matches the reference signal (within 1 mph). When you press and hold SET, the stored reference signal somehow continues to increase and you will observe the cruise speed risingwhenever you hold the SET button in for a few moments. Knowing this, you should be able to gain back your lost 5 mph just by pressing and holding the SET button until you reach the proper speed -no need to go 5 mph faster on the road before setting it. I'm guessing that some capacitor in your control module is failing, likely in the circuitry that delivers the stored reference signal to the internal comparitor.

By far the simplest thing to do is start by using the OBD system to check for any faults detected by the cruise control module. Go the 700/900 FAQ section on Cruise Control and the article on cruise control onboard diagnostics. Perform all the tests exactly as described: Take the car out for a short run. Make sure the speed gets above the point at which cruise control can be enabled (22 mph or 35 km/h). Then, without turning the engine off, go to the diagnostic module and read all the stored codes as described. It doesn't say to try setting cruise control during your road test, but I would do that anyway for a short distance just to make sure it gets above the cruise set point and goes through its paces for a moment.

If the diagnostics are normal then go through the entire 700/900 FAQ Cruise Control section for further tips. Note that references to wiring on 700's may be different from those on 900's.
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Dave -940's, prev 740/240/140/120 You'd think I'd have learned by now