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Well bigger tires will surely make it read different than it did before but it should still work...
Do you mean it seriously doesnt work or its off by a few MPH?
Your problem is not likely due to your tire size but a result of a GM design flaw that causes speedometer/cluster gauge problems to be common in GM NBS tahoes & yukons. There was a factory recall issued but to qualify for free repair, you had to have less than 80K of mileage if I remember correctly. There is a cottage business of folks doing these repairs...