Dealers can't void an entire warranty just because of the existence of any mod. Basically if you have a tuner and something goes wrong, they have to prove the tuner caused the issue, and then and only then can they deny the warranty claim, and it only applies to that specific issue.
Using a tuner just to calibrate the speedometer shouldn't give you warranty issues, but if that's all you want it for, I would check with the dealer first like Power247 said. They may be able to do it without you needing to spend as much money for a tuner.
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