Speedometer way off

I took my sons Hyundai to the dealer yesterday for its first free oil change that cost $24.99, but that's another story. While driving down the highway my instincts told me I was driving around 70, but the speedometer read 91 mph. I was in the middle lane and cars were passing me on both sides so I could have even been as slow as 65 mph. The dealer asked that I bring it back Wednesday night and they will give me a loaner car for the next day. It seems exponential, in the neighbor when it says 25 mph, I feel like it's 20-22 mph.
Any thoughts? Software, or mechanical connection to the transmission? Either way it's covered.

Is the tire size, diameter smaller than what came on the car originally?

Go out on the freeway and compare odometer reading to the mile markers for about 10 miles. I have an Edge programmer on my F150 and can adjust to any tire size.

Yours seems to be the early termination of the warranty coverage setting.

The tire size was my first guess, but they are the ones that came with the car. I take it in Wednesday night and they will have it for the day. Hopefully they find out what the problem is. The service write up person seemed like he has seen this before.

I had the car at the dealer on Thursday. They could not duplicate problem. My wife with the engineer license (PE) took the car on the highway set the cruise control, did some math calculations and concluded that at 65 mph, the car was traveling 68.5 mph. Vehicle speedometers are allowed to be +- 5 mph.

Next time it comes out over 5 mph she wants me to bring it back to the dealer. I recommended she bring it back and show them your math, not just your answer likes she asks her students to do. That didn't go over well to put it mildly.

New tires have the car going faster. As they wear it should get closer to actual. Best method is checking odometer to mile markers for 10 miles or more. Yours is at least 4% which would be 4 tenths in 10 miles. Newer electronic speedometers should match speed to distance. Older cars could be different between speed and distance because of the spring attached to the needle.

These tires are the original that came with the car. On the service report they were at 5/32" or in the middle between new and old.