MPH hour gauge is too low. What could be wrong?

I reset this gauge because I thought it was being weird. I then drove about 20-30 miles on the freeway going 60MPH. MPH, once clocked only went from 12 to 14. What sensors should I be checking or replacing to make sure nothing is wrong?

Car is a 2007 Sonata.

I also have very low fuel economy. About 16mpg, and I drive like a slow grandma. I don’t slam on the gas or brake excessively. My sister is always annoyed to drive with me.
I took all the heavy stuff from my car, and keep the tires inflated to 30-35.

@Val
When’s the last time the air filter, spark plugs, and fuel filter were changed?

Jai said:
@Val
When’s the last time the air filter, spark plugs, and fuel filter were changed?

I’m going to guess the spark plugs need replacing. I’ve driven the car about 20,000 miles, but I don’t know much about its previous life. It was owned by an old man in Florida in my parents’ retirement community who sold it to me for very cheap after the pandemic started. The only paperwork he gave me was from a few years before when he bought it from a dealer, but he only put a few hundred miles on it.

The filters are overdue, but only by a couple of months. I lost my job and put it off.

I’m finally getting some other income, so a tune-up + oil change is at the top of my list.
Thanks for your advice!!

By chance, are your tires larger in diameter than original equipment tires?
That could account for a significantly lower speed and lower distance being measured.

Presley said:
By chance, are your tires larger in diameter than original equipment tires?
That could account for a significantly lower speed and lower distance being measured.

I don’t think so, but I’m not a car person. They’re not suped up, or anything abnormal. They’re economy tires from an independent shop.

@Val
Then your tires are not the issue at all.
Hopefully, someone else can steer you in the right direction. Best of luck!

4 cylinder or V6?

I have the 4 cylinder and only getting 18mpg @ 200k miles in a suburban area. (At best)

It says MPH, nothing to do with MPG, or economy. What is being displayed is the average speed traveled.