My speedometer keeps going off and on, mostly off.

There are a couple of possibilities.

The new sensor is faulty as well.

A bad connection on the speedometer/dash cluster.

A GPS unit will tell your speed as will the phone App 'Waze'.

Most likely the Vehicle Speed Sensor is defective. the fact that the cruise control doesn't work properly Is the strongest link to the VSS being defective. If you want to DIY, go to https://www.hyundaitechinfo.com/ and get a free account to look through the service manuals for your specific model. Additional help is available from folks at http://www.hyundai-forums.com Good Luck!

A speedometer shows speed, which is the magnitude of velocity. It does not show velocity, which is a vector, meaning it includes a direction. So speedometer notworking have nothing to do with the transmission not going into overdrive. Know more diagnostic tool for car.

Most likely the vehicle speed sensor in the transmission has failed.

Usually it's the opposite. The speedometer works and the odomater and trip meter go inop. Most likely the speedometer assembly is bad. If the odometer is working you have a signal from the gray and black wire .

Most likely a speed sensor is going bad (most cars and truck do not use mechanical speedometer cables anymore) most sensors are located near the differential in rwd cars or near the drive shaft on the transmission. Best bet is to get a Haynes manual for your car to help locate the speed sensor. Remove it and take it to the auto parts store so you can visually compare it to the replacement. Install the new speed sensor and it should restore your speedometer and odometer.

It can only be changed by a tech 2 scanned through the ECM program. Most larger tire store has a scanner to recalibrate the speedometer but the smaller tire shop will not, you can go to the transmission shop and have them also reprogram ECM to match your tire and recalibrate your transmission to your tire by raising the transmission fluid line pressure which is a must for larger diameter tire to keep the clutch pack from slipping and burning up. Good luck an hope this helps friend. 

well i am going to tell you that your problem is an easy fix and shouldn't be expensive at all. what it most likely is is that your speedometer cable has a medal rod that goes into your transmission and in most cases it will wear down to where it gets loose and will freely spin and in some cases it will catch but it is most likely the speedometer cable

Taller tire have more centrifugal force because they have more mass (weight). A body in motion tends to stay in motion. They also have a larger outer diameter therefore through all the weight to the out side of the tire. requiring more energy to accelerate and more energy to slow down than a smaller tire at the same speeds. This is why I would recommend up graded in your brakes and also lowering your gear ratio( numerically higher) if you are going to larger tires. A lower gear ratio will act as a torque multiplier which will get you moving faster up to speed and also adjust your speedometer to be more accurate.
When you go to larger tires your speedometer will be inaccurate because taller tire have large outer diameter. A taller outer diameter tire will travel farther per revolution than a smaller tire. This cause your engine speed to be lower with taller tires because your vehicles tires are moving slower do to it larger outer diameter.
There your vehicle can be re calibrated through the computer. 4wheel parts offers parts call true speed sensors which also recalibrate your speedometer accurately that is the way i would go. Also maybe up grade to larger master cylinder off a 1 ton Chevy and go with some drill and slotted rotors. You most likely will have better braking with these upgrades than you would stock. keep it safe!

Good luck and have fun.

Sounds like the instrument panel is missing a ground. Check it out before you purchase a new one. Hope this helps. Keep us posted. Good luck