2020 Hyundai Elantra RPM surging at highway speed

This sounds like the torque converter lockup clutch cycling, not an engine surge. At highway speed the clutch locks to connect the engine directly to the transmission, and a steady 200 to 400 RPM bounce at constant speed is the classic sign it is engaging and releasing.

The important part is the computer commands unlock on purpose for a long list of reasons, including a slight misfire you would never feel. So the cause is often engine side rather than a failing transmission.

Start with a full scan, pending codes and transmission codes included, even though the check engine light is off. Light misfires store before they ever turn the light on. Watch misfire counters and torque converter lockup status while the symptom happens.

Quick road test: at cruise on a flat road, lightly rest your left foot on the brake pedal without slowing down. That drops the car out of lockup. If the pattern of the surge changes, you have confirmed the converter clutch circuit is involved.

Check the transmission fluid level and condition, and on the engine side inspect the air filter, clean the throttle body if it is carboned up, and check the spark plugs. At 48k miles plugs are getting tired. If the fluid is dark or smells burned, that points to the transmission side and you want a trans shop to look at it before it gets worse.

A tach bouncing a few hundred RPM at steady highway speed with no warning light is most often the torque converter clutch, not the engine. Your 2020 Elantra's IVT uses a torque converter clutch that locks up at cruise for fuel economy. When that clutch starts slipping, or the transmission computer cannot hold lockup, the clutch cycles in and out. You see it as the RPMs hunting up and down while road speed stays constant.

An engine issue (dirty MAF sensor, weak coil, vacuum leak) is the other suspect, but those usually show up across a wider range of driving and often leave a pending misfire or fuel trim code even with the check engine light off. So the first step is a full scan for pending codes, not just stored ones.

The clean way to split it is a road test with a scanner watching transmission data: TCC lockup commanded state and converter slip RPM. If the RPM hunting lines up with the converter cycling in and out of lockup, it is on the converter side (fluid condition, lockup solenoid, or the converter itself). If the converter holds steady and the engine is hunting, the problem is engine side.

Also have the transmission fluid condition checked. Dark or burnt fluid makes lockup clutch problems worse.

https://ezcarfix.com/blog/transmission-fluid-maintenance-guide