If the pressure gauge is electrical vs mechanical you can test the gauge by grounding the wire at the sender. Should get a full sweep of indicator needle when grounded. If not the gauge may be defective. You can also test sender with a VOM meter by reading resistance when engine is off and when running. If there is no difference in readings then sender may be defective. Make sure there is nothing insulating the body of the sender from the ground/engine and there is no gunk shorting the body of the sender with the electrical connection on the sender.

20 mph is a huge jump. I can only assume the drive train came from more than one vehicle to be that mismatched. On the transmission end of your speedometer is a plastic gear. There are gears available with different number of teeth on them. By changing to a different toothed gear you can alter the speed reading. Increasing teeth will result in slower speed reading while decreasing the number of teeth increases your reading. Each tooth will result in an apx. 7mph difference. You’ll have to pull your gear to determine your starting point then see what’s available. With a 20 mph jump you may have to replace the drive gear that is inside the transmission.