Why does my van only drive like 40 mph and won't go faster

Sorry to hear about your issue. A reduction in speed can be caused by many things like engine sensors and issues with the transmissions. You will need to start with a complete scan of the system and report back with trouble code numbers. If you don't have the tools for this I recommend taking it to a shop. Here is a link to find a shop near you. https://repairpal.com/repair-shops Alex