The van is going five miles under the speed it is showing on the speedometer.
Speedometer issues are not very common and sometimes can be caused by things we do. For example replacing the tires. Tires need to be replaced with tires of the exact same size because that is the size that is programmed into the vehicles computer. So the speedometer calculation is done based on that. If you have replaced your tires with a different size, the vehicle can be reprogrammed to the new tire size so that it all works correctly. This is something that the dealership will need to do for you as it is a factory setting. If you have not replaced your tires, but are still having issues, it might be caused by the speed sensor in the transmission. Some vehicles have two speed sensors. A qualified mechanic can determine the speedometer issue and replace the necessary parts to get it functioning properly again.