You have one of 5 problems:
1.) You have a blown head gasket.
2.) You have a warped head.
3.) You have a cracked head.
4.) You have a warped block.
5.) You have a cracked block.
Most likely you have a warped or cracked head though since it will only do 35mph. It probably over heats on you fairly fast too. This because the water is not getting into your water jackets because it's going to your oil pan instead.