What happens when my speed sensor goes out on my Peterbilt 378?

My dump truck does not go faster then 50 mph on the freeway and I was wondering if a bad speed sensor could be the problem. If it's not the sensor then what do you think is the problem? I have a Detroit Diesel series 60 engine.|||Could be several reasons there Strawberry, and if your speedometer is reading 50 mph and your actually doing 70 then yes your speed sensor is bad. when was the last time you changed a fuel or air filter?|||I am sorry to say I have no clue because I do not work on cars, trucks, or suv's but if I were you take it to a shop that knows all about cars, trucks, suvs and have them tell you what the problem is and how much it would cost you to get it fixed. That is your best bet so you can get the right answers to the problems you are having and getting it fixed right.