I bought this truck, its a 2012 single axle Prostar with a maxforce engine. The truck starts and runs great. Only has 200k on it. Comes up to temp, has great oil pressure. Everything works in it and has Ice cold AC. To just stand next to it while its running, you would think its a good truck. YOU WOULD THINK ! The truck only does 40 MPH. This shop has had it for 4 months and cant find the problem. First they thought the exhaust was the issue. Clogs in the regen system, whatever. They removed all the exhaust and drove it. 40MPH. Then they thought it maybe in the ECM. Tried a different ECM, still 40MPH. Then they found the Air charger was leaking bad, only building 8-10 PSI. AH HA, we found the problem (we thought). I bought and sent them a new air charger, No change, still 40MPH. He has hooked up his laptop to it and all shows to be good, no codes present. What in the hell could cause this issue? I'm glad I don't need the truck( I bought it to resell), but dam, 4 months and no results, and don't misunderstand me, This guy is a pretty decent wrench, he'll throw a clutch in a truck in 4 hours, build a 60 series, or 3406 with his eyes closed. This is bothering him as much as me. There's like NO obvious issue with the truck. It's almost like the speed is turned down to 40 in the ECM, but it's not, set at 70mph. Any help or suggestions would be appreciated.