2003 Chevy C5500 rollback tow truck start and running smooth but not going over 55MPH

I have 2003 Chevy C5500 rollback tow truck only 122k miles on it was running smooth and go over 75mpg and up before start cold weather but since cold weather start it's start and running smooth but not going over 55 mpg speed.I change the oil and oil filter and fuel filter each time on 3000 miles.any one have Idea what need to do to fix this? some one said need turbo to replace some said need fuel injectors some one said just change fuel harness.I am totally confused any one can help me out please.

Welcome the board!

This sounds like a "limp mode" problem that is causing a power/speed limit.

Are there any trouble codes?

A "limp" occurs when the vehicle computer is sensing a problem with either the engine or transmission that could result in damage to either. A trouble code is always generated for one of these codes.

Jim

There is code or any light on dash.But I don't know what is limp mode and how to find out if on limp mode and how to fix it please help if you know abut it?

Have someone take a look to see whether there are any current trouble codes or if any codes are in the "history".

Limp Mode is the computer's response to a problem it detected that could cause damage to the engine or transmission. Limp Mode severely restricts engine power, and it sometimes limits the transmission to the lower 3 gears. Again, it's all to protect the engine/trans from even worse damage, but allow the driver to move the vehicle to a safe location (off the highway or leave the highway at the next exit. Jim