After using Showroom Lawn mover mod I have had this problem. Showroom mod can change your look to metallic paint, but it can change your engine upgrades too. After using this mod, awhile ago, I haven't had my top speed with lawn mover anymore.
What I have done to trying solve this situation:
- I have installed Showroom Lawn mover mod and returned all stock parts
- I have verified files by Steam client tool
- I have played the Career mode again whose prize was lawn mover
- I have tested the speed with Special event (off-line) and raced on-line (server)
- I have all three engine for it: C, B and A class. All top speed is 106 km/h locally (off-line). On-line C class top speed is 127 km/h