Which model Gli? If its older one, 2001-8, it should be able to reach at least 160 easily, regardless of driving style, if throttle is "pushed to the max".
If its the new one, make that figure 180. But if its doing only 140 on petrol, then either the tyres have ~10 Psi, the air filter is choked, oil level is low, speedometer is wrong or the gear is in 3rd...
Smart statement btw thanks
@pak & @illdude have put forth valid points as well (Y)