Well, the theory says double your 80mph, so 160.
However, it takes a whole lot of extra horses to overcome the massive drag at those sorts of speeds, plus it would probably become quite unstable!
Steve
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XC90 V8 Exec, C70 2.4T Convertible, S70 T5 CD, 850 TDI, 2 x Escort vans and a 2.8i Capri.
Clearout has begun, but must continue!