[quote name='Fluffy01016' date='10 March 2010 - 06:34 PM' timestamp='1268246098' post='982230']
Sorry, I meant 70mph dual carridgeways

[/quote]
Don't worry. Your Prius speedo will probably be reporting 75mph when you are doing 70mph.
So 80mph will be about 74 to 75.
Would be an idea to check it against a few GPS devices (or measured mile) to establish how far out the speedo is.
Reported MPG is usually 2 to 3 MPG over the real figure so keep your own record of fuel consumption to get real data (I use fuelly).
MPG will get better as the weather warms up.