Damn thing posted too soon. Conversion are as follows, mult miles by 1.6 gives you the KM or other way by 0.6. 4.5 liters per Eng Gallon, 3.78 liters per US gallon.
So, 50 km is about 30 miles and miles per hour converts same. 80 is about 50, 90 is 55, 160 is 100 and (the important one!) 320 is 200! Not really though if you do the exact calc it is 200 * 1.609 = 321.8. So the conversions as almost as far out as the speedo would normally be anyway.
Once you are past 340 get a GPS!