This approach is most likely faster than using a divison on a '51, but will result in four levels of nesting, which may or may not agree with your sense of aesthetics. :) <p>

It should be possible to do it without nesting that deeply.

unsigned char bar_value_tmp, speed_in_mph_tmp;

bar_value_tmp    = 0;
speed_in_mph_tmp = SpeedCalculated;

if(speed_in_mph_tmp >= 80)
   {
   speed_in_mph_tmp -= 80;
   bar_value_tmp   += 8;
   }

if(speed_in_mph >= 40)
   {
   speed_in_mph_tmp -= 40;  
   bar_value_tmp    += 4;
   }

if(speed_in_mph >= 20)
   {
   speed_in_mph_tmp -= 20;  
   bar_value_tmp    += 2;
   }

if(speed_in_mph >= 10)
   {
   speed_in_mph_tmp -= 10;  
   bar_value_tmp    += 1;
   }

/* Round. */
if(speed_in_mph >= 5)
   {
   bar_value_tmp    += 1;
   }

Bar_Value = bar_value_tmp;

There. No cycle-intensive divisions or 16 bit operations.