Though the HCH OSD tops at 120mpg, the acutal numbers beyond that are used, if say, you get 150 mpg. There are two reasons for me saything this, one logical, one physical:
1. The calculation you see is, just that, a calculation. The 120mpg is not "stored" and used in a future calculation. The computer (estimates) the fuel used and divides it by the mileage driven resulting in MPG (or whatever the calc is in the metric world). Thus, whether the last mile was driven at 0, 120, or infinity MPG (infinity is for those that use FAS), it doesn't use that data to caclulate the next reading. The computer just has a test for (IF MPG>120, then DISPLAY=120 ELSE DISPLAY=MPG); or some such.
2. One gas station I use frequently is at the top of a nice, long 1.5 mile hill. At about halfway done the hill, my Trip A (my tank mpg) will read 120 mpg. After my speed is reduced on the next 1.5 mile flat to 40
mph, there is a stretch where I must use the engine and my FCD shows 80-100 mpg. The OSD display does not immediately start tracking down. It takes another mile or so until I start to see 119.7, 119.4, etc. It takes a while to get from the 120+ mpg back down into real world numbers. I have managed to have the reading at 120 with the trip meter reading 7 miles.
Sound good?