My 66 reg Toyota Yaris 1.33 Multidrive S (CVT but not hybrid) has a fuel gauge with a similar characteristic. Upon filling the tank and first engine start the range is reset and calculated using the previous average computed mpg and is typically 450 miles. The range number reduces linearly with driven miles down to half tank if the vehicle usage is similar to the previous tankfull. The range is adjusted using the current new calculated average mpg since filling and quantity of fuel remaining. After half tank (typically 250 miles) the fuel level and calculated range drop more rapidly (non linear with miles driven) and the fuel level warning light shows at around 430 miles (the range shows around 5 miles left). At range zero a refuel is about 33.5 litres giving a true mpg of 59 (the computer shows 61). The tank has 42 litre stated capacity and I have three times obtained 500 miles from a 37 to 40 litre fillup. Overall consumption for last 3000 miles is 57 (computer says 59). At 10000 miles the engine still feels very tight so this may improve.