We can use 12x24 tyres instead of 12x20 same vehicle. Kilometer to be change? and fuel consumptio?

The speedometer measures how fast the tire turns. That translates to kilometers based on the diameter of the tire. So if the speedometer works right for 20 tires then it will indicate 20/24 = 5/6 of the actual speed with 24 tires. If it works for 24 tires, it will indicate 24/20 = 6/5 of actual speed with 20 tires.