2000 Montero Sport Odometer Inaccurate

I could probably go farther, but I've gone 30 miles after the light came on and didn't run out of gas. I usually go about 300 miles on a tank and normally the light hasn't come on yet at that point. I normally average 19 to 20 mpg but my odometer is off, so as they say in the car ads "your mileage may vary". I estimate my speedometer as well as my odometer is off about 3 per cent. If my speedometer says I'm going 62 I'm really only going 60. Likewise, if the trip odometer says I've gone 300 miles it's probably more like 290 miles. I don't usually apply this correction to my MPG because 19 mpg is the lowest I want to see. It would hurt to know that I'm really only getting 17 or 18 mpg.