The Quantity one is easy, the fuel pump will tell you that.
For the distance traveled, normally you would use the cars milometer, if you think your change in tire size has affected the milometer, then you will need to work out how far out it is.
You can do that either by using a SatNav and comparing the distnace traveled with your milometer, or by using the mile (km) markers on the hihgway and comparing with your milometer. You can then use the difference to calculate your MPG correctly.
Hope this makes sense.
If you travel 10 miles, measured by your satnav or mile markers, and your milometer shows 10.1 miles then your milometer is reading 1% more than you have actually traveled.
The next time you fill up you put in 10 gallons and your milometer says you traveled 101 miles, you should actually use 100 miles in your MPG calculation.