speedometer of your car is showing 5% less speed then actual
If u r going at 60 car is actualy doing 63.
So the reading u get on the ODOmeter is 5% short of actual distance travelled.
If u multiply the reading of Odometer with 105% then u will get the actual Kms travelled. This will improve thinbgs definitely.
e.g if u are currently travelling 100 Kms in 8 Liters
Actualy u r doing (100 * 1.05)=105 Kms
The average u get currently will be
whereas actual average will be 105/8=13.125
I hope it helps
There is a tyre size calculator available on this link.
i hope it will be helpfull