i had a similar problem lucky for me it was just an old dip stick. Replaced the dip stick first see how u go if that doesnt fix it looks like a piston ring problem.
yep exactly jay95r33 right if you change the wheel size the speedo definatly will be out doesnt matter were the speedo gets the reading you put 10" wheels you will do eg 40 kms in 1st put 20" wheels you will do 60kms just like changing diff ratios