I want to change all 4 tires with the other size. Will it be OK?
It should be OK. The change in ratio between the thickness and the width is ever so slight it won't make much of a difference.
a 75 series is taller than a 70, make sure you have the clearance.
No. A tire with a section height of 70 (all other dimensions being equal) has a smaller diameter which will make your odometer and speedometer read inacurately and change the cars gearing which will make it burn more gas. More importantly though such a tire has less load carrying capacity which has the effect of making your tires under-inflated all the time which will not only reduce fuel economy further but could result in a tire blowout and your maiming or death. Use what the car calls for.
