If stock offset is 41 and new wheel offset is 51, the new tires will be 10mm further under your car, plus half the increased width of new tires, if larger. 10mm spacers will suffice for the wheel difference, but the tires may be unknown until mounted and aired up. The best method is actual measurement from a straight-edge across the inflated tire to the face that mounts the hub. Do both tires this way. The difference is the spacer need to clear as stock tires do.