[jdmccright]

The 70-series tire will be a lower aspect ratio tire than your current 75-series (i.e. the sidewall will be smaller). The aspect ratio number is the percentage of the tread width (e.g., for your 215/75R-15, your treadwidth is 215 millimeters and the sidewall height from the rim would be 0.75 x 215 = 161 mm.

This change from 75 to 70 will result in a change in the effective diameter of the tire/wheel (sidewall reduced to 151 mm), thus the rolling circumference, and thus throwing off your speedometer. If you have a 4wd, and must replace tires, you should get all 4 the same size and recalibrate the speedo if you go with the 70-series tires.

If you wish, you could "plus-size" your tires, where you offset the reduction in sidewall loss by increasing tire width, resulting in a tire that has the same overall circumference. A tire dealer would be able to help you with this, but larger, fatter tires are generally more expensive.

Stick with your original size if you want 2 tires on the cheap.