When it comes to telling the difference between a 7.5” and an 8.5” rear end, there are several ways you can do so. First, look at the axle tag which is usually located on the differential cover or on the frame rail near where the driveshaft connects to it; this should indicate what size rear end you have. If that isn't available, measure from flange-to-flange across both axles using a tape measure; if it measures 66 that means you have an 8.5 ring gear while anything measuring 58 indicates a 7.5. You may also be able to tell by counting splines in your yoke or pinion shafts; generally speaking a 7.5″ will contain 28 splines while an 8.5″ typically has 31 splines per side of its yoke/pinion shafts – but keep in mind these numbers can vary depending on what make and model vehicle you're dealing with so always double check before ordering parts for any repairs! Finally, if all else fails simply take off one wheel and examine how many bolts hold it onto your hub assembly: seven will mean it's a 7 ½ inch differential whereas eight typically signals an 8 ½ inch setup!