Best results (depending on the style of cab and truck) use reference pics or CAD drawings. Scale them up (using known measurments) and divide by the scale you plan to use.
Ex. standard wheel and tire height is 40 inch diameter (approx), next measure the height on a picture (assuming is a flat side or front shot, not perspective) let's say its .5 inches on the photo. The scale we'll be working is 1/25 (same as ALF above).
40/.5=80 (meaning our photo is 1/80 of real size) then divide 80 by our scale (25) 80/25= 3.2 factor.
This means we can take any measurement from the photo and measure and multiply that number by 3.2 and get our 1/25 scale measurement.