Definition: The size or scale of an RC vehicle is its size as compared to the real, full-size version. Scale speed is a measure of how fast the RC vehicle goes in relation to the full size model. A simplified way of looking at it: a 1:10 scale RC would go about 1/10 the speed of the full scale vehicle it's modeled after.
Scale Speed vs. Actual Speed: A 1:8 scale RC listed with a scale speed of 110 mph, probably has an actual speed of around 13-14 mph. A 1:64 scale RC, such as Radio Shack Zip Zaps, with a scale speed of 110 mph has an actual speed of about 1.7 mph. There's a big difference between 1.7 and 14 mph even though both vehicles have a scale speed of 110.
Scale Speed That Doesn't Match Size: Not all RC vehicles will have the scale speed you might expect when compared to their full-size counterparts. RC toys, especially those for toddlers and pre-schoolers are much slower. Some modified RC vehicles may have actual speeds that are much faster than you might expect for their scale size.
