There are essentially only four different scales in which slot cars are made. These scales are the 1:24, 1:32 and the HO size which includes the 1:87 to 1:64 scale. These are frequently referred to as one twenty-fourth and one thirty-second respectively.
The 1:24 scale cars are made in such a way that 1 unit of length (usually an inch or a millimeter) on the slot car model is equal to 24 units on the actual car which is being modeled. So a car that is 185″ long in actuality would end up being about 7.7″ long in 1:24 scale. This makes using slot cars in this scale completely impractical for most home enthusiasts. Most people who race slot cars in 1:24 scale usually only do so at commercial tracks. https://www.worldwewant2030.org
1:32 scale cars are much more practical for home race tracks. They are also commonly used in commercial tracks and clubs as well. It is the most popular scale in Europe. It is equivalent in size to the old toy trains #1 Gauge. The same car as indicated above would be about 5.8″ long if made in 1:32 scale.
HO-sized slot cars come in a variety of scales. These cars were originally marketed as accessories for model railroads in the early 1960s. As slotcar racing in this size evolved over time, the cars were gradually enlarged so they could accommodate larger, stronger motors. In today’s market, they are closer to the 1:64 though they are able to run on tracks that are approximately the same width. They are usually simply referred to simply as HO slot cars. The cars made in this scale are generally not accurately scaled because the proportions must often be stretched in order to accommodate the motor. In this scale, the car noted above would be about 2.9″ long. Racing slot cars of this size is much more easily accommodated on a home race track.
Other scales have also been manufactured in limited quantities. Most manufacturers have different car sizes and tracks. Most cars are easily interchanged on tracks of the same scale.
1:64 scale is traditionally used for all models and miniatures. It can also be called three-sixteenths scale because 3/16 inch represents one foot. It was derived by taking the 1:32 and cutting it in half. The 1:32 scale was the standard scale size in some hobbies.
This size became successful for many reasons one of which was its relative size when compared to other toys. Another reason for its popularity was because it was derived from the 1:16 scale and because it was able to be held easily by the small hands of children.
Cars made in this scale will usually have fewer details than those made in the 1:16 scale. This scale size also coincides with the S scale used in model railroading. This is another reason why it became an established size.