I have not tracked down the original source of the claim that a garden snail managed 0.03 mph. However, I am convinced that it is wrong. It works out as 13.6 mm a second. The fastest annual champion in the World Championship Snail Races managed 2.4 mm a second, and in most years the maximum was about 2.0 mm. One lab trial for this species clocked up 4.0 mm a second. Some slugs move a bit faster, but the fastest that I have been told of managed 5.0 mm a second.
https://www.quora.com/What-is-the-top-speed-of-a-snail
In mathematical terms, the speed v {\displaystyle v} is defined as the magnitude of the velocity v {\displaystyle {\boldsymbol {v}}} , that is, the derivative of the position r {\displaystyle {\boldsymbol {r}}} with respect to time:
https://en.wikipedia.org/wiki/Speed
Snails are known to be generally slow, but research shows that snails move the average pace of 563.21m per hour. The bladder snail, however can reach the speed of 812.7m per h…our. Statistics show, that the speed of snails is getting quicker, and by 2060, the average snail will have the speed of 874.98m per hour.
http://www.answers.com/Q/What_is_the_speed_of_a_land_snail
Galileo defined speed as the distance covered per unit of time. In equation form, this is. where v {\displaystyle v} is speed, d {\displaystyle d} is distance, and t {\displaystyle t} is time. A cyclist who covers 30 metres in a time of 2 seconds, for example, has a speed of 15 metres per second.
https://en.wikipedia.org/wiki/Speed
Different from instantaneous speed, average speed is defined as the total distance covered divided by the time interval. For example, if a distance of 80 kilometres is driven in 1 hour, the average speed is 80 kilometres per hour.
https://en.wikipedia.org/wiki/Speed
Just as an add on, the snail that did achieve 0.03 mph was the champion racing mollusc named Archie. I believe an untrained mollusc would be unlikely to match this speed.
https://www.quora.com/What-is-the-top-speed-of-a-snail
Lifespan The lifespan of snails varies from species to species. In  the wild, Achatinidae snails live around 5 to 7 years and Helix  snails live about 2 to 3 years. Aquatic …Apple Snails live only a  year or so. Most deaths are due to predators or parasites. In  captivity, their lifespan is much longer, ranging from 10 to 15  years for most species.
http://www.answers.com/Q/What_is_the_speed_of_a_land_snail
Speed of Animals. Helix aspersa. Garden Snails move by gliding along on their muscular foot, which is lubricated with mucus and covered with epithelial cilia. This motion is powered by succeeding waves of muscular contractions that move down the ventral of the foot. This muscular action is clearly visible when a snail is crawling on the glass of a window or aquarium. Snails move at a proverbially low speed (1 mm/s is a typical speed for an adult). They produce mucus to aid locomotion by reducing friction, and the mucus also helps reduce the snail’s risk of mechanical injury from sharp objects.
http://speedofanimals.com/animals/garden_snail
The speed of a common snail is about 1 millimeter per second, or 0.002 mph. Some snail species are faster than some slug species, and some snails do not move at all — they stay put at the bottom of the sea and feed on any plankton that may drift their way.
http://www.diffen.com/difference/Slug_vs_Snail
The average speed of an object in an interval of time is the distance travelled by the object divided by the duration of the interval; the instantaneous speed is the limit of the average speed as the duration of the time interval approaches zero. Speed has the dimensions of distance divided by time.
https://en.wikipedia.org/wiki/Speed
what is the speed of a snail
The speed of a snail is 1 millimeter per second, or 0.002 miles per hour. The speed of a snail is 1 millimeter per second, or 0.002 mph. The speed of the snail is 1 millimeter per second that is 0.002 mph.