This is the straight math formula 
25 / ( ( ( X / 16 ) 5280 ) / 3600 )
(x) is the actual speed of the tank you are testing for
Example: Pershing  Max Road speed 25 mph
25 divided by 16 ( 16 represents the scale reduction)
That amount times 5280 (5280 is how many feet in a statute mile)
That amount divided by 3600 (3600 is how many seconds in hour)
That amount divided by 25 (25 is the length of the course in feet)
The final calculation should be 10.91 seconds over a 25 foot course.
[Note: RCACN currently gives a 2 mph waver on scale speed so the Pershing
can make it in stock form and make it easier for others to be in limits.]
There is a table that goes with the speed appendix it has all the speeds calculated out already.
Actual speed (real tank speedRTS), time in seconds over a 25 foot course for 1/16 scale speed.
RTS seconds (1/16 scale, 25 foot test)
Miles per Hour 
10 
11 
12 
13 
14 
15 
16 
17 
18 
19 
20 
21 
22 
23 
24 

26 
27 
28 
29 
30 
31 
32 
33 
34 
35 
36 
37 
38 
39 
40 
elapsed time in Seconds 
27.27 
24.79 
22.73 
20.98 
19.48 
18.18 
17.05 
16.04 
15.15 
14.35 
13.64 
12.99 
12.40 
11.86 
11.36 

10.49 
10.10 
9.74 
9.40 
9.09 
8.80 
8.52 
8.26 
8.02 
7.79 
7.58 
7.37 
7.18 
6.99 
6.82 