Lets say our vehicle is moving at $50\frac{km}{h}$ ($31 mph$) for $15$ seconds only on $X$ or $Y$-axis (also on both). How much does the Geographic coordinates change in that time?
I googled and found, that
One degree of longitude = 111.32 km or 69.17 mi
One minute of longitude = 1.86 km or 1.15 mi
One second of longitude = 30.92 m or 101.45 ft
So if a vehicle is moving $50\frac{km}{h}$ = $13.8889$ which is equal to $14\frac{m}{s}$for 15 seconds which equals $14 \cdot 15$ = $210m/31$ = 6 seconds of longitude/latitude?
Or $\sqrt {3^{2}+3^{2}} \approx 4.24 $seconds at 45 degree angle?
The conversions of angles of longitude to distance that you found are only valid at the equator. They are valid for converting angles of latitude to distance anywhere on the (spherical) earth. For angles of longitude, you would have $1^\circ$ of longitude = $69.17 \cos \phi$ miles where $\phi$ is the latitude. You can see this on a globe, as the lines of longitude get closer together at the poles. Given that you are at the equator, yes, 15 seconds at 50 km/hr is about six seconds of latitude or longitude. If you are traveling northeast, the change in each of latitude and longitude is $\frac 6{\sqrt 2} \approx 4.24$ seconds.