What does "$y$ mph is $t$ times faster than $x$ mph" mean? Given numbers $x,y,z$, $x \neq 0$, which is the more appropriate meaning to a phrase such as "$y$ mph is $t$ times faster than $x$ mph"; should it mean that $$(1) \ t=\frac{|y-x|}{|x|},$$ and we should say things like "$100$mph is $24$ times faster than $4$mph", "$110$mph is $1/10$ faster than $100$mph" and "$1$mph is $9/10$ times faster than $10$mph", or should it mean than $$(2) \ t=\frac{|y|}{|x|},$$ and we should say things such as "$100$mph is $25$ times faster then $4$mph", "$110$mph is $11/10$ times faster then $100$mph" and "$1$mph is $1/10$ times faster then $10$mph" (or something else?)?
My guess is that we should take meaning $(2)$ as expressed above, but if working with percentages, we should take the meaning $(1)$ (ie, "$100$mph is $2400\%$ faster than $4$mph", "$110$mph is $10\%$ faster than $100$mph", and so on). 
Is this correct? Is there even a fixed convention?
 A: If $x$ is $t$-times greater than $y$, that usually means (and I would understand it as) $x=t\cdot y$.
A: In another answer it is stated that:
If $x$ is t-times greater than $y$, that usually means $x=t \cdot y$.
However this is not the only interpretation, though it is a common one, another interpretation would be  
If $x$ is t-times greater than $y$, then $x = y + t \cdot y = (t+1) \cdot y$
To explain why lets say I am riding my horse at 20 mph and my brother passes me in has car at 40 mph.  He his travelling at 20 mph faster than me.  This is one times my speed so it would seem reasonable to me to say he is travelling at one times faster than me or 100% faster than me.
Similarly if he passes me at 30mph then he is doing half my speed faster than me or half times faster or 50% faster.
I usually try and avoid statements like this because of potential confusion as it appears to be interpreted differently by different groups of people in much the same sense as 1 billion means 1,000,000,000 to some people 1,000,000,000,000 to others.
When my brother was driving at 40 mph I would say he was travelling at two times, or twice my speed.
When he was driving at 30 mph I would say he was travelling at 1 and a half times my speed. 
