On an expected value inequality. Given $X$ a random variable that takes values on all of $\mathbb{R}$ with associated probability density function $f$ is it true that for all $r > 0$
$$E \left[ \int_{X-r}^{X+r} f(x) dx \right] \ge E \left[ \int_{X-r}^{X+r} g(x) dx \right]$$
for any other probability density function $g$ ?
This seems intuitively true to me and I imagine if it were to be true that it has been proven but I can't find a similar result on the standard textbooks, even a reference is welcome.

Unfortunately, your intuitive conjecture is INCORRECT.
Let $f(x)$ be the PDF of the random variable $X$ and $F(x)$ be its cumulative PDF, so that $F'(x)=f(x)$, or
$$F(x)=\int_{-\infty}^x f(t)dt$$
Similarly, let $g(x)$ be another PDF with cumulative PDF $G(x)$. Then the expected value of the integral
$$\int_{X-r}^{X+r} g(x)dx$$
is equal to
$$\int_{-\infty}^\infty \int_{x-r}^{x+r} f(x)g(t)dtdx=\int_{-\infty}^\infty (G(x+r)-G(x-r))f(x)dx$$
By using integration by parts, we have that
$$\int_{-\infty}^\infty (G(x+r)-G(x-r))f(x)dx=\int_{-\infty}^\infty (F(x+r)-F(x-r))g(x)dx$$
Consider this simple counterexample. Let $r=1$, and suppose that
$$f(x)=\frac{1}{\pi}\frac{1}{1+x^2}$$
Then, if your conjecture is true, for no function $g$ will the integral
$$\frac{1}{\pi}\int_{-\infty}^\infty (\arctan(x+1)-\arctan(x-1))g(x)dx$$
even surpass the value
$$\frac{1}{\pi^2}\int_{-\infty}^\infty \frac{\arctan(x+1)-\arctan(x-1)}{1+x^2}dx\approx 0.1475$$
However, suppose that we let
$$g(x)=\frac{4}{\pi}\frac{1}{1+4x^2}$$
Then the value of our integral is equal to
$$\frac{4}{\pi^2}\int_{-\infty}^\infty \frac{\arctan(x+1)-\arctan(x-1)}{1+4x^2}dx\approx 0.3743$$
which disproves your conjecture.
