If you want to test your maximum bandwidth using wireless is often a poor choice. Wireless g has this theoretical speed of 56mbit, which is in practice at the very least halved. Wireless n might have enough bandwidth to cap out a 75/35 connection, but only if the laptop is really close to the router.
The ethernet result shows that something is wrong. It can be a hyperactive anti-virus, a virus, a cheap router unable to hit 75mbit, congestion, routing at the ISP side, or a bad signal for the modem, etc. Definitely worth investigating, since you're not even close to what you're paying for.