Here is a link to the relevant Texas Statute: Title 7, Subtitle C, Chapter 545, Subchapter A:
Sec. 545.351. MAXIMUM SPEED REQUIREMENT. (a) An operator may not drive at a speed greater than is reasonable and prudent under the circumstances then existing.
Combine that with:
Sec. 545.352. PRIMA FACIE SPEED LIMITS. (a) A speed in excess of the limits established by Subsection (b) or under another provision of this subchapter is prima facie evidence that the speed is not reasonable and prudent and that the speed is unlawful.
This section mentions, as an example, among other points:
(2) except as provided by Subdivision (4), 70 miles per hour on a highway numbered by this state or the United States outside an urban district, including a farm-to-market or ranch-to-market road;
What this means is that if you are driving over 70 miles per hour on a type of highway mentioned in (2) above then you are, by legal definition, driving at a speed that is greater than is reasonable and prudent.
When you are cited for speeding you are not charged for driving at a specific speed in an area posted at another speed. You are cited for violating a broader law, such as in Texas, driving in an unreasonable or imprudent manner. The mention of the speed is merely a recordation of the facts that support the state's case against you.
Let's take the proposal to demonstrate the speed is too low to its logical conclusion. You plead not guilty and it comes out during testimony that you weren't driving 80 but really 85. The judge will still find you guilty of the underlying charge as the facts in the case still support that finding.
I have personally witnessed mistakes in tickets result in dismissal. Those mistakes have, however, been related to other facts about the case though: time of day, date of offense, etc.
I've also witnessed people attempt to claim a lower but still illegal speed. For example, "I wasn't going 85 I was only going 80." These resulted in findings of guilt.