This is information is kind of interesting too -
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http://www.mit.edu/~jfc/laws.html
The "limit" column lists the maximum speed limit in the state. In most states this maximum limit is set by law. Where state law allows for limits higher than currently posted the speed is listed in parentheses.
Almost all states have laws defining a speed limit that is in effect when no limit is posted. This default speed limit is called the "statutory speed limit". Except as noted below it is the same as the maximum speed limit.
Four states have no statutory speed limits on some categories of highways: Arkansas (controlled access highways), Hawaii (state highways, but 80 MPH state speed limit goes into effect in 2007), Vermont (Interstate highways), and West Virginia (controlled access highways). Alaska has no legislative statutory speed limit but the state DOT has established unposted speed limits by regulation. In these states the DOT could raise speed limits to any value that could be justified by an engineering study, or abolish speed limits entirely on the indicated classes of highway.
Some other states have statutory speed limits but permit that speed limit to be increased without bound by posting signs. States where increases have been posted are Delaware, Massachusetts, New Jersey, and Rhode Island. As an exception, state law defines the speed limit to be 65 MPH on certain highways in Delaware and Massachusetts. States where the DOT has not used its authority to increase speed limits are Alabama, Indiana, Minnesota, North Dakota, Oklahoma, and South Carolina. Montana law allows limits speed zones to 50 miles in length, so although the limit could be increased above 75 it would not be possible to increase the limit on the Interstate system as a whole.
Several toll road authorities have greater power to raise speed limits than the state DOT, but none of them have used this power to raise the maximum speed limit. Except where the legislature has ordered the speed limit to be 65, the Massachusetts Turnpike can post any speed limit or none at all. The Ohio Turnpike explicitly has the power to disregard state speed limits, ORC 5537.16, and has used this power to post a uniform speed limit for cars and trucks. Pennsylvania law may allow the Pennsylvania Turnpike to eliminate speed limits, but prohibits speed limits above 65. Penn. C. S. Title 75 § 6110.
A few states permit the DOT to raise speed limits up to a legislatively-set maximum, only on freeways except as noted: California (65 to 70), Connecticut (55 to 65), Florida (55 to 60 on two lane roads and 70 on freeways), Maryland (55 to 65 on freeways and expressways), Mississippi (65 to 70), New York (55 to 65 on state highways), North Carolina (55 to 70), Oregon (65 to 70), Texas (70 to 75 on state highways in sparsely populated counties), Utah (65 to 75), Virginia (55 to 60 on certain divided highways, 65 on freeways, and 70 on I-85), and Washington (60 to 70 on any state highway).
Except in Texas and Alaska the maximum two lane speed limit is lower than the speed listed here: typically 65 in the west half of the country and 55 in the east.
This is the most logical way I've been able to use to determine the true speed limit of any given area.
