Okay here is some info:
Speed limits in the United States are set by each state. Speed limits are usually:
25–30 mph (40–50 km/h) on residential streets
35–45 mph (55–70 km/h) on urban arterial roads
50–65 mph (80–105 km/h) on major highways inside cities
45–65 mph (70–105 km/h) on rural two-lane roads
55–70 mph (90–110 km/h) on rural expressways
65–75 mph (105–120 km/h) on rural Interstate highways
Western states generally have higher limits than eastern states, and some limits fall outside these ranges. For example, some two-lane rural roads in Texas have 75 mph (120 km/h) speed limits, and there are two stretches of Interstate in West Texas with a daytime 80 mph (130 km/h) speed limit for passenger vehicles. In contrast, the highest speed limit posted on Interstates in Hawaii is 60 mph (95 km/h).
In response to the 1973 oil crisis, Congress enacted a National Maximum Speed Law that federally mandated that no speed limit may be higher than 55 mph. The law was widely disregarded, even after the national maximum was increased to 65 mph in 1987 on certain roads. In 1995, the law was repealed, returning the choice of speed limit to each state.
85 mph limits
The legislation creating the Trans-Texas Corridor allows speed limits of up to 85 mph (140 km/h)[49] on roads built under the program. However, no such roads have been built as of September 2006. The language of the statute does not prohibit the Texas Transportation Commission from raising the truck speed limit or the night speed limit on these roads.
Night Speed Limits:
While the "basic rule," which requires drivers to drive a "reasonable and prudent" speed at all times, is usually relied upon to regulate proper night speed reductions, night speed limits generally may be established on roads where safety problems require a speed lower than what is self-selected by drivers.
Texas
Texas is the only state with a broadly-applicable legislatively-specified night speed limit. Texas statutorily prescribes:
a blanket 65 mph (100 km/h) night speed limit on roads with a speed limit of at least 70 mph (110 km/h)[50]. While the Texas Department of Transportation has the power to lower this night speed limit or raise it to 70 mph, it in fact rarely does, so nearly every 70 mph or higher speed limit sign has an accompanying 65 mph night speed limit sign.
a 55 mph night speed limit for trucks on farm to market roads, complementing the statutory 60 mph truck day limit on these roads. (This is a holdover from Texas's truck speed limits.)
a 55 mph night speed limit on county roads (except for Harris County Toll Road Authority-owned toll freeways).
Other states
North Dakota, Montana[51], and Oklahoma also have night speed limits, but they are only generally applicable to rural, non-Interstate-class roads.
Minnesota once had a night speed limit on sections of MN 61, between Duluth and Two Harbors, due to the danger of rocks falling off the cliffs alongside the road. Tunnels have since been constructed, bypassing the dangerous areas under the cliffs and making the night speed limit unnecessary.
In addition to the legally defined maximum speed, minimum speeds may be posted. There is little evidence to suggest they are enforced.[citation needed]
Connecticut limited access divided highways have minimum speed of 40 mph (65 km/h),[66] but they are not always posted.
Florida interstate highways have minimum speeds of 40 to 50 mph.[citation needed]
Hawaii has a minimum speed posted along much of Interstate H-1 of only 10 mph below the speed limit. The minimum speed is usually 45 mph when the speed limit is 55, and 40 mph when the speed limit is 50.
Illinois interstate highways are posted with both minimum and maximum speeds in many cases. The minimum speed is 45 mph.[citation needed]
Iowa rural interstates have a maximum speed limit of 70 mph and a minimum speed limit of 40 mph, and U.S. Highway 20 between Interstate 35 and Dubuque also has a 40 mph minimum speed, even though its interstate-quality road is signed at a 65 mph maximum. Other four-lane divided rural highways are signed at 65 mph, with no minimum speed. Urban minimum speeds are becoming increasingly uncommon in Iowa, however, until the recent reconstruction of Interstate 235 in Des Moines, a 40 mph minimum speed was signed.
Michigan freeways are usually posted with both minimum and maximum speeds. The minimum speed is currently 55 mph for cars, despite a maximum speed limit of 60 mph for trucks.[2]
Mississippi has a minimum speed of 30 mph on four-lane U.S. highways when no hazard exists. Strangely, there is no law for the minimum speed of the state's growing number of four-lane state highways. The minimum is 40 mph on Interstate highways and on 4 lane U.S. designated highways which have a posted maximum speed limit of 70 mph, but this minimum shall be posted.[67] In 2004, Mississippi posted minimum speed limits (40 mph) on all rural Interstates, but this minimum speed limit was already state law before the widespread posting.
Missouri and Minnesota interstate highways have minimum speed of 40 mph.[68]
A small portion of Interstate 95 in New Hampshire has a minimum speed of 45 mph.[citation needed]
In New York State, a minimum speed limit of 40 mph has been set on the entire length of Interstate 787.
North Carolina has blanket minimum speed limits on interstate and primary highways only when signs are posted. The minimum is 40 mph if the maximum is 55 mph. The minimum is 45 mph is the maximum is at least 60 mph. These minimums do not apply to vehicles that are towing other vehicles.[69]
Oklahoma has minimum speed limits posted on more or less all interstate highways that are 25 MPH below the maximum speed limit. For example, on the turnpikes, which have a maximum speed limit of 75 MPH, they are nearly always accompanied by a sign stating a minimum speed limit of 50 MPH.
The Pennsylvania Turnpike has a minimum speed limit of 15 mph below the posted maximum speed limit. Therefore, the minimum speed is 50 mph when the speed limit is 65 mph, and 40 mph when the speed limit is 55 mph. The Pennsylvania Turnpike Commission also states that vehicles that cannot maintain this speed on a level road may not use the turnpike. This minimum speed is not generally posted.
In Tennessee, a minimum speed limit of 45 mph (70 km/h) is posted in some metropolitan areas where the speed limit is 55 mph (90 km/h) or 65 mph (105 km/h).[citation needed]
In Vermont Interstate 89 has a 40 mph speed minimum at least around the Burlington area, as well as Interstate 189.
In Virginia the Chesapeake Bay Bridge-Tunnel has a minimum speed limit of 40 mph.
In Utah, there is a minimum speed limit of 45 mph on interstate highways, when conditions permit.
Some jurisdictions set lower speed limits applicable only to large commercial vehicles like heavy trucks and buses. While they are called "truck speed limits", they generally do not apply to light trucks.
[edit] Theory
Since trucks are far heavier than other vehicles, they take longer to stop, are less adept at avoiding hazards, and have much greater crash energy. Therefore, it follows from basic physics that limiting truck speeds could reduce the severity and incidence of truck-related crashes.
However, the research record is mixed. A 1987 study finds that crash involvement significantly increases when trucks drive much slower than passenger vehicles[70], suggesting that the difference in speed between passenger vehicles and slower trucks could cause crashes that otherwise may not happen. Furthermore, in a review of available research, the Transportation Research Board, part of the United States National Research Council, states "[no] conclusive evidence could be found to support or reject the use of differential speed limits for passenger cars and heavy trucks" (page 11) and "a strong case cannot be made on empirical grounds in support of or in opposition to differential speed limits" (page 109) [71].
Two thirds of truck/passenger car crashes are the fault of the passenger vehicle[72].
Hope this helps you good luck!!!!!!!!!!!!!!