Many Americans are slowly transitioning to metric. There was a big push in the late 1970's during the gas crunch that set the program back into the stone age. Back to that in a moment..
Compatibility with existing rules regulations and infrastructure are keeping much of our measurement on current standards. Dealing with dual standards for the transition is too difficult. This is why our plumbing is still English. Too many building codes would have to change and dual stock of both standards would be too much hassle for retailers.
Hardware stores carry both metric and English hardware. This of course brings it's own set of problems when close but not matching threads are forced..
Grocery stores carry many items in metric sizes such as bottled water, soda pop, etc.
Back to the setback with Gas.. During the gas crisis in the 1970's, many gas pumps became obsolete as the 3 digit display could not be set above $0.99.9/gallon. About 1/2 the gas stations upgraded and put in metric pumps. Price comparison became difficult as finding the better deal and those who did mange to bother with the math found the 37 cent gas was no bargain. It soon became common knowledge the low price gas was much more expensive and even when prices evened out, the general knowledge was that was the expensive gas because you got so little of it. The metric stations quickly converted back or went out of business.
Calculating gas mileage was a problem too. Miles / Litre was neither all metric or all English.
High priced metric pumps was probably the biggest roadblock to the US adopting metric in the middle of a good push to convert.
Metric is making big advances in fields not bogged down by this problem. American cars have Metric engines, so mechanics now have dual tool sets. Consumer electronics is mostly all metric, such as the bolts needed to mount a flat screen TV to the wall bracket, etc.
Despite the passage of laws in the 1970s, the USA has not completely gone to the metric system. We still buy gas by the gallon, land by the acre and rope by the foot.
Specifically on gas
Gasoline by the liter in Minnesota, from the USMA Newsletter, Sep-Oct 1979.
At its 2–3 May 1979 public hearings in Washington, DC, the USMB considered the issue of gasoline pumps that were unable to handle prices higher than 99.9¢ per gallon. The 107-page report resulting from the hearings concluded that the U.S. would save at least $94 million by converting pumps to sell gas by the liter instead of converting the pumps to handle prices above $1 per gallon.
At its 21–22 June 1979 meeting in Boston, the USMB voted 13 to 1 to approve a resolution to support conversion of retail motor fuel pumps to dispense fuel by the liter:
The petroleum retailing industry generally indicates a willingness to dispense gasoline by the liter. Several states are taking independent action in requiring or recommending liter dispensing. Therefore, the United States Metric Board declares that:
This is an opportune time for the development of a planned and coordinated voluntary program of dispensing gasoline by the liter and the Board urges all affected parties to participate in the planning process. It calls atttention to the need for adequate public information in connection with liter dispensing.
Without taking this action, metric usage is likely to proceed in a haphazard fashion leading to public confusion, disparate end results and a negation of the positive cost advantages that a nationally planned and coordinated program offers.