Is the United States the only country not using the metric system?
Burma and Liberia don’t 
A number of other countries use non-SI units in certain circumstances, as well.
The last time I was in the UK (about 4 years ago) they were still using imperial units for some things but metric for most.
They measure distance in miles and speed in miles per hour, but weight in kilograms and volume in liters.
Miles and inches I can deal with. But does anyone outside the US use °F?
There are some people who use imperial units informally, but legally everything here has to be metric. For example a unit of beer is still a “pint” rather than 568ml.
A lot of US stuff is metric such as chemistry and the medical field.
I thought road distances were still marked in miles on the signs in the UK.
Quite right.
This is a country full of anomalies and contradictions.
In the UK, distances are miles, speeds are miles-per-hour.
Most people will state their weight in stones and pounds, and their height in feet and inches (although the medical uses metric).
Food and petrol are all sold in metric.
In some ways the UK is even MORE archaic measurement-wise than the US. “Stone” comes to mind - why not “stones”, to start with? Does anybody but the Brits use that one?
I give my height as six foot one, not “feet”. In Canada, all measures are supposedly metric, but people still use feet and inches for height and pounds for body weight. Also body temperature is invariably in F, although air temperature in C.
About 30 years ago, I spent a year living in an apartment in Zurich that had an outside thermometer that gave the temperature in C and R (Reaumur), which is exactly 80% of C. So I discovered that degrees C + degrees R + 32 = degrees F exactly.
Sort of like using “Pound” as in “it costs 7 Pound”. Never got that one.
As for the miles and mph, in Britain, I always like to tell the story of the first time I drove there. Got into my rental car from the airport, figured out driving on the left wasn’t so difficult, got on the A1 (or maybe it was the A4), and figured I’d go about 100 since that’s about 65 mph, and I didn’t want to speed much on my first drive. After flying by everyone else for a few minutes, it dawned on me that Britain wasn’t metric for distances and speed.
Actually the United States is heavily using the metric system. It’s just not apparent in the day to day consumer market.
science and technical fields use it.
companies that do international trade have to use it.
How is fuel efficiency of cars measured in the UK? I thought I saw adverts on TV there saying such and such car has a fuel efficiency of X miles per gallon. I assume that is imperial gallon? But isn’t fuel sold in litres on the street? 
Or am I going crazy?
The illegal drug trade is all about the metric. There was a comedian who had a joke about that - you know, grams and such, and then you shoot them with a 9mm.
Because the former is used to sell the car and the later is used to measure goods, which pretty much sums the entire situation up.
Myanmar and Liberia and the US are the only 3 countries that still use the English system with Fahrenheit
Burma doesn’t use it but Myanmar does 
And as for Liberia, well they use it too, they pretend like they don’t but you just know when no one is looking they are talking in kilos and meters