I thought the 2.0 engine in the UK version of the car was the same one used in the US version, but the UK version manages 158bhp and the US one 167bhp. How come?
Possibly based on premium unleaded there and normal unleaded here.
Plus differences because one number is SAE (US; engine power is tested stripped of ancilliaries) and the other DIN (EU; engine is tested with ancilliaries connected).
And their gallons are smaller too. Eight pints in a gallon - same as us - but look at the pints. 16 fluid ounces to their pint, 20 flozs in ours.
Further to that UK Gallon = 4.55 litre, US gallon = 3.78 litre. Therefore UK Gallon = 1.2 US gallons
The power output off the cars is the same but the Yanks just get the data wrong.
Lets be honest they can’t even get the date right 9 - 11 is the ninth of October in the rest of the world.
Technically, there is nothing wrong with their gallon; they use the US Gallon, which was standardised long befo.re the Imperial Gallon. And the Imperial Gallon changes quite regularly; its not so long ago that the UK adopted the Canadian measure for the Imperial Gallon. And we used to use the same date format as the US until about 1910, reverting to the European practice.
SAE is correct for the test conducted. Its Mazda who sometimes mess up, given the debacle that lead the Japanese overstating the power output of the Mk2 in the US (they used numbers for the Japanese market variant, which was more powerful than the US (different inlet manifold) in order to deliberately overstate the performance of the MX5, Mazda was forced to compensate owners, or buy back cars).
Err — I make it the 9th of November in my world… 
And they only have ponys, not proper Clydesdales…hence the phrase Pony Cars.