A lot to unpack here
1) I don't quite recognise the numbers: possibly a mix-up between Brent and WTI. But it is true that crude prices have fallen very sharply since the beginning of October. Brent peaked at over $85 last month, and was below $64 just now
2) It's not really accurate to give most of the credit to Saudi Arabia, which has suggested that OPEC and its allies should cut production: ft.com/content/1e3639…
One of the most important factors has been US policy: its decision to ease off on Iran by allowing waivers for the eight countries that buy most of its oil to keep buying - at least for a while - despite the reinstated sanctions
The other big issue has been mounting concerns about global growth, as reflected in falling stock markets: ft.com/content/59dfad…
3) Falling oil prices are indeed like a tax cut for oil-importing countries. But for the US, which is now the world's largest oil producer and has cut its imports sharply, the net impact is less obviously positive
The evidence of the past five years suggests that the net effect of falling oil prices on the US economy is roughly a wash, with the boost to consumption being entirely cancelled out by the hit to investment
That's the conclusion of this Brookings paper from Christiane Baumeister and Lutz Kilian. It's from 2016, but I haven't seen any more recent research that contradicts this assessment: brookings.edu/bpea-articles/…
While the overall rate of growth in the US may be broadly unchanged, if the fall in crude prices is sustained there could be significant distributional effects. Investment and employment in the oil industry, and in the parts of manufacturing that rely on it, would be hit
In short, it could look a lot like what @Neil_Irwin has described as the "mini-recession" of 2015-16: nytimes.com/2018/09/29/ups…
This chart from @bhgreeley shows how hard US business investment was hit by the oil slump of 2014-16. Does President Trump really want to go through that again? Source: ftalphaville.ft.com/2018/11/20/154… (ENDS) 
