Also, the minimum Octane fuel we get in the UK, and most of Europe, is 95 RON, which converted to US is 90.7, and for premium it's 97/98 RON which again, converted to US is about 94. Think I got that right anyway...;)

As far as I know, neither octane rating nor cetane rating is directly related to energy content per unit volume.

Octane rating is a measurement of resistance to preignition or detonation for gasoline. Higher octane ratings allow engines to be run at higher compression ratios, thus higher efficiencies. If used in the same engine, and under conditions where the higher octane is not required, low and high octane fuels produce the same power and economy.

Cetane rating for diesel fuel is a measure of its ease of ignition, in a sense the opposite of octane rating. Higher cetane fuels will ignite more easily, thus making cold starts easier and reducing engine "slugging" at light loads. The "slugging" is what causes some of the diesel clatter at light loads and on cold starts. It is caused by slight and variable delay between the time fuel injection begins and fuel ignition occurs. When ignition does occur, the fuel already injected into the cylinder ignites quickly causing a pressure spike and noise. Higher cetane fuel ignites more quickly and reduces this effect.

Premium petrol can be slightly lower than regular in energy content; depends on what is used to up the octane. It can have better economy in a high compression engine. The longer power stroke extracts more energy from the burning fuel, but there are numerous variables that effect economy.

The 1.4 turbo in the Sonic and Cruze was designed for Europe and higher octane. I got better economy on the one tank I tried. With the current price difference between regular and premium, it isn't worth the cost, though. I figure the break even gap is 25 cents, but most places are near 40 cents here.

Is that the 1.4 turbo from Fiat? I had that in my last car and did almost 60,000 miles in it, 99% on regular. MPG ranged from 26 MPG to 58 MPG depending on how I drove ;)

It's a GM Ecotec, and it is getting replaced by a newer model with direct injection.

Hang on, another 6% drop in oil, gasoline should continue it's downward trend, come on $2 gas!

Yea yea you keep praying for your $2 a gallon, and i'll keep my figers crossed for $8 a gallon! Lol ;)

Just today, 12/3/14, gas was $2.659 at a local truck stop. Diesel was $3.649 cash, $3.699 credit. That makes diesel (cash price) 37% more expensive. Most diesels sold here struggle to get 30% better mileage than comparable gassers. I'm wondering whether some of the truckers might switch back to gasoline. Particularly when the "joys" of diesel particulate filters and urea injection are factored into the equation.

Whoop There It Is! Gasoline is selling for under $2 per gallon in Oklahoma City!

Thats the case in the US, different all together here. If you look at 3 of the top cars on fuelly as an example, VW Jetta, gas version (very rare here, only 2 for sale in the UK at the moment) it does an average of 45 MPG, the diesel does 67, so that's 50% better. Honda Civic, same story, 52 average for gas, 78 MPG for diesel, about 50% again. Ford Focus, gas does 47 MPG, diesel 67 MPG so about 44%. Thats before you've factored in the pollution tax which is always cheaper for diesels, insurance is cheaper too, and servicing is required less often. As for DPF/Urea problems, not somthing we need to worry about here either.