let's do the math on this:
Currently, I hit 5.2l/100. Litres per hundred km is a linear relationship.
If I get 5% higher consumption numbers, then I'm looking at 5.2+(5.2*0.05), so 5.46l/100.
If it were 10%, that's 5.2+0.52. 5.7l/100.
I get around 800km to a tank... 8 * 0.52 = 4.16 litres more fuel used, so around €6 on a tank if the fuel is €1.50 per litre (which it approximately is).
I get more variance than that from day to day, with the same trip, on the same road, at around the same time.
Price per km: assuming 5.2l /100 vs 5.7l/100.
5.2 liters @ 1.5 each --> 7.8€ for 100km, so 8c/km
5.7 liters @ 1.5 each --> 8.55€ for 100km, so 8,5c/km.
A 0.5c per km increase is minimal.
"people could check", yes, they could. But scientific tests are going to be thin on the ground. Back in the day, there were tests done by What Car claiming 10% worse numbers (though the numbers don't seem to have been published). The consumer institute here did some pooh-poohing; yes, there could be a mild increase in consumption, but only in relation to the different energy content of ethanol vs petrol. From a logical standpoint, if Ethanol contains half the energy of Petrol, you've reduced the energy content by a *maximum* of 2.5%. That it's 40% means that number is slightly higher, but it's still not 10. Having dragged Excel out, the specific energy content should be around 96.9% of E5 (90*100)+(10*40) vs (95*100)+(5*40).
It used to be that 98 was around 2c/l more expensive than 95. Now it's around 7 to 9 c / l.