Years ago I tested a variety of octanes in my 82 XJS. I chose 95 octane as apposed to 91 in the V12 due to slight pinging. I didn’t go to the 98 because it is considerably dearer. The 95 oct. has consistently given me about 13.5 l/100klm city-highway loop and with a brilliant performance.
On a recent 6 hr trip (1 hr city-5 hwy) I noted a drop to 12.3 l/100klm after topping up the tank ( about a 1/3) with 98 oct
On the return trip I used only 98 oct and recorded 11.1 l/100klm.
I’m aware that running the V12 too lean can cause an increase in running temp so I carefully recorded the head temps throughout the trip. I’m using 82 C thermostats and recorded head temps 81-85 C throughout the trip. Traffic and hills. For most of the highway we were travelling at 110 klm /hr.
So I guess that my set up for 95 must have had some inaudible pinging which was making the V12 work harder than it needed.
I have not used the 98 before due to the added cost but on reflection (and some calculation) the fuel cost for the long trip was 15% cheaper using the 98 oct.
If no one suggests there is a problem I have not considered using the higher octane then it becomes a no brainer but to switch.
I found similar gains with my XJR-S and other high performance cars, like my Subaru 3.0L R spec B. The extra cost of the high octane fuel was outweighed by the economy gain, and also quality fuel has better additives for longer engine life - it’s not all snake oil. I’ve not bothered to mess with the ignition on the V12 so there are probably more gains to be had, but I don’t want to screw with the settings too much as I don’t have any diagnostics for the Zytek ECU. Modern cars, like the Subaru, should automatically adjust when the octane rating changes.