I think I've mentioned this somewhere in another thread, but, at least for me, the question of which octane to run and possible tunes (if applicable) is a rather multifaceted one. It's one thing to run various octanes and engine tunes for power and performance reasons, but when some guys throw in what kind of MPGs their getting, I think we need to take a step back and reassess...Originally Posted by 07JeepXK
While it's common to talk about MPGs and their improvement with octane and tunes (or how they go down the crapper with lifts and added weight), the bottom line for the discussion is really money. I ask, what's the difference between 18mpg running on 87 octane and 18mpg running on 91 octane with a tune? Well, it's still 18mpg, but the difference is the cost of the fuel. I once did some calculating and came up with something like this (for my commute to work):
$27 (to fill half the tank) / 138 miles (my round-trip commute) = $0.1957 per driven mile
So, yesterday I paid $3.29 for 87 octane. It costs me about $0.20 to drive 1 mile, on average. Premium (91) at the same station is $3.59, so it would have cost about $29.50 to fill up. That translates to $0.2141 per driven mile. While this calculation does not take in to account driving styles (burnouts!) or conditions (city, hwy, traffic...), I think it provides a better comparison across setups. So, when the question is asked, "is the 4.7L more fuel efficient than the 5.7L with MDS?" or "which tune (and octane) should I run for better mileage?", I think it's better to just ask "which setup saves me the most money?" So at roughly 1.5 cents per mile difference between running 87 and 91, across 10,000 miles (yearly?) that's $150.
So, at least for me, I'm not looking for power gains, since the XK is my daily driver. I am happy with the stock performance, so I look to save some $$$ where I can. Running a higher octane does not work out for me financially.
I once did a calculation to compare the cost of running my XK vs my wife's turbocharged Subaru Outback. You would think there would be a big difference, but since the turbo requires premium fuel, the costs are almost the same!
Reply With Quote