LC mileage, forget mods folks, you brought that on yourselves, is highly dependent on mileage. It's all about the aero.
I routinely see a 2+mpg drop from 75-80 mph for example. After 6000miles it is is obvious.
Therefore, I give you a range which accounts for variations in temps, winds etc that I have witnessed. On a + note, the onboard calculations are always +/-.2-.3mpg. Impressive. These are based on CC regulated speed
80mph 17-20mpg
75mph 21-23mpg
70mph 23-25mpg
65mph......too boring to try but I have routinely seen 25++mpg in the hypermiler zone....and this is precisely the speed the EPA testing is done at!!!
Snow tires have traditionally reduced MPG on any vehicle I have owned by 1 mpg.
Sport modes kill mileage in several ways.
1. They hold shift points to give one a "sporty" sensation..often regardless of actual effective and often with a disregard to torque peaks. great for towing and hilly terrain if loaded. Does NOTHING for passing ability etc.
2. They are nothing but a throttle controller hyped up making a vehicle bounce on and off boost unless your inputs are deadly smooth. I ghave hated every TC installed car I have ever tried. STUPID MOD!! TheyNEVER make a car faster...same people thought Vornados helped hehe.
A very detailed review of our LC called out the accelerator as aggressive btw, so sport mode exacerbates this behavior. I routinely make students shut off sport mode when teaching till they learn to modulate smoothly. It is a major weakness in the driving skills of many. Affects mileage, car control at the limit, winter skills esp icy conditions and MILEAGE. Scott Dixon (Indycar) is the absolute master at these skills and a prime reason he is among the winningest racers of all time.
Drive with the battery and boost gauges to teach yourself how to be smooth. That is my second set of gauges and I used it A LOT while learning vehicle....it helped me and I have 25yrs of track skills behind me. If it helped me, I am sure it can help you low mpg folks unless you are always 75mph and above, THEN YOU WILL DRINK FUEL.
Finally, so far, my mileage differences are minimal between regular and premium and this makes sense as the octane really only comes into play at WOT/peak power, high heat, towing conditions etc.
Just food for thought without too much science, but a fair amount of skills and experience.
For the record, 18 v 20s NO DIFFERENCE (SAME OD and weight VERY CLOSE). My setup is stock Yokos on the 20s and Michelin X Ice 3s on the 18s....both treads pretty optimal for highway and snows pulled off when temps broach 40 to save them when they DO get their most inefficient (soft and squishy). Michelins may actually be quieter much to my surprise. My pressures are always 33/33 or less cold. (31/31 bout as soft as I start but ride is exponentially better) More pressure just kills the ride esp at low speeds where all that unsprung weight gets jousty. Notice the minimal number of variables.
My gut says a moderate AT terrain would be like a snow in the mileage department. An aggressive one far worse and a E Load worse sill (unsprung weight is ALWAYS the enemy). While fine climbing in deep snow, they will NEVER compete with true winter rubber unless VERY heavily siped, even then I have never seen a test where they are close.
Hope the summary helps a few folks...in English, if your mileage sucks, you ARE NOT SMOOTH, live in hilly terrain that OF COURSE, kills mileage, lots of redlights/stopngo, jack rabbit starts etc. Try driving like a limo driver and I say that seriously because I did that one summer....you CAN be smooth AND FAST!!!.......ask Frank Perdue.......torque is your friend.