I took my first long (450 miles) road trip and averaged around 80MPH. I averaged around 16.6MPG on my (265/70R18 SL) Falken Wildpeak AT4W's. I'm using Premium fuel and driving in Eco mode at this time.
One thing I noticed that got me thinking was that the speed in my Waze app (79) was slower than what was on the dash (82).
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I'm not sure how to account for this. The new tires are within .1 inches in diameter of the stock tires so I don't think it could be related to the circumference of the tire. If anything, I think the slightly larger Falken should make the speedometer read as slower than the actual speed because it would take fewer revolutions of the tire to go the same distance. This difference in speed was consistent for the whole trip and not just a fluke.
I bring this subject up because it also seemed like my odometer was off and showing fewer miles traveled between tanks. I started recording a "track" on Gaia GPS when I filled up and also reset my trip odometer. When I filled up next, my trip odometer read 241 whereas Gaia said 246. So, the computer is thinking I've traveled less distance for the amount of fuel I used. The dash said I averaged something like 16.6MPG but simple math of dividing the number of miles according to Gaia (246) by the number of gallons (14.452) = 17MPG.
I am definitely not happy with either 16.6 or 17 MPG. I knew I was going to take a hit with the AT's but nearly 25%?