Am I doing the math right?

So I have a 2015 crewmax Pro (26 gallon tank) and I am tying to figure out my average fuel range on a full tank. I just need someone to double check my math.

I reset a trip meter at full tank and was able to get 240 miles before gas gauge was resting on E (80% city driving).

I did go up in tire size, stock 275/65r18 now 295/70r18. Speedo was not calibrated so it's off by 6.38%.

Would be correct in saying that the actual milage on full tank was 240 + 6.38% ? That would result in 255 miles.

I assume same would go in MPG calculation, truck is showing me a consistent 11.8mpg but in reality add 6.38% and we end up 12.55mpg.

Am I correct in this way of thinking?

I just like to have an idea since estimated range displayed at fill up is way off and tank is small enough too be a concern when planning trips etc. After refueling I was only able to squeeze in 19.9 gallons which means at E there is still 6 gallons in reserve....I know there are multiple threads on this but I find that part absurd.

 

Looks good to me. If the 6.3% figure is accurate. Your range is about 325 +- depending on a bunch of factors. Personally I am conservative When planning when to fuel up. The truck fuel gauge indicators on these trucks seem to keep a lot of fuel in reserve vs what they indicate. I have the large tank and, even with the gauge showing low there is a 6-8 gallon reserve when I fill up. That being said, the psychology of driving with the needle and computer showing low fuel is gnawing. Not fun.