[gone9]

So Im pretty comfortable with my current trucks fuel economy. Been tracking since 2016 but I had a really odd occurrence in July I just never got around to asking about...

I purchased a 33ft car hauler wedge trailer for a business venture (not hauling cars ironically but its going very well thankfully). Anyways it was about a 5.5hr drive there and about 6.5-7hrs back. The trailer is 33ft long, and on the scale with 4950lb. Now here is what is interesting...

I got 18-18.3mpg between 3ish tanks. on the way there. Just my 1999 7.3l doing about 67mph on average and just cruising. Not really trying and not flooring it either. On the way back stopping at the same fuel stations I got just over 17.2mpg. going between 55-65mph max

Is this due to the upside down boat design and the fact the top of the trailer sits just below the cab of my truck? Truck is around 7900lb with me and full tank but i had about 450lb of gear including the dog with me.

In winter my truck averaged 14-14.5mpg with winter blend diesel but using full synthetic oil and a fuel additive really helped. The full synthetic reduced warm up time immensely and the diesel kleen cetane booster helped but only with winter fuel blends. Roughly a .75mpg improvement in winter now. I have not found a measurable difference using any fuel additive in summer. my numbers are just rounded because I hate all the extra digits.

I triple checked my numbers because I thought it was odd getting 17mpg+ on a near 7hr trip weighing little over 13k.

Anyways figured Id run it by the experts and get some thoughts?