Interesting fuel economy phenomenon

[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?

[aerohead]

Originally Posted by justinooo9

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?

Without more data all I can offer is that you're experiencing a 'wake-rider' situation. The TV is displacing the atmosphere, such that the trailer is essentially fully-submerged in the TV's turbulence.
Completely enclosed, and with a flexible gap-seal between TV and Trailer, you'd no doubt see even better numbers.
It's been demonstrated that a trailer can actually increase the TV's solo mpg, if everything is properly designed. Higher range while pulling the trailer.

[gone9]

Ok thanks, that makes sense actually. I did even consider zip typing pieces of chloroplast to the sides to "enclose" the sides and making little skirts but the sheer cost of the material kept me from experimenting beyond thoughts of grandeur lol.

Appreciate the info and I will research it more. I am just a noob at best but im ocd with tracking numbers, trying to hit the same fuel stations, and at least staying consistent in how I track lol.

[MeteorGray]

So you were going "67mph on average" without the trailer and averaged 18-18.3mpg, and you were going "55-65mph max" burdened with the trailer and averaged 17.2mpg.

I would say that a major reason there wasn't a larger fuel usage penalty for towing the trailer was the faster speed you ran without the trailer in tow. It appears you were running at more than 10% faster without the trailer. Air resistance due to speed is a major factor relative to fuel consumption on the highway, and it builds up exponentially as the speed increases.

Another major factor that really helped you was the configuration of the trailer: it is not very high, so it presented a relatively low-resistance figure moving through the air. If you load the trailer with a high, box-shaped cargo that presents much more resistance to movement through the air, you likely will see a fuel usage differential that is much less kind to the wallet.

But there are a lot of of other things that could have come into play on your trip, especially the direction and velocity of the wind you encountered and any grade differences between the place you started from and the place you got the trailer.

The "wake-rider" factor mentioned earlier could help, but at the 33-foot length of that trailer, I wouldn't think the "wake" would extend far enough back to make much difference.

And don't hold your breath for that trailer improving your mileage to the point it is better when towing it than not, unless, of course, the grade is all downhill on the tow :-)

[freebeard]

Fineness ratio. It might lose less mileage if it weren't so long.

Box it in and you have the world's longest boat tail.

[gone9]

Thanks for your folks input. I kind of thought it had the boat tail look but thought more or less double checked my double checked numbers and they were spot on. I can't really remember a headwind it was a fairly nice day clear skies and un-eventful drive except on the way back... rock slide/detour and some white knuckle driving on very sharp turns/narrow roads till back on the highway. I definatley made some followers very mad but I didn't haven't any opportunity to pull over safely for about an hour .

I will say it was a nice surprise and im certainly not expecting to take the trail on any road-trips in the hopes of it helping but just a fun little story i guess. The trailers primary service is a mobile business in summer/early fall and then i park the trailer and do drop-offs by appointment in winter/early spring. I run a rust prevention business using Woolwax. Super popular in Canada/NorthEast but the west coast didn't start using all the chemical de-icer fluids until around 10 yrs ago. Now folks are seeing the carnage in the undercarriage. I on average hand calculate 14mpg for the very stop n go summer driving wile in service but not much I can do about that outside of driving easy and trying to think ahead more for the appointments and schedule.

If I ever get a hankerin i may try a couple shorter trips with and without the trailer and report back to see if it stayed consistent. We have some really nice long straight stretches with very few stop lights/hills that are all under 1 hour.