I've had 2 of these trucks, both with the I-6. The first one, an '83, had a 2.47 rear and a 3 on the tree. The one I have now, an '85, has the C6 and a 3.55 rear. The '83 used to get as much as 22 mpg on the highway. My '85 gets about 13. At 65-70, the '85 is churning at 3000 rpms. Generally, especially with these older trucks, you get better gas mileage from a standard shift that you would an automatic. Also keep in mind that your 351 will probably not rev as high as an I6 at 70 mph, due to the extra power. Frankly, I would go with the standard shift. Hope this helps.
I don't know exactly how the hp usage is worked out - it can't be a constant, so maybe the highest loss isn't until full-on at 5-6000 rpm?
The more I dig, the more the 60ish hp figure comes up. And around 15hp for the 4 speed transmissions.
And yet one guy with 302 in an older F1, swapped from a 4spd to a C6, & said he couldn't notice any power loss driving..........he wasn't sure on his mileage.
I can't help much either but I'm fairly confident that the 55 - 60 hp loss is from drag racing guys comparisons & their most likely pulling down 500- 700hp or so on the dyno. I doubt the drag at cruise would be as hugely significant.
I have no doubt it probably does take some more turning at cruise than a t18 have no idea how much but my truck has a c6 & its not like you can feel it pulling the motor down at cruise or anything I average 11.7MPG absolutely loaded to the hilt. don't know what it dose unloaded it nearly never is.
Ken, A little off topic....but I really like the picture of your truck. I am looking to repaint mine, which is the same color, but different 2-tone style than yours. I like the two tone style of your truck better. Would you be able to create an album of your truck. I want to see how your tailgate is painted. Thanks.
I've added some photos showing the tailgate to my existing album '83 F250 XLT' (via my public profile), that were sent to me by the trucks previous owner when I was looking at buying it.
Let me know, I can take some better quality ones if you would like.
To be honest I'm not sure, I used a conversion calculator on the net to convert it & I don't remember which one.
Our speed zones & such are in metric here so we also talk metric figures for fuel consumption. It uses 1liter per 5 kilometers, what we call 5 kays per liter. I forgot to I run 3:5 diff gears for economy reasons, acts like an overdrive so my figures won't be comparable anyways.