As far as I know it should be a 10.25 if it's on an OBS
. From what I understood the 10.5 wasn't around till the Superduty came out & the 10.5 uses a metric bolt pattern.
Personally I'm a firm believer in a free flow exhaust & intake. It might not get another MPG
or two but it let's the engine breath better which is always a good thing. It also paves the way for a chip if you want one. I've delt with & driven a few Powerstrokes stock & modded & there is a night & day diffrence with the chip. I know my buddy just put a DP
Tuner on his truck with the 80 ecno he has no problem merging onto the expressway usually without any smoke either. But with the stock setup he would sometimes have to floor it to make it down shift & wait till it got up to speed to merge with traffic. In other words the engine doesn't have to work as hard to get moving up to speed. Not sure MPG
wise crusing, we drove about 70mph on the Ohio Turnpike & got 19-20mpg the whole time with his DP
Tuner set in 80 ecno. We didn't try it with it set in stock to see what it would do but with the amount of traffic the added power was nice to have if we needed to get out of the way in a hurry.
On another story when I got the IDI
it had a 2.5" exhaust coming down to a gasser muffler then the tail pipe was only 2.25" big. The tail pipe was smaller than the manifolds...
It also had a large fiberglass topper on the back that was promptly removed.
With just a straight pipe & a normal open pickup box I could see 16 mpg
on a regular basis in the summer. The last owner said the average he normally saw was 14mpg. So I'm not sure what did more, the straight pipe or removing the topper but all I know is that it picked up a MPG