'86 6.9 with C6 and Banks turbo, 3.55:1 gears 4x4 crew cab. Prior to new pump and injectors it was averaging 11-12.5 MPG the previous few months, the two tanks since it has done 10.19 and 10.91. I suspect because I'm using the additional power as it was terrible gutless previously, basically could use the pedal as an on/off switch and it was still ridiculously slow.
Over 21k miles I've seen:
Worst of 8.25 (gained 2-3 MPG since that first measured tank by advancing pump timing from 6*-8.5*)
Best of 15.22 (on 45-55 MPH two lane blacktop)
Average of 11.85.
The key here is most of my driving is either around town stop and go or highway 65-70 MPH. The slip of the slushbox torque converter and lack of overdrive kill MPG in those conditions. ZF5 is currently in the works and I expect highway MPG to improve to mid teens.
IMO a turbo probably won't make a huge difference in MPG but may help a little. You'll notice improvement in power and EGT though. What you really need is a ZF5 if you want more MPG.
On the econobox, even when fuel was $3.50-$4.00/gal is would take a LOT of miles for a decent econobox to reach ROI particularly when factoring insurance costs (NJ is very high primarily resultant from socialist laws), taxes, and maintenance. Crunch the numbers and see how far off ROI is. I restore classic bikes as a hobby and primarily ride for half the year. I don't consider TCO, ROI as it's not there for the time and money involved in a restoration and the miles I travel, but riding costs $.04/mi in fuel these days while the IDI costs $.15/mi. Cost of restoration, maintenance, taxes, insurance for the bike would exist either way but every mile I ride instead of drive saves me $.11 these days. Varies based on fuel prices... Average cost per mile since I've been tracking is $.26.
Point is, if your goal is to save an appreciable amount of money, crunch the numbers and weigh your options. I always laugh when people at work go out and buy a new $30k econobox with payments and interest and brag to me about how much money they're saving. I tell 'em stop right there unless you want to hang around for a bit while I crunch the numbers on how money you're losing per month and over the time you expect to own the vehicle. Perfectly fine if they just want a nice new luxury, we all have things we value, but it annoys me when they try to mask their extravagant luxury purchase (which they usually can't afford, hence the loan) as some sort of great frugal endeavor.
Rant over... Overdrive will get you the most bang for the buck on MPG improvement with a fairly short number of miles to ROI.