Fresh 4.0 ohv install poor gas mileage!

[Danno87]

87 b2. 91 4.0

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[Danno87]

I have an 87 b2 I recently put a 91 4.0 auto swap.. using auto engine,harness and 00' m50d . 91 manual ecu, every component was swapped with the exception of k&n air intake tube/filter, Flowmaster 44s 2.5 inch exhaust, and a be cool 24×18 radiator with 3k cfm fan.. at this point I seem to be getting 9 mpg with 33s and 3:73 gears. Any ideas how I can boost mpg even a little bit?

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[Danno87]

The gas recorded was gps clocked in town over 100 miles and a fill up. Did this twice with same results.

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[JerryC]

I've never done one of those...

So just basic engine stuff.
How was the engine doing before the swap?
Does it feel like it has the correct amount power?
Have you done any service work on the engine, tune-up, etc...
How do the spark plugs look?
Are all of the sensors connected? It only takes one to screw up the fuel mgmt.

Have you or can you pull codes?