Hi Tom,
   On a new installation I would assume that things like the air filter and
spark plugs were installed new or at least inspected.  Was an OBDII plug
included in the installation?  Check engine light?  If so, Autozone will
read the codes for him.
   The O2 sensor is one of the usual suspects for poor mileage.  With a DVM
he may be able to monitor the signal voltage while it's idling.  It should
go up and down from .1V to .9v or so.
   Another possibility is blocked exhaust due to the inside of the cat
breaking up.

I would expect a bad cam or crank sensor to cause serious stumbling if the
engine runs at all.  The MAF sensor or its equivalent might also be dirty
or defective.

Good luck,
Larry

On Sat, Jul 30, 2016 at 12:03 PM, Tom Stone <tstone8359@aol.com> wrote:

> My son is driving his '84 Vanagon with a newly installed 2.5 Subaru engine
> in it back from Colorado to Ohio.  He has noticed a decrease in fuel
> mileage as he comes east.  He says it runs great and accelerates normally
> but he is concerned about the mileage drop, from 22 to 16mpg so far.
> Sounds like a sensor problem but I know nothing about Subarus.  Anyone have
> any suggestions?
>
> Tom
>