Picked up a 98 Saab 9000 CSE...
 

A few days ago I bought this car from a private party for $2800. It's in great shape, heated leather, power everything, sunroof, and... TURBO which provides 225 horsepower at 5500 rpm and a monstrous 258 lb/ft of torque at only 1800 RPM!

.gov says it gets 18 city 24 highway.

When I went to the guy's house to test drive it, the car didn't seem to go into boost. I asked him what he puts in the gas tank and he said 87 octane fuel. The car REQUIRES 92! I figured the ECU noticed knock and backed off on the boost. I took it for a day trip to Philly, not doing anything special, just keeping it between 45 and 65 MPH and averaged: 36.9 MPG :D

The next day I filled up with 93 octane fuel and hoped it would start boosting but it didn't. I got frustrated, started checking hoses, and wires. Then I poked the wastegate actuator with a screwdriver and it wobbled. It came off the wastegate lever. Maybe this is why the highway fuel economy was so good? 5th gear is fairly tall on that car, and it's not a lightweight by any means, but for most of that trip, I was operating at rather wide throttle openings and the turbo was idle, probably barely spinning along. I reconnected the actuator and took it out for a joyride. This thing is QUICK, like faster than a WRX. :D

I haven't tested the highway mileage with a functioning wastegate, but I'm sure it will be lower, so now I'm trying to rig up a device to hold it open with the flick of a switch.

I'm just happy to report that I found a luxury sedan that can hang with my gutted economy car in the MPG department, while blowing its doors off in every other category. :)

it's also a heavy car, we have a 2000 Saab 93 turbo at work for bidding jobs and it is alot of fun to drive as well, has the dash mpg gauge and I can get 33-35mpg with it, otherwise it's in the 25-28mpg range when other people drive it, still better then the 12 mpg full size truck.

Investigate to see if it has a knock sensor. Traditionally, that's the sensor that retards the timing and reduces boost to save the internals from detonation.

There's a fine line between octane and FE. Many times, lower octane fuel forces better FE, but reduces performance. Sometimes, the higher octane fuel allows better low-RPM operation (and consequently improves in-town FE) but costs more per gallon.

It may require some testing to see what is both most efficient and cost-effective.

RH77

Don't be so sure that the fuel economy will be lower with the wastegate connected. I would expect it to be the same if you don't use turbo boost. When I drove a 1994 Saab 9000 Turbo with automatic transmission, the computer reported that it averaged 43 MPG after driving about 100 miles. I occasionally used neutral when going down hills. There were few stops on this trip, maybe about 8. The 9000 seems to get about 10 MPG more than the Saab 900. The 2.3 liter engine must be very efficient.

Why wouldn't it do better with the wastegate open? Even under manifold vacuum, I can hear the hissing of pressure building up behind the throttle plate and a subdued "psssshhhhhh" when I let off the throttle. That energy comes from somewhere, no? What about the fact that I can achieve much higher throttle openings without the turbo?

Ok, maybe there would be a small reduction in pumping loss (from lower exhaust manifold pressure) with the wastegate open. I think the throttle is before the turbocharger inlet. That would be more efficient than having it between the turbocharger and the intake manifold. If you can measure intake manifold vacuum accurately you can see if there is a difference at a steady cruising speed between having the wastegate open or closed. If there really is a significant improvement in economy when the wastegate is open (indicated by more vacuum in the intake manifold), maybe you can connect something to open the wastegate at light throttle. It might involve blowing pressurized air into the servo unit.