This is a small part of an article that I used for information for choosing a turbo housing for my towing. It may be of interest as it is opposite from info for racing. It worked for me:

Install a turbo boost gauge in the exhaust manifold before the turbocharger. Now you have two boost gauges, one for the intake boost pressure and one for the exhaust manifold pressure. What are these gauges going to inform you of? During cruising speeds on the level the exhaust backpressure should be approximately 66% of intake pressure. While pulling moderate grades the exhaust backpressure will increase more than the intake pressure will however it should still be less than the intake. At wide open throttle the exhaust pressure may be greater than the intake pressure but only by 2 to 3 pounds of pressure.

What does all of this mean? If most of you trucking is on level ground or rolling short hills, its best to have a larger turbine housing to minimize the back pressure. If the majority of your trucking is in the city with a lot of stop and go driving, than a slightly smaller turbine housing is preferred. The smaller housing is good for quick turbo response and instant boost, however on the open road the backpressure may be too high.

High altitude operation of 5,000 feet or more will also require a smaller turbine housing because at 10,000 feet elevation there is one half of the molecules of oxygen than at sea level and a loss of 3% horsepower for every 1,000 altitude your climbing. So at the Eisenhower tunnel on I-70 in Colorado the altitude is 11,400 feet and your 400 horsepower engine is now 268 horsepower. The pyrometer heat or exhaust gas temperature will be about 200 degrees hotter at the top of the mountain also. If you spend the majority of your time at high altitude you may have to experiment with several different turbos to find the one that meets your particular requirements.

High humidity displaces oxygen so that can throw another monkey wrench into the equation.
Now that you have this additional knowledge you’ll find out that there is no perfect turbocharger for all situations. You have to compromise. What’s good for the city driving is not perfect for highway driving. What’s good for the open level road isn’t perfect for high mountains.

Wastegated turbochargers are not the answer. The wastegate is set to open at a predetermined PSI setting and at cruising speeds. The backpressure is usually equal to the intake pressure. Wastegated turbos usually use a very small turbine housing for quick response. This is great for city driving, but not ideal for over the road. At high altitude why would you want to dump the exhaust overboard when you need the extra boost to keep the engine cool.

 

Cool, thanks HID! How's that 18.5 working out for fuel mileage?

 

I get overall better pulling power. I get some slight mileage gains in great towing conditions, ie no wind, warm temperatures, not too many hills. The harder it is worked, the better it does for the economy. In bad conditions it is a real good savings at least 1 mpg but more. I will know better later, the weather has been bad.