I have the same setup, (99 Z71/5.3/3.73), and completely agree with the comments
re the 5.3 not being a good motor for towing. These motors are based on the
5.7L LS1's, which were designed for F bodies & Vettes, and they are awesome in
that application. In a pickup, I'd much rather have an old 350 smallblock.

More importantly, though, regardless of what the manual tells you, it is not a
good idea to tow in D. The factory overdrive servo in the 4L60E does not hold
the band tight enough to prevent slip under moderate loads. Slip means heat,
which spells an early death for an auto trans. Also, the frequent up and down
shifting generates even more heat. You can have larger servos installed to
tighten up the band in 2nd and 4th gears, and a shift kit will crisp up the
shifting ("clunky" shifts = less heat). That still won't help the downshifting,
though, and it has to downshift because there's no torque down in low RPMs.

That said, I really like the motor for driving around unloaded, and it gets
good gas mileage (16+) unloaded. My boat is around 4500 lbs loaded, and I live
in Texas (relatively flat). I can't really justify a diesel, but I've owned
them in the past and they are awesome for towing. Not much fun around town,
though.

Trace

The Corvette servo is nice, but it only affects 2nd gear. I put one
in mine when I did the Transgo kit. You still need to upgrade the OD
servo to tow in OD. I didn't bother on mine because I never tow in
OD - I seriously have to be going downhill with a tail wind for it to
stay in OD and not lose speed. I bet a supercharger would really
help with that! :)

If you start tweaking the shifting, you're probably going to need to
reprogram, at least to delete the torque management function.
(google it) If you go with a dyno tuner that knows what they're
doing (westers, pcmforless.com,...) they can give you about 10 more
hp and about 1 mpg better mileage to boot. Hypertech does not have a
good reputation.

One thought I had on the guy with the intermittent clunky shifting -
check your TPS. The transmission looks at the TPS signal to analyze
demand, which determines pump pressure / shifting firmness. If the
TPS is off, the shifts will become excessively harsh or sloppy.

A good resource for this stuff is also fullsizechevy.com.

Good luck-
Trace

Here is a link for the trailer hitches:

http://www.etrailer.com/products.asp?model...06&make=Che
vrolet&t1=&h=e

hopefully this helps. I dont think there is a wiring
harness for the HHR though. you may have to tap into
one of the back lights to get your power, ground, back
up, and brake wires. I had a customer who wanted a
hitch, but not for towing, they wanted to put their
bike rack on it.

Seeing that the HHR is supposedly built off of the
Cobalt 'frame' I wander if you could use one that
would fit the Cobalt? I believe the Cobalt can tow up
to 2000lbs, but don't quote me on that.

Dave