Gearing has a lot to do with highway mileage.The '93 Cavalier had a 2.53 final drive ratio with a 1:1 3rd gear.My '88 2.8 Cavalier had a 2.73 final drive with a 1:1 3rd gear.The HHR has a 4.17 final drive along with a 5th gear ratio of about .70 which would make the ratio a 2.92-more revs for a give road speed.
BTW,the average mileage for my '93 Cavalier was always 22-24 m.p.g.

 

Very true. I had no idea that the older j bodies had such high gearing. That combine with specific driving situations and possible lean a/f ratios from the factory could offer a resonable explanation for the much better than sticker mileage. Too bad (yeah I hear the HP lovers cringing) we did'nt have a higher gear ratio...or 6th speed (see im not so bad after all).

The new cavy gets 36 on the highway, so if I drove that and had 20% better mileage than the sticker like I get in the hhr, 43mpg highway should be possible?????

 

It was done because of the lack of an overdrive automatic trans that would fit in the car.When the J car was revised for 1995 room was made for the 4 speed automatic.Only the early (1982)manual trans cars were 4 speeds.Five speeds came in during the 1983 model year.

 

Just some notes i have over the last 10k miles...

2008 LT2 Panel
2.4L Auto
70-80% hwy 75mph or less.

The trans seems to stay locked when getting out of the throttle, so i've found that bumping it to neutral helps...about 1mpg. This was an annoyance to me, so that's my fix for now. Anyone know if the TCI transmission computers works on the auto? I'm looking into reprogramming the shift points and TC lockup to keep it from not upshifting and unlocking when it should be.

Mileage:
Winter: 24.5mpg.... a few minutes of warm-up each morning.
Spring: 28.5mpg on the last tank.
Summer: looking for 29+ when it gets hot out....i don't run the A/C much.

Speed: this is definitely a biggie.
45mph~40+mpg
55mph~35 mpg
65mph~33 mpg
70-75mph~28-29 mpg
75-80mph~23-25 mpg
...all from the DIC on level ground.

As for high mileage in the city with lots of stop and go... i find it hard to believe that any non-hybrid/unmodified vehicle with similar power and weight will get much better than 20mpg under those conditions...just about any vehicle with a DIC will show that when taking off from a stop around 4-10mpg no matter how fast you accel. The laws of physics still apply here on earth...someone let me know if i'm wrong here.

And for anyone that is thinking about a HHO generator to improve mileage....they do work! 10-20% improvement from what i've seen. You just have to be careful on the other modifications that are needed to make sure you don't use more gas or have a meltdown. Also, be very leery of some of the poorly built units as they have design flaws that could cause serious damage. Do your homework on these...which is what i'm doing now. Oh and the HHO isn't helping like you may think, it's really helping the combustion efficiency of the gas that is already in the chamber.

 

I respectfully disagree with the HHO. All I can say is that everything in my Chemical Engineering degree cries foul when people bring this subject up. I don't want to write a thousand pages here. So let me simplifiy.

Its crap.

No offense to anyone who has this system, or wants, or believes. No matter how you cut it, they don't help. I can't say if they hurt. But please don't waste too much money here. If you can get it to work on my vehicle and I see 39-43mpg on the highway (10-20% improvement over my current flat ground warm weather, no wind average mpg over extended trips) Ill pay you 5 times what you paid for the system. No questions asked.

People lie to you. I started a thread about this.

Again I repeat, by all means do what you want with YOUR hhr, i am not telling you what to do.....just what WILL and WON'T work.

 

The whole problem is one of PEBSAGP (Problem Exists Between Seat And Gas Pedal). Yes there are some other issues out of our control like wind, gas quality, MTBE additives, traffic and some in our control like tire inflation, weight in the vehicle, extra attachments on top of the vehicle and use of the cruise control. But the biggest way to get the best gas mileage is to drive consciously. So many people put their mind on cruise control when they drive they are not aware what they do to kill good gas mileage.

I drive a 2LT with 2.4 and 5 speed manual. My commute is about 35 mile round trip, with about 20 miles on the highway. Over the last four tanks I have gotten 27 – 28 MPG. I use the cruise control, coast and keep my foot out of the pedal as much as possible. I shift at 3000 rpms and do not start hard from the light.

So my best and really only advice is to Drive Consciously.

 

True...True. As for the stop and go, if your car is warmed up, and the road is relatively flat, and you are not in NYC-DC-SFBAY etc parking lots called interstates just push and glide. The 4-10mpg during the push will be balanced out with the 99+mpg during the glide. Youll be suprised how far this car will glide with just a small blip of the pedal. I do routinely get 25+ mpg in the concrete jungle.

 

I have found that 17,000 miles was the break in point for my HHR.

Up until that I would travel to St. Louis (about 300 miles) between 65 and 70 and the most I got was 29.9 mpg.

Now the car has 18,000 miles on it and the mileage has started to go up. This last trip to St. Louis it I was looking at 33.2 mpg. Would have been better if the wife hadn't had to make 3 different pit stops :)

Cruise control still sucks with it. The slighest inclines the car thinks it has to grab a gear to get up up it and sometimes it thinks it needs to grab two gears and tach 4200 up the hill.

I have learned to anticipate the hills and tap the break to turn cruise off up the hill and turn it back on at the top of the hill.

 

I forgot, thats also with 89 octane.

 

Have a 2007 2LT - consistently get 30-31 on hiway. Use the cruise control which is a must with any car to get best fuel economy. If you have a lot of hills which cause downshift, you will see a drop in mpg. One of the advantages of a lrger engine is less downshift and better economy. My G6 consistenly gets 31-33 highway with the 3.5l engine. The 2.4l in the HHR gets lsightly less.