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First of all, this is a tl;dr post, so scroll to "Final thoughts" if you don't want to read all of it.

Before I start, a quick listing of some relevant information:
Vehicle:
1997 Nissan Altima 5-speed
2.4L straight 4
EPA New: 21/28, 24 combined, Old: 24/30, 26 combined

My main commute is 6 trips a week to and from a local tech school about 6 miles away with a roughly 50/50 mix of city/highway by distance. Most other trips end up being mostly city driving with a small amount of interstate driving here and there.

Before using SGII:
I was getting 24-25 MPG on my trips to school and 28-29 on a previous job in a city about 15 miles away by way of the interstate. These numbers increased a bit in early summer (no AC) and dropped a bit in late summer (using AC.) My previous record was 34 MPG on a roughly 330 mile trip to the north end of the state that was mostly highway driving. I usually got 31-33 on those trips otherwise. I didn't employ any hypermiling techniques but always give myself a decent following distance which leads to less use of the brakes. I usually shifted around 2500 RPMs and accelerated at a rate that maintained 5 in. Hg manifold vacuum. (I "installed" a vacuum gauge on my dash for some diagnostic tests a while back and decided to make it semi-permanent rather than remove it. I'd highly recommend anyone trying to improve fuel mileage do that, I'll explain below.) I usually wouldn't upshift unless I was over 2000 RPM.

First impressions:
The SGII was on my list of devices I was contemplating picking up for computer code reading and diagnostic purposes. I was weighing the pros/cons of various devices all the way from basic code readers up to advanced data loggers. In the end I settled on the SGII due to the cost and the fact it is designed to be mounted as an additional gauge on the dash and used day to day. I'm still kicking around the idea of getting an OBDII to USB cable so I can use my laptop as a data logger for diagnostics. (If anyone thinks those things aren't worth the roughly $75 they cost feel free to let me know as this purchase isn't planned for a number of months.)

The SGII doesn't have the update speed I was hoping for. Even on the fastest setting it only updates roughly once a second. After using it for a while I've come to realize this speed is fine for hypermiling use but I believe it would be a bit slow, although not completely useless, for advanced diagnostic use. This is where a good vacuum gauge comes in handy for maintaining a steady load during acceleration as the load readout lags a bit in higher gears and is 100% useless in lower gears whereas the vacuum gauge gives instant feedback on what throttle position to use. (From my use I've determined that on my car at least a particular load number in a particular gear matches a particular vacuum readout over any range of engine RPM. 75% load on my car is roughly 5 in. Hg vacuum in 5th gear, 6 in. in 4th, and 7 in. in 3rd. I don't worry about 1st and 2nd as I try to get out of them as quickly as safely possible.)

The first test:
My first test was the aforementioned long distance trip to the north end of the state. I didn't have the SGII calibrated yet so during the trip I just attempted to get the avg readout as high as possible. I used this display configuration:
AVG LOD
MPG LP
I employed the DWL and NICE-on techniques on hills (I can't do NICE-off due to a faulty idle air control valve, see end of post for explanation) and raised my tire inflation to 35PSI front 39PSI back. (I don't recommend you do this if your roads are in need of repair. You WILL feel every slightest imperfection in the road especially if your suspension is tighter. I have since dropped the back tires back down to match the front.) When there was traffic behind me I used a modified DWL where I set the cruise control at the speed limit and added throttle during downhills to keep the manifold vacuum below 20 in. Hg and thus avoiding engine braking. (My engine has rather high compression (KA24DE if anyone wants to look it up) and has quite a bit of braking power even in higher gears, so in my opinion allowing the cruise to close the throttle completely was wasteful of energy in much the same way as using the brakes is.) I did this to be at least a little considerate of those behind me and allowed them plenty of opportunities to pass.

The trip normally took me between 5.5 and 6 hours on my usual Interstate and highway route. I took a different route that was a bit shorter and seemed to lead me through every little small town in the state. It took about 7.5 hours this time, but I blew away my old mileage record and got 37.2 MPG. (I probably could have gotten it higher if I could pulse and glide on a roughly 30 mile section of road that is next to a railroad grade. 10 miles of it is almost salt-flats in grade. 100 feet elevation difference from one end to the other according to my GPS.)

During this trip I noticed my average would increase considerably going through towns and only increase slightly or hold steady on the highway. (It seems DWL targets city mileage and only helps highway in very hilly areas.) I found that optimum steady speed is around 42 MPH in my car, getting low to mid 40's MPG readouts.

On my trip south it took me about 8 hours and I got 37.3 MPG. I would have taken less time and gotten better fuel mileage but I was stupid and followed my GPS (please put down the cluebat...) down a wet gravel road. (It felt more like I was in a boat than a car...) I also had to reroute around an accident (and resulting interstate parking lot) which added some distance to the trip.

Day to day use:
Using DWL and NICE-on and changing my shift pattern to be more in line with what's in the owner's manual for the car instead of the factory service manual (the factory service manual says shift at 2500 RPM but the owner's manual puts the shift points around 2000) has raised my average mileage to the low to mid 30's for my daily commute to school and other various driving. I'm currently at about 34 MPG estimated for this tank and near the 1/4 mark where I usually fill.

Other things of note:
I've noticed a few quirks with the SGII. If it's set to gas for fuel type and I attempt a NICE-off the gauge will shut down even though the vehicle is in motion. Changing fuel type to hybrid seems to fix this. At idle it will tend to switch to injectors off (9999MPG readout 0.00GPH) even though that clearly isn't the case. Giving it just a little bit of throttle switches it back to normal operation. (It does report open loop when it does that, so perhaps I've got an O2 sensor that is approaching the end of its life.)

The speed readout seems to be within 1 MPH of my GPS without calibration. It seems to fluctuate from matching exactly to being 1 MPH above. The tank distance currently lags my trip odometer by amount 10 miles on a roughly 300 mile tank. (I'll update this with the exact discrepancy when I fill the tank.) I can't comment on tank mileage accuracy as I don't think I've done enough calibration fills yet.

When scanning the computer, it will sometimes give me a code of P0325 (knock sensor) then lose it on a re-scan (without clearing the codes.) Sometimes the code will "stick" and still be there even after clearing the codes. The check engine light is never lit during this. I'll probably check the sensor this summer some time. I haven't yet sabotaged the engine to try to get a code out of it (I believe it'll throw a code if I disconnect the TPS before a cold start.)

Final thoughts:
After a couple of weeks of using it I've found that I use the vacuum gauge more for an instant MPG readout than the SGII. I expect that as I tune my driving style more and better refine my hypermiling techniques it will become less useful for fuel mileage purposes. I will not be selling it, however. The other features that it has will more than make up for the loss in usefulness for hypermiling! I don't think I can stress enough how valuable a vacuum gauge on the dash board is for fuel economy. If you do buy a gauge don't get one of those tiny dashboard mount gauges. Get a decently large diagnostic use gauge and find someplace to mound it with a bit of velcro tape. I don't think I'd be 40% over EPA combined without mine. (I'll post some pics of my dash later in another thread on a more appropriate board, perhaps the introductions board...)

A few offtopic questions:
1. Would it do anything for fuel efficiency to buy high octane no-ethanol gas and advance the timing a bit? Right now it's set dead-on at 20* BTDC as given as spec in the factory service manual and Haynes book. Another question is would it be worth it? 4% gain, for example, woudln't be as even though I'd be burning less gas I'd be paying more for it per mile.

2. What effects does replacing the catalytic converter with a resonator pipe have on the system as a whole? (Apart from emissions that is.) I ask because this was done a month or so back due to a "misidentification" of the converter as a resonator pipe (I haven't yet decided if it was deliberate or not) by a small shop. The converter developed a leak right at the back where the pipe joins it so it had to be replaced. The rear O2 sensor on my car is oddly just in front of the cat instead of behind it so this hasn't caused the check engine light to come on. I haven't corrected the problem as there's no emissions testing in my state and I haven't noticed any issues die to it.

Notes:
Normally with a stick shift NICE-off would be a piece of cake, but the fault with mine causes it to refuse to idle after doing a clutch start. It runs fine afterward, but the moment I push the clutch in the engine will die unless I feather the gas. It starts to idle normally again after about 30 seconds of this. I'd fix the problem but I'd have to either pull the engine (lots of work) or take off the top half of the intake manifold (and the injectors, fuel rail, a bunch of vacuum lines for the EGR valve, and a bunch of other little things that seem to be in the way) to get to the part.
Now that I think about it, I probably should have used this as a sort of automatic FAS on that flat section of highway that killed my time. I don't do it normally as it's hazardous to have it happening in traffic.

If you decide to install a vacuum gauge remember to find the manifold vacuum and not ported vacuum. The line to the brake booster is always a safe bet. Keep in mind that you may need to buy some brass fittings to make yourself an adapter T junction to connect your gauge hose to whatever line you tap.

Hi TH:

___Just a quick reply... Your Fuel cut is set to low if you are seeing 9999 mpg or 0.0 gph at idle. Next, when moving from Gas to Hybrid, you have ICE-Off recording in most vehicles. Next, what is your instantaneous FE with a VAC gauge? Not a god sign as it’s different for each gear, speed, and condition.

___Regarding bypassing a CAT, would you pipe your cars exhaust into your home with or without the CAT hooked up? Please consider this question carefully as we all breathe your exhaust as you have to breathe ours.

___I hope that helped.

___Good Luck

___Wayne

Thanks for the reply xcel. I'll try increasing the fuel cut and see if that fixes the problem of 0.00gph at idle.

I'll have to take some notes on the vacuum vs. instant FE readout, but I know off the top of my head that in 5th gear 15 in. Hg is around 39MPG. Lower gears give lower FE at the same vacuum. I'm sure this will change over the summer once it gets warmer as the air density drops as the temperature increases.

Regarding your second point, I wouldn't pipe the exhaust of any ICE into my home, cat attached and working or not. Also, there are many vehicles on the road without converters. My neighbor has a truck from the 80's that didn't have one from the factory. Every time he starts it up in the summer I can smell it. (It's far worse than my car though as he has only minimal emissions controls on his engine compared to mine.) I'll probably get it fixed later this summer by a reputable shop though. Right now it's looking like I'll have them replace everything from the exhaust manifold back as I've had more than a few patches to it. Wisconsin winters tend to be hard on the exhaust...

Let me clarify (upon re-reading that section I don't think I was clear) that it wasn't my choice to remove the cat. (Hence the quotes around misidentified, as I've read that cat theft is a growing problem due to the value of the precious metals in them.) It was pointed out to me as a leak in a "resonator" and replaced with such a pipe. I only started comparing the exhaust to what was listed in the Haynes book a week after the repair was done when I noticed the smell of the exhaust when working on the car. Also, the fact the part that was chopped out was behind the rear O2 sensor led me to believe what I was being told by the shop was correct. Cat is usually in front of the rear O2 sensor and will trip the computer if it is removed. This was behind it and didn't trip the computer after removal so I didn't think anything of it at the time.

Hi TH:

___Please get your exhaust system fixed for the sake of you, your family and the rest of us. What somebody does down the street, up the block or down the road a piece has nothing to do with a car emitting hundreds of times its rated emissions specs due to a faulted exhaust system. I purchased a PZEV from across the country for this very reason. Please, the least you could for everyone involved is make sure the OEM exhaust and emissions systems are in working order before doing anything else. When you read up on PZEV's, you would much rather have one of those piped into your house than any ill-fitting/ill-working exhaust and emissions control system on anything else.

___Regarding the SG-II, along with the calibrated i and aFCD for fuel consumption and distance traveled, we use the included IGN, LOD, TPS and a number of X-Gauges to maximize FE in any number of vehicles. I you want to watch the MAP, that is available to be displayed as well.

___Good Luck

___Wayne

If you're that worried about my exhaust system I can PM you my address and you can ship me a cat. I simply don't have the $100+ to have it fixed right now. I'd scan and email receipts and take before/after photos of the exhaust system to prove it was installed. Continued postings on a web forum harping on me won't get it fixed any faster. I plan on getting it fixed after I get some money ahead from work this summer. Not everyone still has a well paying job. I've very happy for the fact that you do and I hope you continue to survive the layoffs that are continuing with no end in sight.

To answer the rest of your post, the SG won't read MAP on my car. It shuts down if I try to set that. (MAP and FPR are the only two that do that afaik.) The vacuum gauge is basically the inverse of the MAP. Higher manifold vacuum is lower manifold pressure. Zero vacuum means full atmospheric pressure in the manifold. When I DWL I attempt to hold the same reading on the vacuum gauge, thus holding close to the same FE up and down hills.

I did some more looking into the 0.00 GPH at idle problem and it appears to be related to my engine. It seems to be going open loop at idle. Giving it a tiny bit of gas (not even enough to change the TPS) pops it back into closed loop. Releasing the throttle causes it to fall back into open loop. The TPS reads 0 at idle, 90 at WOT.

Hi TH:

___Sell the SG-II and have you CAT fixed. It is that important and if I made as much as you, that would be the first thing I would do.

___Good Luck

___Wayne

Well, I can see this is going nowhere. Like I said, if it's that important to you, you're free to send me a cat to install, or you can buy my SG for $185 if that's more what you want to do. (Price includes shipping.) While you're at it, I think you should also send a cat to my neighbor to put on his truck as it puts out far more pollution than my car. Please either drop the matter or put your money where your mouth is. I don't appreciate being preached at.

Not having a cat has negligible effect on me. The SG helps me improve my FE, thus saving money in fuel. It will also help to diagnose problems that may develop later on as tends to happen with older vehicles. So tell me, why would I get rid of something that is saving me money over time with a small positive delta on that savings and get something that may actually cost me a small amount over time?

Well... the title of the site IS "clean" MPG. As regards to the second O2 sensor, I've not seen one behind the cat. Both my Kia and Hyundai put the second O2 sensor in front of the cat. It's really just there as decoration anyway, as I'm not sure it'll ever trip a CEL. That seems to be the job of the upstream O2 sensor. The main job of the downstream sensor is to see if there's any oddness happening after the air is past the first sensor, and to adjust settings accordingly. At least, that's what I found when my upstream cat started its slow death march.

Hi TH:

___Goodbye and good riddance. Too bad you paid $35.00 to much for the SG-II as well. That $35.00 would have gone a long ways towards fixing your car and for the rest of us not having to deal with your blatant disregard and violation of the law regarding clean air.

___Wayne