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Old 10-15-2012, 02:37 AM   #1 (permalink)
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My 2000 Jimmy challenge

Ok, gonna document what I do here, and hope to get advice/suggestions/ideas from those "in the know".

Vehicle: 2000 GMC Jimmy SUV 4-door. Previous years were called the "S-15 Jimmy", same as Chevy's "S-10 Blazer". 4.3 liter V-6 engine, 4-speed automatic overdrive transmission, 4 wheel drive & about 160,000 miles. Tires are well-worn Uniroyal Liberators from Wal-Mart. Everything is "stock".

Instrument: brand new ScanGauge II just bought.

I was in such a hurry to get started, I didn't even wait for my 2nd fillup and the ScanGauge "calibration". First off was to run a "baseline" to see what it would do at different speeds. I only did one set of passes (one northbound and one southbound) per speed over a course about 4.5 miles long. Cruise control was set going into the first (northbound) leg, then "resumed" after turning around going into the second (southbound) leg. The results follow:

Speed - - MPG - - 2nd
35 - - - 30.65
40 - - - 32.2 - - - 32.4
45 - - - 31.4
50 - - - 29.6 - *30.9 with tires over 50psi - 32.38 with tires 50+ and grille block*
55 - - - 28.35
60 - - - 26.9
65 - - - 25.5
70 - - - 24.85
75 - - - 23.7
80 - - - 22.15

* after second day testing - added for comparison*

When I was done, I did another 2-way average at 40 mph to confirm. It actually went 0.2 mpg higher than the first time.

I then went and read MetroMPG's thread on proper testing, realized that these single sets weren't exactly "statistically relavent", nevertheless, I now think I have a good idea where my best MPG will be found at (40).


Last edited by wmjinman; 10-15-2012 at 02:43 PM.. Reason: to make corrections, add more info
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Old 10-15-2012, 03:42 AM   #2 (permalink)
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Needing to burn some more gas so I could fill up and "calibrate" my new ScanGauge, I decided to try a couple of things. First off, to get a better baseline, I did 3 sets of two-way passes. Also, to make it go a bit faster, and thinking it would show aerodynamic differences better, - and to "fit" in with traffic in the 45, 50, and 55 mph speed limits along this section of road, I chose 50 mph to test at rather than the 40 mph "best mileage speed" my earlier test showed. The results were:

speed - MPG (3 set average)
50 - - - 30.95
50 - - - 30.95
50 - - - 30.8
- - - - - - - - - - 30.90

OK, good, consistant data. Now to check something I've always been curious about - will "folding my mirrors in" improve mileage at all, and if so, how much? So, without turning off the engine or the cruise control, I "folded the mirrors" and did another 3 sets. The results were:

speed - MPG (3 set average)
50 - - - 30.95
50 - - - 31.05
50 - - - 31.05
- - - - - - - - - - - 31.02

Good, consistant data here, too. So - it appears to be worth about 0.1 mpg. Hmmm... But to be a "good boy", and check that the conditions hadn't changed, it was time for the second "B" in the "A-B-A" test.

speed - MPG
50 - - - 30.65

OK, back to "worse" with the mirrors back in "stock" position. Next to test my cardboard grille block. There were also 2 holes in the lower bumper/air dam for the tow hooks, so I covered them, too. I also used a couple pieces of "plexiglass" to continue the contour from in front of the grille across the headlight area to the side of the vehicle. Results:

speed - MPG (3 set avg.)
50 - - - 32.4
50 - - - 32.4
50 - - - 32.35
- - - - - - - - - - 32.38

So - grille block seems to be good for 1.5 mpg. However, it made the engine run hot enough to cause concern (I think) - 225 to 230 degrees. So I guess I need to come up with the smallest size "vent hole" now to keep the engine temp down a bit. Question: what temp is best for MPG?

I should have quit at that point, but decided to try the rear "fender skirts" next:

speed - MPG (2 set avg.)
50 - - - 31.85
50 - - - 31.6
- - - - - - - - - 31.725

Weird - mileage down!?!?! So, better do the final "B" test:

50 - - - 31.8

Hmmm.... But guess what? I'm now down to 1/4 tank. Tomorrow I can go fill it up & do the ScanGauge "calibration"! By the way, the engine and cruise control remained on and "set" the whole time, so each of thse runs were done under "non-human controlled speed".
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Old 10-15-2012, 05:41 AM   #3 (permalink)
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Quote:
Originally Posted by wmjinman View Post
Question: what temp is best for MPG?
The best temp is when your vehicle is at normal operating temp. You want it to be as hot as it can be without the fan turning on automatically at a set temp.
For example my car runs 186F with and without the grill blocks, Although now with the grill blocks in stop and go city driving i have seen as much as 200F.
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Old 10-15-2012, 12:56 PM   #4 (permalink)
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Your themostat doesn't open until 195. Between 180 and 220 is normal operating temps.

The best speed for MPG is approximately 40mph; This is the minimum speed OD can be engaged at, if you were curious. With careful throttle management it's possible to maintain OD down to 35mph.

I am also doing some trialing for an EGR mod allowing greater EGR at cruise. This should provide a small increase in fuel economy at cruise and is easily done.

I have a 99 Blazer 4x4 2 door myself with some ecomods done on it. The dual mirror delete only has very marginal effects. I have an air dam, a battery relocation with a lighter battery, E-Fan, roof rack delete, and AC delete. I see high teen's to low 20's city and not much better on the highway.

The air dam.



Here you can see the mirror and roof rack removal.

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Last edited by raylit20; 10-15-2012 at 09:36 PM..
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Old 10-15-2012, 02:33 PM   #5 (permalink)
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I neglected to mention I pumped up the tires between the first "baseline" tests at different speeds and the tests yesterday. I attribute the 1.3 mpg improvement to that. Wow, now that I think about it, that's almost as much improvement as the grille block!! And it's not as though my tires were "low" before (all over 40psi and one of the the fronts almost 50). Before yesteday's tests, I put the fronts at almost 55, and the rears at 50.

When I saw my engine was getting hot (after the first set with the grille block), I used my roll of duct tape as a guide & traced out a 5" hole in the middle of the air dam to let air into the radiator. As I mentioned, I left the engine running the whole time yesterday, and when I put that cardboard up to the grille, it got "sucked" onto it, so I knew it was blocking airflow. Afterwards, the temp stayed around 228 -229, sometimes dropping to 224 F. - and it was "boiling over" and draining onto the ground after I got home & parked. And I needed to put more water in it once the radiator sucked the reservoir dry. So before starting it this morning, I enlarged the vent hole to 6 1/4" and will see how that works.

raylit20;

Thanks for that info confirming my 40 mph "best speed". One thing I noticed was that the RPMs seemed to be the same at 35 as 40. I thought maybe the "lock-up" torque converter just hadn't locked up or something. But not going into OD explains it, too.

Low 20s on the highway is what I would usually see, however the speed limits around here (depending on which road I take), can be up to 75 mph. My first day's test showed 23.7 at that speed, and that's probably about what I'd usually get.

Mine is a 4-door, though. Otherewise, they're probably about the same. I've also got the factory roof rack. Thinking about seeing what difference it makes taking it off, but I'm gonna guess it will be very little - similar to folding the mirrors back. However, I don't use it nearly as much as I use my mirrors! LOL!! Did you see any improvement when you removed yours?

Last edited by wmjinman; 10-15-2012 at 02:53 PM..
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Old 10-15-2012, 03:51 PM   #6 (permalink)
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More cool testing, wmjinman. It's fun, eh?

Please consider: Your Garage: [ Add new vehicle ] Also, it would be great if you giys kept fuel logs here too!
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Old 10-15-2012, 04:50 PM   #7 (permalink)
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Thanks, MetroMPG!!

Yes, I love this sort of thing. Just never seemed to get around to being "serious" (seriously scientific) about it before.

I'm not sure how much I intend to "push it" on the Jimmy's MPG. Just had to have an excuse to play with the new ScanGauge until I got my "Gnat" (1999 Suzuki Swift) back for the SERIOUS mileage attempts. Hence, not sure if I should start an entry into the "garage" and then possibly just let it sit....

Same thing with the fuel log. Once I swap the Jimmy back for the Gnat, I don't see any way I can keep the fuel log with someone else driving it. Is there a fuel log "tool" here? I see some people have what i'm assuming is a "running report" on "best tank", "last 90 days", and "lifetime" MPG averages, so I'm guessing "yes". I would be interested in possibly doing that when I get my Gnat back.

Thanks again for having this site. I'm on it enough to piss-off my fiancee, so that must mean I really like it, huh?
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Old 10-15-2012, 04:56 PM   #8 (permalink)
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when you enter a vehicle into the garage there is an option under your newly created vehicle data to add into the fuel logs
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Old 10-16-2012, 02:23 AM   #9 (permalink)
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OK, I did it!!! Filled up ($65 - YIKES!!!) and did my ScanGauge calibration. It said (estimated) I used about a gallon less than I actually did. It also estimated my gas mileage for the tank was about 1 mpg better than it was (due to the estimate being "off", I suspect). Actual was 23.682 mpg and the ScanGauge estimate was 24.7 mpg.

So I entered the information in the "garage" and now am #81 for best pecentage above EPA (EPA=16 combined, 23.682 actual for 48% above). I can see how this can be contagious!! More tests, or just drive around at 40 mph with the grille block & mirrors folded in for a tank!! HAH!!

Oh, with the enlarged "vent hole" in the grille block, the temp didn't go above 188 deg on a 30 mile round trip including a hill. Maybe I should pump up my tires to 60 for this tank - gotta get on the first page of best % over EPA. HAH!! You've created a monster. Bwah-hahahaha!!!!!
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Old 10-18-2012, 01:46 AM   #10 (permalink)
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So after reading the "roof rack delete" thread, I decided to go out and try my own "A-B-A" tests. This time, instead of Washoe Valley, I went out on US-50 east of Dayton. I had work to do out that direction, anyway. So I found a pretty straight, flat, 4-lane 3.8 mile section of road with easy turn-arounds past each end, and set my cruise control for 50. First, the "A" tests, grille block still on, roof rack on:

speed - E-bound - W-bound - average
50 mph - - - 27.9 - - - 33.6 - - - 30.75
50 mph - - - 28.2 - - - 31.1 - - - 29.65
50 mph - - - 28.2 - - - 33.5 - - - 30.85
overall average - - - 30.417

I noticed the mileage was a couple mpg less than the 50 mph tests in Washoe Valley a few days ago. Two possible reasons; 1) I "calibrated" the ScanGauge between those runs and these. It estimated less fuel use than actual, so I'm guessing the mileage is lower with the new "calibrated" readings. 2) Dayton Valley is probably close to 1000 feet lower in elevation than Washoe Valley, so that should mean the air is thicker = more drag?

There was a fairly stiff breeze blowing out of the east, so I attribute the difference between eastbound and westbound to that. Without turning off the engine or the cruise control, I quickly removed the roof rack & stashed it inside, then headed back out. Four sets this time:

speed - E-bound - W-bound - average
50 mph - - - 28.6 - - - 33.1 - - - 30.85
50 mph - - - 29.1 - - - 32.8 - - - 30.95
50 mph - - - 29.0 - - - 32.5 - - - 30.75
50 mph - - - 28.7 - - - 32.8 - - - 30.75
overall average - - - 30.825

Good deal! Looks like over 4 tenths of a mpg - and I hardly ever use that roof rack. Good riddance - exept, I need to do the final "A" test. Oh, the stiff east breeze was still blowing. OK, one last "A" test with the rack back on:

speed - E-bound - W-bound - average
50 mph - - - 28.7 - - - 31.7 - - - 30.2

When added to the first set of "A" averages, we get an updated 30.3625 mpg with the roof rack on. That makes it a 0.46 mpg improvement. I like it!!

One final observation; with the newly enlarged 6.25" diameter "vent hole" in the cardboard grille block, the engine temp. stayed around 193 deg.


Last edited by wmjinman; 10-18-2012 at 01:58 AM.. Reason: to add "observations" at the end
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