KJ Fuel economy thread

[mikey]

Originally Posted by Pipeliner

Speedos (no, not your budgie smugglers) are always inaccurate: this is to make sure that you can't sue the car manufacturer when you get a speeding ticket. Expect it to read 4-6kph higher than your actual speed when doing 100kph.

So just how accurate are our GPS units???

[cmohr]

Originally Posted by KJ_andy

Just in case I have something wrong, which happens on occasion, aren't the 235 and 245 figures representing tread width. Its the 70 or in my case 75 figure, the profile, which indicates overall tyre diameter.

Yes, the profile is a percentage of the width. So in a 245/70 the side wall is 70% of 245 or 172mm, with 245/75 its 184mm. This sizing is the tyre carcass size, then the tread depth is on top of that. I have ATZ's in a 31/10.5/15 they are 770mm overall diameter, compared to my MTZ's ( Mudders - deeper tread blocks) 31/10.5/15 at 789mm overall diameter. Same designated tyre sizing, but, phyically different. Then theres SLR (or Static Load rating) that changes just what the actual distance travelled by the vehicle in 1 tyre rotation. Its not the actual phyical circumference of the tyre, its based on the circumference of a circle that is approx equivilant to radius from hub centre to the ground (notice that buldge of the tyre where it meets the ground, its shorter than when there is no weight on the tyre) when the tyre is spinning as it will be larger when at speed due to centrifical forces than in a stationary position .... Its all very technical. most manufacturers have in the specs for their tyres a revs per mile( or km) at 45mph so as speed perametres can be set to suit their tyres. This of course is at a certain reccommended tyre pressure and temp. blah blah blah..

So in closing.

If you have 245/75/16s, get them to do a "Pinion reset" to 677 revs per mile or the closest tyre size that appears on their scantool "245/75/16" if that is not there, go 245/70/16, don't go over the size of the tyre. Also, get them to check in the ABS settings to see if on your ECU they have to change it as well, simply match the change made to the Pinion reset.


I think up to 04 requires the revs per mile set in only one place, 05 onwards can be either revs or tyre size and may be required to change in two places.

cmohr..

[KJ_andy]

Thanks Cmohr, as mine's an 03, i'll get the "Pinion reset" done. No ESP here, just LSD (all smiles)

[cmohr]

Don't expect it to go back to "standard", as to go that far you'll need to do gear ratios (big dollars), but it will certainly improve things a lot.

[Jimmyjojo]

Called Essendon Jeep here in Melb and they didn't know what I meant by having my Pinion Reset. He just said that he would re-adjust my speedo..

Called city jeep and he knew exactly what i was on about.

I'm getting 17.5/100 (3.7ltr) atm with 245/75/16 muds.. wife isn't happy! but a cheap $66 dealer job will fix that..

[Nervous Nellie]

According to the owner's manual the computer displays the average fuel consumption "since the last reset". Strictly speaking the displayed consumption would therefore be a cocktail over weeks or months of freeway, suburbs, lo-range, city, 4wd and highway driving, unless you reset between trips (unlikely). In my (humble) opinion this would be a fairly useless figure. I would have thought most people would want to know their consumption on a journey by journey basis.
Does anybody know how the computer calculates the consumption??? I think a more useful figure would be something like "ave fuel consumption for last 100 km" (or 50 or 200 or whatever).
Would the last "reset" be a reset of the EVIC or a reset of the trip meter on the odometer???

[Fouros]

the reset would be a reset of the data you are looking at only not the total info in the evic,

in 03 they removed the instang MPG feature and kept the average