For the Downunder Losties - I have started a thread on Ausjeep to determine the quantifiable affects on fuel economy of tyre upgrades and lifts. There was a thread on LOST awhile back that suggested that the Rotational Mass of a tyre would negatively impact fuel economy in city driving cycles (thanks to Mr Newtons Law that states that an object at rest want to remain at rest) but actually improve it on the highway cycle (due to interia affect I suppose - that once in rotating motion Mr Newtons law would say it wants to remain in motion). The second part of this research is to pin down the fuel economy impacts of lifting a KJ, specifically whether a lift increases frontal area or simply increases dirty air affects under the vehicle. As I don't have a wind tunnel, I was after some real world feedback.

Here is the link to the Ausjeep question. If one of the guru's of this site is aware of LOST research in this area please let me know (I did do a search but found little of relevance).

http://www.ausjeepoffroad.com/forum/sho ... hp?t=83247

I can tell you my experience. I actually went down slightly in tire diameter. Went from stock 225/75R16 to 235/75R15. I also added the 2.5" Daystar lift and my MPG went down about 1.5 MPG. So I would guess it is aerodynamics related in my case.

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2007 Light Khaki 4x2 Sport
- Debadged - LTT CAI with Amsoil Filter - JVC HD Radio - Infinity BassLink
- Iggee Seat Covers - Black Moabs - 265/75-16 Cooper Discoverer ATP - 4.10's
- DIY Franky sitting at 23.75" - Al's 4.5 UCAs - Trimmed F&R Bumpers
- Removed Rear Sway Bar - Removed Rear Vibration Damper
- 75k miles so far...

I'm pretty sure there's been no effort to collect data - we do have individuals with data points - and it seems anything you do - makes your KJ get worse mileage.

However theoretically

Larger diameter tire should cause worse mileage intown - acceleration of a lower effective gear - causes the engine to work harder to provide the necessary torque to accelerate.

highway - below ~60 mph (100 kph) it should be better - lower effective gear ratio - therefore lower engine speed for the same highway speed.

above that ~60mph - aerodynamics will cancel any improvement.

heavier mass - will always cause worse mileage - however if you're at steady highway driving it will be lost in the noise.

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2005 CRD
stuff
Skeptic quod gratis asseritur, gratis negatur

I went from stock 235/70/16 to 245/75/16 and added a 2.5" lift at the same time as the tyres and the fuel consumption went from 12.8ltrs per 100ks to mid 13's average over many thousand K's. I have now fitted the 4.1's and trutracs but have not done enough K's yet to get a fair comparison, I'l wait for a few months to tally it all up.

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'04 sport, 3.7, dobinsons 50mm lift, full skids, mopar rock rails,DTT's, 4.1's. 245/75/16 MT ATZ's