What size tyres for best power & economy

[multihull]

Here's an interesting conundrum, my JK has low diff ratios of 4.88 to suit my 34.5" muddies when off road however as I use the car as a daily drive I swap them for stock 31" Goodyear Wranglers for my driving around town. This gives me a smoother drive and does save on fuel, about 1 to 2 lp/100. It also takes a huge load off all the steering and running gear . My 3.8 litre Auto JK also is loaded with extra weight such as H/D front & rear bumpers, tyre carrier, winch, drawers, fridge, roof rack, 2nd fuel tank etc... With these smaller tyres it does raise the engine revs slightly so when doing 100kph the engine sits on 2300 RPM. One issue I do have is these tyres look weird against a 3" lifted Wrangler. I was considering moving up to 33" Highway or ATR tyres for my daily driving.
Now the questions are:
Q-what is the standard engine Revs for a stock 3.8L auto JK at 100kph?
Q-Will taller highway tyres give me better economy because the the revs come down?
Q- Or with slightly taller tyres I will loose some power so will economy get worse because?

I look forward to your feed back

[Nanook]

I went with 265s on my JK to keep the revs as close as possible to what is was with the original tyre size. I would have loved to have gone to 285s but it would have meant a loss of power especially when towing if I stayed with the original gearing.

My TJ had 31's which is larger than stock and I found 5th gear was useless unless I was doing 100kph+. So around town on 80kph roads I had to stay in 4th to have any kind of power otherwise it would labour in 5th.

[multihull]

Bump - does anyone have an answer?

Q-what is the standard engine Revs for a stock 3.8L auto JK at 100kph?

[humdingerslammer]

Originally Posted by multihull

Bump - does anyone have an answer?

Q-what is the standard engine Revs for a stock 3.8L auto JK at 100kph?

2200 RPM in mine.

Cheers. For each centimetre in lift, except about a 10% reduction in fuel efficiency (10% increase in fuel use, that is).

Hope this helps.

[mike_nofx]

Just a thought with regards to your fuel economy between the 31s and the 34.5s. You say around 1-2L/100km difference? How did you calculate this difference, just by using the L/100km read out on the dash?

Since there is roughly an 11% tyre size difference between the two, it's also safe to assume that your odometer reading and L/100km read out will also have an 11% difference.

So say on 31s you average 12L/100km, on 34.5s to get the exact same economy you'd expect 13.3L/100km. I hope I have the math right here and not the wrong way around!

[PureCaboose]

Originally Posted by mike_nofx

Just a thought with regards to your fuel economy between the 31s and the 34.5s. You say around 1-2L/100km difference? How did you calculate this difference, just by using the L/100km read out on the dash?

Since there is roughly an 11% tyre size difference between the two, it's also safe to assume that your odometer reading and L/100km read out will also have an 11% difference.

So say on 31s you average 12L/100km, on 34.5s to get the exact same economy you'd expect 13.3L/100km. I hope I have the math right here and not the wrong way around!

Maths aside, unless you reset the tyre sizes in the computer your trip computer will be wrong. I have 2 JKU's, with the same size tyres (theoretically) and only has been adjusted. My speedo is out by the mandated 10%, while the Mrs is bang on at 100km/h (unadjusted).

[multihull]

Originally Posted by mike_nofx

Just a thought with regards to your fuel economy between the 31s and the 34.5s. You say around 1-2L/100km difference? How did you calculate this difference, just by using the L/100km read out on the dash?

Since there is roughly an 11% tyre size difference between the two, it's also safe to assume that your odometer reading and L/100km read out will also have an 11% difference.

So say on 31s you average 12L/100km, on 34.5s to get the exact same economy you'd expect 13.3L/100km. I hope I have the math right here and not the wrong way around!

No I have a Diablo tool and I change the tyre size in the ECU each time I swap tyres and I've done a speedo check against my GPS so the speeds are spot on.