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Jeep Wrangler (1992)

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Jeep XJ Cherokee 4WD Sport 4-door (1999)

Owner: Moses Ludel

Added: 15 September 2013 - 01:16 PM


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4x4 Oversize Tires, Gearing and Fuel Efficiency

general discussion 4WD four-wheel drive sharing ideas off-road

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#1 Moses Ludel

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Posted 02 February 2014 - 12:51 PM

Mounting oversized tires is common for Jeep, 4x4 truck and SUV models.  Member Bamafan1 sent a question that begs attention.  This is a general topic that has importance.  There is a "trickle down effect" when running oversized tires...These issues have solutions...

 

See Bamafan1's note below, my reply, and join the discussion!

 

Moses



#2 bamafan1

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Posted 02 February 2014 - 12:57 PM

I'd like an opinion on mpg, I run 33x10.50x15 BFG MT's (on it when I bought it). Mostly in town driving as far as street use goes and I checked mpg ob last tank and averaging 12.6 which seems significantly lower than average of 15 mpg. Just wondered if "misfire" is contributing to fuel consumption too?          



#3 Moses Ludel

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Posted 02 February 2014 - 01:50 PM

Bamafan1, you bring up a universal issue with our vehicles.  If the 33" diameter tires went on your Jeep without a change in axle ratio gearing, the effect could be damaging to fuel efficiency.  First, it's important to note whether your speedometer is accurate with the oversized tires, which create an additional "overdriving" effect.  I did an article on the gearing subject for those interested:

 

http://www.4wdmechan...Gear-Ratio.html

 

To begin, is the speedometer accurate?  Also, in town driving is a very difficult way to assess fuel efficiency.  To determine "normal" mileage for your Jeep TJ Wrangler first requires a baseline.  This would be a steady run on the highway, optimally flat and without grades, for optimal fuel efficiency.

 

The change to oversized tires often drops fuel efficiency.  Your Jeep came equipped with 30" or so diameter tires (31" for the TJ Rubicon).  Most manual transmission models have either 3.55:1 or 3.73:1 gearing, which you will find noted on the window sticker or at the axle tags.  Either of these ratios with a manual/overdrive transmission is considered "tall gearing".  This is low emissions oriented gearing and supposedly helpful to fuel efficiency. However, the low engine rpm on the highway at cruise speed in overdrive is anything but efficient.

 

As a rule, engine efficiency is best at or near the torque peak of the engine. On the 4.0L inline six in your Jeep TJ Wrangler, peak torque is around 3000 rpm.  If you notice while driving, your engine rpm at a 60 mph cruise in overdrive is more like 2000 or less rpm.

 

Fuel efficiency might drop even further with oversized tires.  Oversized tires, instead of adding a useful additional overdrive effect, actually drops fuel efficiency.  Why?  Because the engine is even further off it's peak torque rpm and under an increased load due to the oversized tires!  To restore the engine rpm on our XJ Cherokee to stock (similar to stock tires with the OE 3.55:1 gearing), I had to install 4.10:1 gears.  This gearing corrected speedometer calibration, too.

 

So, to restore fuel efficiency with 33" tires, even to the stock level, you need at least 4.10 gears.  Frankly, in my experience, with all the added weight of accessories and add-ons, I should have gone a step further and switched to 4.56:1 gears on the XJ Cherokee.  Many go to 4.56:1 on the Jeep YJ or TJ Wrangler with 33" tires and a manual/overdrive transmission.  We take a different approach with the 3-speed automatic transmissions to 2002.

 

So, confirm your axle gearing and speedometer accuracy.  If the speedometer is correct, let's look at the engine rpm at 60 mph in 5th gear/overdrive.  (I keep assuming you have a manual transmission, is that correct?)  Once you note the engine rpm at 60 mph, we can talk about ways to improve performance.  As for your concern for an engine misfire, one issue for misfire can be a lugging engine—too low an engine speed under load and not efficient enough to burn fuel properly.

 

I did take a recent initiative with the XJ Cherokee and was very happy with the results.  Though historically shy of engine programmers and PCM tuners, I chose to test a Hypertech performance software upgrade.  Set for unleaded regular fuel, the single most impressive gain was that my 4.0L six, similar to yours, now delivers peak torque at 2000 rpm, a full 1000 rpm less speed than stock!  This means that I can cruise in overdrive at 60-70 mph without lugging the engine.  In fact, with only exhaust system improvements that were already in place, both the engine's performance and fuel efficiency jumped up.  See my article and details at:

 

http://www.4wdmechan...ge-Cummins.html

 

This actually was a surprise and real compliment to Hypertech's engineering.  There is no "ping" with unleaded regular gas under load, and the XJ's curb weight is hefty!  I would not recommend a software tuning remedy if an engine is worn badly or out of basic tune.  At the current 145K miles on the engine, which is in good condition still, the gains were substantial, including fuel efficiency and much improved acceleration across the power band.

 

Moses



#4 biggman100

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Posted 02 February 2014 - 01:56 PM

Moses, I will add my one and only experience here. I have a 1991 Dakota, that came with factory steel wheels and 225/75/15, and got right around 18 MPG, but when I swapped to 31/10.50/15's, with stock 1998 Durango aluminum wheels, I found I was only getting 16 MPG. After a friend suggested weighing each tire and wheel separately, as well as weighing the steel and the aluminum wheels with tires mounted, I decided to try it, and I found that even though there was only maybe a pound or 2 difference in the steel versus aluminum wheel, there was almost a 7 pound difference between the 225/75/15's and the 31/10.50/15's, which works out to an added 28 pounds of weight. Im not saying that alone caused my fuel reduction, because at the time it also could have probably needed a tune up among other things, but the more weight you add to a vehicle, the more that weight can affect things like performance and fuel consumption. In the case I presented, think of it this way, where it may take a light pedal to get the vehicle to initially move and keep it rolling, the extra weight of the larger tires may take a bit more pedal pressure to get things rolling and keep them moving, which may translate into added fuel consumption.



#5 Moses Ludel

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Posted 02 February 2014 - 02:10 PM

Biggman100, you bring up an important point, and I'll add a few comments.  Oversized tires are not only more weight per wheel/tire, they are usually a hint that the vehicle has other modifications, too.  And that adds even more weight!

 

Our 2005 Dodge Ram 3500 4WD Quad-Cab had a factory curb weight of 7800 pounds.  It now has a curb weight of 9000 pounds.  A great looking, great performing truck, however, it now goes down the road empty with an addition 1,200 pounds onboard!  That's like towing a small trailer...

 

On another note, the Dodge/Ram truck, your Dakota or Bamafan1's Jeep TJ Wrangler are now standing tall and, I often like to say, "Pushing billboards down the road!"  This increased wind resistance also impacts fuel efficiency.  Even with the Hypertech tune on the Cummins engine, we're down about 2 mph over the stock height, stock tuned truck before the added 1,200 pounds, a lift kit and accessories.  I'm not knocking it, 21 mph at sensible road speeds when "empty" is still quite good for a truck of this size.

 

Moses





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