Optimum Tire Pressure

slikjr · **Posted:** Sun Aug 13, 2006 11:43 am

A quick question; I am running BFG Mud Terrains on my '02 KJ and was wondering what the best pressure is to run them at for normal, everyday, on road driving. They are an "LT" tire so max pressure is 65 psi but at much heavier loads than a KJ.

I have them at 40 psi right now. Anyone else running them or with a lil' know-how please let me know what you have found out. Thanks for the help!

raceinfan · **Posted:** Sun Aug 13, 2006 1:53 pm

I run my BFG's AT's 245/70/16 at 40 psi, & was recomended around here for better fuel economy

jeepkj02 · **Posted:** Sun Aug 13, 2006 5:47 pm

Is it true that you get better fuel economy at the max psi, but your tires wear out faster?

elephantrider · **Posted:** Sun Aug 13, 2006 6:01 pm

im running 45 psi in the goodyear mt/r's.

no wear showing now, and i still get 20mpg on the highway, 16 - 17mpg in city.

FreedomKJ · **Posted:** Sun Aug 13, 2006 7:11 pm

The optimum tire pressure depends on the size of the tire and the weight of your vehicle. If you really want to know the best pressure to run at, weigh your KJ on a truck scale and then call the tire manufacturer and ask them.

jpzkj · **Posted:** Sun Aug 13, 2006 7:22 pm

I run our MTR's @ 40 on the road and 20 off the road

Treadwear has been pretty much normal for my KJ
Ballance of less rolling resitance and decent ride is what I'm looking for on road. Offroad I want good float with the least chance of loosing the bead.

Hope that helps

05JeepKJ · **Posted:** Sun Aug 13, 2006 8:45 pm

I have the mud-terrains on 35 psi. So far it says im getting 15 mpg but I have not recalibrated my speedo so im not really quite sure what im getting.

Jeepjeepster · **Posted:** Sun Aug 13, 2006 10:26 pm

05JeepKJ wrote:

I have the mud-terrains on 35 psi. So far it says im getting 15 mpg but I have not recalibrated my speedo so im not really quite sure what im getting.

Id say it doesnt get off that bad. It may actually be more accurate.

corwyyn · **Posted:** Sun Aug 13, 2006 11:21 pm

jeepkj02 wrote:

Is it true that you get better fuel economy at the max psi, but your tires wear out faster?

Yes. You'll have less rolling resistance since you'll have a smaller contact patch, but conversely since all of the weight of your Jeep is being transfered to the road on smaller contact patches the center section of your tread will wear a lot faster. You should only approach the max. pressure stated on the tire when you have heavy loads; anything else is just wearing them out faster. Also your handling in wet conditions will be degraded running max pressure with regular load.

Jeepjeepster · **Posted:** Mon Aug 14, 2006 10:59 am

I keep my Generals at 35psi. I cannot stand the ride at anything higher than that.

Elwenil · **Posted:** Mon Aug 14, 2006 11:16 am

The only real way I have discovered to get the correct pressure for a tire on a vehicle with tires other then the OEM ones is to draw a wide line across the tread with a piece of chalk. Drive forward and then backward (no turns) slowly and then check the line. If it wears in the middle, you need to drop some pressure, if it wears on the outside you need to add some pressure. Do this for the front and the rear with tires that are near new and have level tread and with all the normal stuff you carry in the vehicle and a full tank of gas. It is normal for the front and rear pressure to differ. Record the pressure for later use.

HyperKJ · **Posted:** Mon Aug 14, 2006 6:01 pm

i keep about 40 psi for daily driving in my MTR's,, and im able to tolerate it no-prob... and i definately air down when i hit the trails to about 20 psi..

DaveKJ02 · **Posted:** Mon Aug 14, 2006 8:12 pm

05JeepKJ wrote:

I have the mud-terrains on 35 psi. So far it says im getting 15 mpg but I have not recalibrated my speedo so im not really quite sure what im getting.

Tire differences
Stock 235/70/16 715 RPM
Your sig size 245/75/16 682 RPM

Your odometer/speedometer is off by 4.8%. You just boosted your MPG 4.8% if you werent adding that in before. :lol:

Joe Jeeper · **Posted:** Mon Aug 14, 2006 10:21 pm

Elwenil has the correct method for checking the tire press vs the foot on the road! I ran my 245/75R16's Mtr's at 30 psi and had a full foot on the ground, it's also what I run my current 235/85R16's and get a full foot or all the tread evenly on the ground. Too much pressure you wear out the middle, not enough you wear out the outer edges. As far as wheelin' we don't go that far so it depends what the terrain is I run at 20-22psi. Joe p.s. you can figure the correct pressure by looking at the max. load per full pressure, weighing your vehicle and dividing until you get the right pressure per your weight like someone mentioned.

Jeepjeepster · **Posted:** Mon Aug 14, 2006 10:31 pm

Your MPG will actually be better than what the computer says. Thats b/c you are going more miles than the odo says. So that means you would be dividing into a bigger number than what you are. :wink:

slikjr · **Posted:** Mon Aug 14, 2006 11:12 pm

Thanks for all the great help! I have them at 40psi right now and all seems to be good. I am going to do the "footprint" test. That's a great, easy idea that didn't even cross my mind. Thanks a lot people!

FreedomKJ · **Posted:** Tue Aug 15, 2006 6:01 pm

Elwenil wrote:

It is normal for the front and rear pressure to differ. Record the pressure for later use.

In four wheel drive vehicles the front and rear tire pressures must be the same.

Goglio704 · **Posted:** Tue Aug 15, 2006 7:25 pm

FreedomKJ wrote:

Elwenil wrote:

It is normal for the front and rear pressure to differ. Record the pressure for later use.

In four wheel drive vehicles the front and rear tire pressures must be the same.

Why?

Elwenil · **Posted:** Tue Aug 15, 2006 7:50 pm

FreedomKJ wrote:

Elwenil wrote:

It is normal for the front and rear pressure to differ. Record the pressure for later use.

In four wheel drive vehicles the front and rear tire pressures must be the same.

Negative. I know what you are thinking. You are thinking that with the same pressure, the tires will have the same diameter and therefore will not harm the 4WD system. I doubt you could find a set of 4 tires on any vehicle that are all the same diameter. Furthermore the 4WD system will handle a little variance. Most Dodge 4x4 trucks had different gears front and rear. My '84 Ramcharger had 3.23 gears in the rear and 3.21 gears in the front, same with my '88 W150. My '74 W100 has 3.55s in the rear and 3.54s in the front, my '74 W200 has 4.10s in the rear and 4.11s in the front. Either way, with the different amount of weight pressing down on each axle due to the natural balance of the vehicle, different pressures would be needed to keep the tires the same diameter anyway. If a vehicle weighs more in the rear, it will carry more weight on the rear axle and therefore the rear tires. If the front has less weight, it will carry less on the front axle and tires. If you put the front tires to the correct pressure for the weight on them, and then applied that same amount of pressure to the rear tires, the rear tires would be low on air due to the increased weight pressing down on them. They would need more pressure to support the weight and maintain the same diameter as the fronts. The opposite can also be true of vehicles with large engines and low weight on the rear, such as a Cummins Turbo Diesel powered Dodge Ram. They have much more weight on the front than on the rear. That is why the chalk test works the best since it puts the pressure to the exact spec required for that vehicle with the amount of weight it generally carries on each axle.

Eddo · **Joined:** Thu Jul 07, 2005 7:01 pm **Posts:** 1214

Jeepjeepster wrote:

Your MPG will actually be better than what the computer says. Thats b/c you are going more miles than the odo says. So that means you would be dividing by a bigger number than what you are. :wink:

It's the other way around. Still dividing by the same number of gallons used. Just dividing into a bigger number. It's miles/gallon.

Optimum Tire Pressure

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