[nickv]

98 TJ Sahara, 4.0 motor, auto trans, stock gears, 32" tires, AEM Cold Air intake with throttle body spacer, electric fan, new plugs, wires, high capacity water pump, new cat + all exhaust sensors replaced with brand new parts.
Currently getting 8.4 MPG? Any ideas would be appreciated

 

[moose]

98 TJ Sahara, 4.0 motor, auto trans, stock gears, 32" tires, AEM Cold Air intake with throttle body spacer, electric fan, new plugs, wires, high capacity water pump, new cat + all exhaust sensors replaced with brand new parts.
Currently getting 8.4 MPG? Any ideas would be appreciated

First off, I'd like to see datastream info. Fuel trims, MAF readings, etc. Then compression test; dry and wet. Then we can try to figure it out.

 

[Mboelts]

Since replacing all these parts have you reset your ecm?

 

[havasu]

Those 32" tires with stock gears did it to you. Re-gear or drink lots of fuel.

 

[nickv]

Have not reset ECM. Any suggestion on what programmer to use?

Concerning tire size and gear ratio, I checked with Randy's and 4Wheel Parts, who both think I have the correct gear ratio. They claim bigger tires should yield better mileage, but less acceleration?

 

[havasu]

When I changed from stock tires to 33" tires on my auto YJ with stock gearing, my MPG went from 15 down to 9. Bigger tires don't yield better mileage as long as you have to start and stop the revolution.

 

[Mboelts]

Don't really need any tools to do it, just pull the positive connector on the battery and hold that to the negative side for a couple of minutes.

 

[spencer97tj]

I've got a 97 4.0 on 35 inch tires with factory gear and I get better gas mileage than that I'm at 10.9

 

[Chris]

Bigger tires will yield you better mileage but only on the open highway of which is flat when it can keep you at a lower RPM. It is just like gearing say from 4:10 to 3:73 but on take off it will use much more fuel then before to get the larger tire turning and the vehicle up to speed. So if most of your driving is around town it will hurt your mileage quite a bit but if all you do is long haul trucking it will benifit you unless you live in the mountains where hills will get you because you will have to push that pedal more to keep speed up.

Hope this helps a little.

 

[Mboelts]

I was thinking about this, might also need a good seafoam cleaning, here is a good write up of how to do it (I know its from a LS1 forum aka Chevy guys, but it works for anyones engine basically):

http://ls1tech.com/forums/general-maintenance-repairs/534376-how-seafoam-your-car.html

 

[swells18002]

One thing to remember, is that with a tire change your odometer will be inaccurate. How are you getting your mileage figures when computing your MPG.

 

[Chris]

On my F250 if I calc the mileage off my odometer it shows about 7 mpg, if I do it off my GPS it is closer to 10.

 

[rrent]

First 80 mile trip in mine - I checked milage and immediately knew my speedo was off, due to larger tires. Need to get a GPS to be accurate.