So I feel like my 97 3.9 has been getting worse gas mileage as of late. The last time I filled it up, I got somewhere around 13.8, and that was on about 170 of 200 miles on the highway. I know they don't get great mileage, but I thought that seemed a little low. So my question is what would be the best way to improve mileage? before I got the pickup about a year and a half ago, my Uncle said that he did a tune up on it, but I have no idea what exactly he did. (I mean just plugs?, plugs wires cap and rotor?). I can also tell you that I changed the air filter about 2,000 miles ago, and ran a bottle of stp through it around 3,000 ago. Please let me know what you guys think.

ps, it does not have a cat.

Thanks!

-Kyle

 

Are the sensors still there where the cat used to be? As far as gas mileage there are tons of factors. Tune up,brakes in good working order,all the sensors operating as they should, terrain,speed,tranny condition,engine mileage,weather,altitude. You would be really surprised how much this can cause. Tune up should include plugs,wires,cap and rotor. Clean the throttle body and sensors(use a cleaner safe for sensors) Make sure calipers and wheel cylinders all work properly, Tires are inflated to correct psi, try bumping up a grade of gas(instead of 87 run 89) Speed greatly effects mpg slowing it down a little will help, a slipping tranny will cause poor mpg. Also as the weather gets colder mpgs will drop.

 

I only wish I got close to 14 MPG. I'm usually in the 10-11 range with my 5.2 rolling on 33's.

 

I'm not a mechanic, but my 3.9 5 spd manual gets in the high teens around town and 21 on the interstate. I'm wondering why you do not have a cat, and whether things are out of whack with the sensors and PCM because of that.

 

He has a 97, so the missing cat is a normal mod. The downstream can be zip-tied to the framerail until the 2000 computer change.

 

Yea the cat off isnt a big deal, like the guy below you said for the 97 you can just have the sensor behind the cat off and it wont mess with the computer. Do you know if there is any way the o2 before the cat could be causing this? Its not like im running big tires or anything. I have a set of kumo 15's that look more aggressive than a highway but they are nothing compared to a lot of the ones I have seen.

Also, what gear ratio are all you guys running?

Thanks!
Kyle

 

I'm on 4.10's with my 3.9 and I get about 16 on the highway if I keep my foot out of it. I got 18-19 with the 3.55's. But around town, I picked up 1.5 mpg with the swap. Seeing as my commute is 12 miles of suburbia, I didn't mind.

 

I'm guessing I have the stock 3.55's. And that last milage reading was on the hwy! Do you have anything else that I should check?

 

any known history on the throttle body sensors? TPS, MAP, IAC?

 

I've got 3.55's, 5.2, 5 speed and 33" tires. I get MPG in the low to mid teens.