So I noticed the last few fill ups that my milage has significantly gone down. On the highway I usually average around 17-19 or so, but lately Im down to 14-15mpg. I know my trans (4l60) is not long for this world, but I don't feel it slipping that I can tell. Could I be slipping in 4th without realizing? The truck does feel a bit more sluggish when it's in drive.

 

I feel the same thing. Like when my trucks in 4th and you try to give it some gas in the 50mph-60mph range its like I dont move anywhere. Honestly I dont think its the transmission, I think its just the gear your in lacking power. It was common in a lot of vehicles I have had.

 

It's not just in fourth that it seems lacking though, its all through the gears. It doesn't seem to want to shift out of third, and when getting up to highway speed I need to really get on it to get it moving.

 

It could be hurt if it takes feathering the gas to get out of 3rd. However, I suspect that the engine is more likely lacking in power for some reason, maybe it needs a tune up? If it's slow in all the gears it's not going to be slip, you'd know it was slipping and it wouldn't go very far in such a condition.

Also, if your tune is not spot on (and really only way to get one is with an excellent log or an in-person dyno tune), you could be having fueling issues making the engine feel boggish.

 

I did a full tuneup this past fall, so unless the plugs are defective, I think it should be good there. My tune is a Nelson mail order, I know it won't be spot on not being live, but usually he gets pretty close.

 

A bad transmission could absolutely rob power and make your truck feel sluggish.

 

I think you might not have read it, but a bad transmission would be DEAD at this point. You don't lose power for very long on a 60E or even an 80E before it goes nowhere.

 

You would know if your 3-4 clutch pack or lockup clutch(converter) was slipping. It would be revving quite a bit higher than normal 4th gear locked. Sometimes it even slips enough to feel like it never even shifted out of 3rd. I've had more than my fair share of that particular problem.

If your truck still feels like it has all 4 gears, and lockup, then your problem is not the transmission. I'd look elsewhere. Have you checked your fuel trims? Have you monitered the O2 sensor activity? Do you have any exhaust leaks ahead of the oxygen sensors, or close to them? Your best bet is to get a scan tool or data logger of some sort on the truck and go for a drive.

The wife's Yukon was getting crappy MPG's. Tune up was good. I found the driver's side exhaust manifold bolt closest to the firewall was broken off in the head. There was a very small exhaust leak because of it, the gasket showed evidence of the leak when I removed the manifold. The leak was never noticed or heard. But it was there.

 

when's the last time you changed your fuel filter and air filter?

Also as stated check the o2 trims. If an 02 goes bad it will demolish your mileage. I had one go bad and was getting 200miles per tank.

 

I did a fuel filter change last fall, air filter a few months ago. I need to get a log on it and see whats what. Hopefully it's nothing major.