I have done about an hour of research and can't find an answer. I have a 2009 jeep jk x with a 6 speed manual, stock transfer case, and rubicon axles. Im running 37's with 4:88 gearing. Recently I noticed my gas milage plummeted. At the same time my rpm's went from around 2500 at 65mph to around 3300 at 65... Any ideas of what could cause that? TIA!

 

Only thing that could be is clutch slip if the increase is the 800rpms you stated. Won't last long doing that slip.

 

I just had the clutch replaced by a reputable shop less than 2 years ago and I haven't really beaten on it so I'm not sure it's that. I will definitely keep that in mind if there are no other causes.

 

You staed the increase in RPM - is it steady or can you lift your foot slightly and get the rpms to come down without losing speed. Only other thing it could be is tachometer has gone stupid.

 

It bogs down and decelerates pretty quick as soon as I let off the gas. I almost never use 6th gear so a majority of my driving is in 5th. As far as I know 5th is a 1:1 ratio so I wouldn't expect as much slowdown just by letting off the gas.

 

OK let me restate the previous post - can you go along at 60mph, flat ground, 6th gear put your foot down hard and get it to slip the RPM higher? If this is the case there is a clutch issue and could even be the throwout bearing pushing slightly on the pressure plate fingers causing the slippage seeing as you had a replacement clutch installed. (Hate these new clutch setups now and prefer the old manually adjustable setups) but first you need to identify where the problem is. I calculated that in 5th gear at 65mph you should be at 2880RPM so pretty close to 2500. I would do the checks asap because if there is slippage you will wreck your clutch plate quickly.

 

I tried what you suggested and the rpm's didn't change disproportionate to the speed.

 

Ok no clutch issue something wrong with the tachometer. I would start with a scan first then start with checking grounds and there are many under the hood and inside the cabin. Never heard of a tach going sideways even mine works through an interface to make an 8 cylinder read on a 6 cylinder tach and it is right on.

 

I have a traildash 2 installed to correct for gearing and tire size. The only code its throwing is a p0420. But it goes away for like a week after clearing it. The rpms remain high even when the engine light is off. I'm going to take it to a shop as soon as I get a chance. Thanks for the advice.