Something don't seem right
JK Super Freak
Joined: Sep 2009
Posts: 1,591
Likes: 2
From: annapolis, MD
Never heard the heat thing. Doesn't make sense, either. It's the AC that cuts mileage from the compressor running. Ever notice that vehicles are "slower" web the AC is on? Doesn't happen with the heat.
JK Enthusiast
Joined: Sep 2010
Posts: 406
Likes: 1
From: Hot Springs, AR
I consistently get 21 mpg hwy and 17 -18 mpg around town. I would love to get 37's but I am afraid I will end up with 12-15 mpg , and the way gas prices are going up it just wouldn't make me a happy Jeeper.
JK Enthusiast
Joined: Nov 2010
Posts: 160
Likes: 0
From: Omaha, NE
You can in theory get better mileage with the installation of slightly larger tires. Look at the ever so referred to gear ratio and tire size chart. If you go from a 32" tire, to a taller tire, say 33" your RPMs at speed will be slightly lower. Running lower RPMs uses less fuel. Now, going from a 32" to a 35" on 3.73 gears will make the engine rev several hundred RPM lower, and the engine will not be running in the main power band and performance MAY be adversely affected.
JK Freak
Joined: Dec 2008
Posts: 712
Likes: 1
From: Shreveport, LA
The defrost kicking in the A/C clutch makes sense. I think most cars do that...not unique to JKs. That was what was strange to me...the heater causing the A/C clutch to engage.
On the old CJs we used to use the York A/C compressors for OBA setups and they worked great...but when engaged you could tell a difference in the engine RPMs. With the EFI set up on the AMC 360, I was able to tell the GM ECM that the compressor was engaged and it would kick the idle RPMs up a bit to keep them in the normal to above-normal range. End result? More idle RPMs=more power=more gas used. Plain and simple. The same thing happens when an A/C clutch engages on a JK.
On the old CJs we used to use the York A/C compressors for OBA setups and they worked great...but when engaged you could tell a difference in the engine RPMs. With the EFI set up on the AMC 360, I was able to tell the GM ECM that the compressor was engaged and it would kick the idle RPMs up a bit to keep them in the normal to above-normal range. End result? More idle RPMs=more power=more gas used. Plain and simple. The same thing happens when an A/C clutch engages on a JK.
JK Enthusiast
Joined: Dec 2010
Posts: 111
Likes: 0
From: San Jose, CA
Originally Posted by Pushrod
I consistently get 21 mpg hwy and 17 -18 mpg around town. I would love to get 37's but I am afraid I will end up with 12-15 mpg , and the way gas prices are going up it just wouldn't make me a happy Jeeper.