Sluggish Highway performance... (Not a BS gear thread)

Hey. I've got an 01 xj. 4.0, aw4.
Stock motor besides a snorkel, IRO cat back exhaust.
4.56 gears, turning 265/75/r16 Treadwright tires. All terrains, at 32 psi. Only 4 inches of lift, no roof rack.

The tires are freaking heavy for being 32x10.50.
60lbs each. With Moab rims, each wheel is about 80 pounds. (Even the maxxis bighorn in that size is between 49-53 pounds)

I tach at 2500rpm at 65 mph (speedo corrected).

However, its so HARD to get it up to 65. It's really fast around town, but I feel like I have to floor it to go past 55 mph on the freeway. And the tranny downshifts no problem, but it feels like it's running out of steam, even in 3rd gear. Overdrive kicks in at 40 mph.

Once it's up to 65, It seems that you have to use a lot of throttle to keep it there. The overhead console says I get 18 mpg at 55mph, 12-14mpg at 65. Horrible. I actually think it's over geared. I know aerodynamics are bad, but I could get 19 mpg all day long on 3.55 gears, at 65 mpg. (Although it sucked everywhere but the freeway.)

I'm think going to 255/85/16 Maxxis Bighorn might help, its a 33.3x10.40r16 tire, and its only 53 pounds. Lighter, and less rpms. (And less rolling resistance.)

Any advice? Kinda sick of taching so high, yet it feels so weak.

Sick of the poor performance, and the bad mpg isn't good either, gas is $4.19/gallon here. (Highest average in the US.)

Thanks.

Having no personal experience with this, if you think it's over geared borrow some bigger tires and see if your mileage goes up.

Hmmm. I think trying someone's bigger tires might be worth looking into. I have stock 3.55's, and 31" BFG KM1's on 15's. I'm actually very surprised at the economy and drivability on the highway. From my house to Tech in Blacksburg is about 350 miles. I averaged 16.5 mpg, at an average speed of 83mph. From about 72-80mph, the power was perfect. I could sustain the speed fairly easily without having to mash the pedal up hills. Same went for about 50-60mph. But right in between, from 60-70ish, I was amazed at the loss of power. Really had to stomp on the pedal to keep it going, and the trans would downshift if I pushed too hard. I think 3.73's would be absolutely perfect for my application. Tach around 2500 at 80mph. Btw this is totally gonna turn into a bs gear thread haha.

I do want to try some 33s. And yea, it's an abused topic, but this is also about tire weight. I've had 4.56s for about 8 months, but it seems like my mpg and power is just not right.

If you're serious about your wheel/tire weight, 80lbs is EXTREMELY heavy for that size. I have a 31" first gen MTR on a steel wheel laying around, I'll see what it weighs. It's heavy as sin, but I guarantee not anything near 80lbs. Maybe the fact that it's a retread really does matter?

I know, it's horribly heavy. Rotating mass is very, very important. I could save 14 pounds per axle by going to the bighorn.

Yea, please post that weight up.

Treadwrights are dreadfully heavy tires, I ran them on my KJ for a year and a half. Tough as nails and did great off road but on the road they sucked the gas down. I got a 3 mpg improvement from my 245/75/16 Guard dogs load range E, to the 265/75/16 yokohama geolander at's load range P that i run now. Make sure you check the load ranges on the tires you buy as the higher load rating the more plys hence more weight the tire is going to be.

Lol I try to save oil by buying retreads, but I use so much fuel it doesn't matter.
They are load d.

srmitchell said:

Lol I try to save money by buying retreads, but I use so much fuel it doesn't matter.
They are load d.

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FIFY?




Tapatalky

Yeah, that.

80lbs? Damn. I thought my 32s (BFG ATKO) with eagle alloys were heavy and they came out to just under 38lbs per mounted and balanced wheel. When I picked up the 36" TSL Radials it felt like I was carrying all four 32s.

I know your pain... 285/75r16 treadwright ATs on 16x8 cragar V5s here! Never actually weighed them but they are pretty heavy.

I'm on 3.55s and get about the same gas mileage as you, or did before my downpipe got abused badly. Fixing that soonish.

Have you adjusted your throttle valve cable? Calibrated your speedo?

Anything with steel wheels is heavy. If you have 32's with aluminum wheels and you're still coming to almost 80 lbs per tire, you must have lead in the equation somewhere lol. My 35" MT/R-K on 15x8" aluminum wheels weighed just around 80 lbs (weighed by me on a scale, not the company's weights).

I'm gonna say something else is wrong in the equation here. I had 32's and 4.56s for quite a while and I had power at any RPM. I could easily get up to 80mph on the highway if I wanted without a problem (as long as my drivetrain and everything didn't shake apart). Going to bigger tires is definitely not going to help the problem. 35s and 4.56s are really sluggish and I'm in 3rd gear quite a bit. Going up hills on the highway sucks because I always have to downshift. Never did with 32s.

I can do basically whatever speed I want, but it is a bit sluggish accelerating.

sunburned said:

Anything with steel wheels is heavy. If you have 32's with aluminum wheels and you're still coming to almost 80 lbs per tire, you must have lead in the equation somewhere lol. My 35" MT/R-K on 15x8" aluminum wheels weighed just around 80 lbs (weighed by me on a scale, not the company's weights).

I'm gonna say something else is wrong in the equation here. I had 32's and 4.56s for quite a while and I had power at any RPM. I could easily get up to 80mph on the highway if I wanted without a problem (as long as my drivetrain and everything didn't shake apart). Going to bigger tires is definitely not going to help the problem. 35s and 4.56s are really sluggish and I'm in 3rd gear quite a bit. Going up hills on the highway sucks because I always have to downshift. Never did with 32s.

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I support this message. :0

yeah not to be a gear thread,BUT,keeping the rpm's a bit hi is not bad really.I run 3000 rpm or so on 35" bfg all terrains with an nv3550 in 5th, 4.88 gears @ 75 mph.doesn't mater hills only make me put a bit more throttle into it.I get some where between 14-16 miles per gallon.might explain why it feels better for most 4.0's to be around 2500-3000 rpm's for power and mileage.Plus plenty of power getting up there too.Aww hell I can usually bark second gear!!!!! with the aussie locker...:eyes:

There's just something wrong. It actually feels good at 71 mph+, but theres like a throttle dead zone between 55-69 mph.

Throttle cable is adjusted, and the speedo gear is the one for 4.56/32's.

My rig is pretty aerodynamic too, no roof rack, and the roof line is about 6'3".

Does anyone know if there is a definitive answer-
I could go to the super skinny 33, which weighs 53 pounds.
Or the 265/75 load c tire which weighs only 49 pounds.
Would I be better off reving at 2300 at 65, or 2500 at 65?
Same tire, Maxxis Bighorn.

I wouldn't call it a definitive answer unless I'd done some repeatable experiments with similarly configured XJs in a controlled environment. That said, I would go for the lightest tyre you can in the 31-33" range if you're not satisfied with your current kit. From my experience, I'd prefer to be in the 2500-3000 RPM range at 65 MPH. The power band is there/nearer and it makes for a more exciting drive. I'd keep the choice skinny but that's just me! You can see how adamant I am about "skinnies" (not Somalians[well, them too!], the rubber that meets the tarmac) in some of my other posts. LOL!

Yeah, I understand. I need to find a buyer for my tires.