18" will be the wheel size, not the tire size. whats the tire size? what type of tire?
was the speedometer gear in the TC changed? if not, your not reading the correct mileage. have you verified speed using GPS?
assuming its a 4.0L
what gears are in the diff?
im guessing its a combination of inaccurate speedometer and undergearing for the larger tires
So your odometer is off then.
But adding bigger tires without regearing also affects mileage.
Jeep's start off with generally poor mileage (you are driving a brick), adding bigger heavier tires will always affect gas mileage.
This post is more for future researchers with some of your questions. I am pretty sure you already know some (if not most) of this. Please be patient as I explain.
Everything you messed with during your lift is part of a engineering/mathematical package. It has all been calculated out at the factory to behave in a certain way. Brand new TJs were doing well when they got 17 mpg on the highway. (Mine still does, but I had to do a lot to get it there, which I will explain below.)
• Firstly, tires have a specific length of tread based on how big around they are. If you increase their size you will travel farther on each rotation.
• Tires have a rotational weight, as do the rims, brake rotors, wheel bearings, etc. More rotational weight equates to a LOT more grunt required to start, maintain speed and stop. Larger tires mean the vehicle has to work harder to turn them.
• Rim diameter does not factor into this much, except in how tall your sidewalls are, which affects you some in lane changes at speed, stopping, and general handling, with taller sidewalls meaning generally more squishy response. However, we *need* taller sidewalls so we can absorb sharper impacts and more easily conform to the broken surfaces we like to roll on over smooth pavement. We like to air down the tires when in loose stuff, mud or in rocks. The tires then have a larger contact patch with the ground and we get better performance. We air back up on the road to help with handling and MPG. For the TJ the original rim diameter was 15" and many of use still prefer that over larger rims if we air down a lot. I have 15s on my TJ and 17s on my JK. I am considering bumping the JK down after having years of experience with both off road. When you get up to 20s you seriously help the jeep on the highway, but hamper it terribly off road if you run on uneven, rocky ground.
• Larger tires eventually will require bigger brakes. This is costly and time consuming but very much worth the money and time if you decide to do it. There are kits for sale for this. There are also rear disc brake conversion kits if you did not get a Rubicon and have drums on the back. I have one of these and I like it a lot for working on the TJ, but have never seen any real braking distance reduction in an emergency stop in that vehicle, so I am not sure how I feel about that upgrade. The big brakes I have on the front are really great, though.
• Lifting the frame higher above the axles is to allow for the clearance of larger tires. It seriously screws with everything and it takes a lot of tweaking and experimentation (as well as tons of reading in places like WF) to make it drive well again. Many lift kits are missing stuff that a lot of us would consider to be very important. Some have legacy parts that we have not needed for decades but no one has seen fit to pull from the kits. (Read up on dropped Pittman arms and what they were designed for and why you most likely do not want on on your TJ.)
• Lifting your frame higher off the axles alters the very specific angles of your upper and lower control arms, and this can cause all sorts of weirdness in steering and other things. We generally use adjustable control arms to allow them to be lengthened or shortened to restore the angles of those control arms to something similar to the factory specs or whatever solves our issues. Generally, the steeper the control arms the more twitchy the steering and handling, with more shallow angles giving easier steering that can end up being sluggish and heavy. (I am speaking from experience with this. There are lots of other issues that are very complex and expensive. The TJ's super-short rear driveshaft with its slip yoke is the main concern when you lift and is worthy of a massive amount of reading to understand.)
• When you increase tire size I stated that you make them heavier and that they turn at a slower speed than what your odometer is expecting. To account for this you must replace the speedo gear with one with the correct number of teeth so both the speedometer and odometer read correctly. There are charts here for calculating that and with the part number so you can order from your dealer of online. This is important because you can get a ticket, you can miscalculate the gas left in your tank on a long trip in the middle of nowhere, or you can have an odometer that displays incorrect miles on the vehicle, which can be a PITA if you sell later and you are an honest person.
• Your engine has no gearing. It is a simple device that turns at the RPM you tell it to. The transmission is full of gears, as it the transfer case. These all have very specific numbers of teeth to rotate in exact ratios with the other gears so you know what is happening as you drive. The axle diffs are likewise geared, but they only have one, fixed ratio and cannot be adjusted. When you alter the tire size the load on the axles changes. They still want to rotate in sync with the transmission but you have altered the relationship, mathematically. Now they move more slowly (or you go farther with each rotation) but they are under more stress. In order to restore how the engine runs on the Interstate you have to alter the gears in both axles so they match up with the tire specs and your transmission's gears. For example, my 2003 Sport came with 3.73 gears from the factory. It also came with the "30" Wheel and Tire Group" so that was better than the 3.02 (? I am guessing, here) gears that came with the standard 27" tires. But when I went to 33" tires I had a hard time getting into 5th gear on the Interstate, and maintaining 70 MPH on hills and in a headwind became more of an issue. My MPG also took a dump on me, going from 17 highway to about 12 highway. Fixing several small things helped restore some of that, but it took regearing my axles to 4.56 made it drive like it rolled off the factory floor. Acceleration and maintaining speed, and PASSING people all improved and now I get 17 MPG on the highway again.
• Regearing Dana axles is a massive PITA best left to a pro if you do not own some very spendy tools and have some prior experience with easier axles. The issue is that the many paper-thin shims needed to dial in the ring gear from right to left and the depth of the pinion gear have to go on BEFORE the hydraulically pressed on bearings. Since you generally have to adjust the height of each shim pack many times before you get a tooth contact pattern that will be both quiet and not overheat (as well as one that won't see you chipping teeth all the time) the easy to damage bearings have to be removed and reinstalled over and over using a hydraulic shop press. To make this a non-hellish experience we end up frequently making "setup" bearings, which are old bearings that have been bored out a little in the center so they just barely slide on and off using bare hands. I own a set of "dummy" bearings from a dealership for each of my specific axles. They are like a miracle. They are solid tool steel that are machined to the same tolerances of an actual bearing, but with the center bored out perfectly smooth and straight. You almost can't do it wrong with these. Anyway, what I am saying is that until you re-gear and put in a new speedo gear all your info will be off regarding speed, miles driven (or left on your tank) and your MPG. If you do not do these things you will have to learn over time to recalculate everything in your head, like when in 5th gear and rolling at 2750 RPM on the tach you are actually going at XX miles per hour instead of what your speedo says, that your already very nearly useless gas gauge (okay for 75% of the tank and garbage for the last 1/4) has become something that you never even look at because it has become that inaccurate. You will also possibly need to have your front brakes looked at.
• The magic height for a TJ used to be a 3" suspension lift. Body lifts were a no-no for years. More recently with improvements in what we can purchase it seems to have become a 4" suspension lift with a 1" body lift. Every quarter inch alters the numbers, so you have to lift with care and make sure your kit includes everything you need to make it work. Look at the really expensive kits to see what is available to make a lift a smoother process to install and to live with. Then look at the really cheap kits. Not only are the components themselves of a lower quality (usually) but there are just a lot of parts not there. You need to look at what others have decided is *needed* to make things play nicely together. Then look carefully at all the parts included with your kit and see whether you want to look at purchasing some additional bits to help make things work better as a system.
I have no idea what you have installed, in the lift kit, the tire diameter, and all that, but these things, no matter how poorly installed or incomplete, tend to work well enough for daily driving if you are careful. Do drive of pavement with no danger and a lower chance of heading home in the passenger seat of a tow truck that you had to pay tons extra to get your rig out of the backwoods, I would seriously look at what I took the time to write up to help you out. If you know some of this already, please understand that I have no idea where you are in your "jeep education" or what is installed. I am guessing, and to help the most I am guessing you are more or less a novice, here. All of us were at one point.
I wish you luck. Message me if you want any info privately. I am happy to help when I can here in the thread, too. Also note that I am an older fart and have not been on this forum in some time. I am not up-to-date on all the most current suspension geometry ideas and on what the current state of the art for suspension and brake parts and kits are. Last time I was here it was still a slugfest between Curry and Metalcloak. I am sure that has moved on some. I am back because I am about to rebuild my 4.0L engine and need a ton of specs and advice and such. I am sure I will find out how "quaint" my TJ is all of three years later, heh, heh, heh.
If I have put up any info that is known to be incorrect I am sure someone will gleefully set my pants on fire. (This place was headed down the hill of becoming another Pirate 4X4 when I last visited regularly. I am hoping this has changed and the old, sane WF is still available as a resource.
Best of luck. Post pics of your TJ for us. Jeep pics are like porn to people here. We all want to gawk at each other's rigs and post pics of our own. HAHAHA!!!
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