Spare tire location - Aerodynamics and centre of gravity

Very little in the way of discussion on this. I don't care that "it's a Jeep". That's a bogus excuse for poor engineering.

So my XJ's 5000lbs right now, sitting on 35x12.5's. I've no idea how that happened. The mass just crept up on me.
I've got a full size roof basket as well as a swing out tire carrier. Both look bad-ass as hell, but the basket is overkill for almost everything I ever do with it. The swing out carrier blocks my view (which only bothers me on the trail), increases my COG, and I land on the spare tire all the time on the trail even though it's centre line is pretty near half way up my rear window.

My COG is getting stupid, and the aerodynamics suck. With the roof basket and carrier removed, tire mounted inside, I go from 17mpg highway to around 23-24mpg. Yea, not a HUGE deal, but combined with the increased COG and hanging up on the tire, I think it's worth fixing.

The real question is, what's the most aerodynamic approach to the spare tire? I can move my 52" light bar above the front bumper, and my side lamps can all be moved either under the Jeep, or brought inside and mounted facing out the windows. I still need a roof rack with a full floor, but I can build something much lower profile and lighter.

Assuming a lower profile roof rack, what about the full size spare (that I'll probably never need but I'm paranoid and I often end up needing it for other peoples rigs). The carrier itself isn't as high as the roof, but it adds 60 or 70 lbs that's definitely above my roll axis, as well as puts an extra 150 - 170lbs on the ass end of my Jeep (which is already ass heavy). The aerodynamics of a spare on the back are horrible, and I almost suspect both my COG would be lowered as well as increasing aerodynamics by mounting the tire on the roof. Yea, you're moving the spare up higher, but you're also losing 70lbs of carrier from above the centre line.

What do ya'll think? Who's done both and actually got a feel for how it affected COG and aerodynamics?


P.S. No, there's no room inside the cab. In any situation where I may need my spare, I also need my tools, recovery gear, fluids, and usually camping gear.

blistovmhz said:

Very little in the way of discussion on this. I don't care that "it's a Jeep". That's a bogus excuse for poor engineering.

My COG is getting stupid, and the aerodynamics suck. With the roof basket and carrier removed, tire mounted inside, I go from 17mpg highway to around 23-24mpg. Yea, not a HUGE deal, but combined with the increased COG and hanging up on the tire, I think it's worth fixing.

The real question is, what's the most aerodynamic approach to the spare tire?

Assuming a lower profile roof rack, what about the full size spare (that I'll probably never need but I'm paranoid and I often end up needing it for other peoples rigs). The carrier itself isn't as high as the roof, but it adds 60 or 70 lbs that's definitely above my roll axis, as well as puts an extra 150 - 170lbs on the ass end of my Jeep (which is already ass heavy). The aerodynamics of a spare on the back are horrible, and I almost suspect both my COG would be lowered as well as increasing aerodynamics by mounting the tire on the roof.

What do ya'll think? Who's done both and actually got a feel for how it affected COG and aerodynamics?


P.S. No, there's no room inside the cab. In any situation where I may need my spare, I also need my tools, recovery gear, fluids, and usually camping gear.

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Post up a picture of your set up so people can visualize and make a better suggestion.

Nothing on top of the roof is about the best you will get. I can't imagine there being better aerodynamics with a 12.5" tall tire sitting up there. The tire out of the way on the back where it isn't dragging air is the best way you can keep decent aerodynamics.

It's a jeep man. It is a square body lifted into the air catching more resistance. If you are getting 17-19mpg I think you are doing fine. If you can't handle that get a liberty uke:

17-19 ain't bad for a 350hp Jeep, but I've played this game before. Ask a mileage question, and unless you're getting less than 12mpg, nearly everyone will say "get a prius".
Fact is, a stock XJ in good working order can see up to 25mpg (don't ask me how, but I've had 3 very high mileage XJ's and all got 23-25mpg, while my lower mileage always got 19-21).

With the V8, I KNOW that with the roof rack and carrier removed, I get much better than my current 17-19mpg. Removing the spare tire from the carrier alone, nets me a full 2mpg increase on average, so while it seems intuitive that something directly in the wind would cause the most drag, that just ain't how it works. Air compressing in front of your vehicle and air being forcibly decompressed behind you, are just as damaging to aerodynamics. Many will argue that the back of your vehicle is actually more important (which does show up in a lot of very aerodynamic vehicles).

I'm aware that I'm driving a brick. Tall, wide tires and lift ruin aerodynamics, but fact is that even with them, I can still pull an easy 23mpg. The roof rack and spare tire carrier have had a much larger impact to fuel economy than tires and lift ever have for my Jeeps thus far.
The COG question is just as important. I won't throw the tire on the roof if it increases my overall COG and makes weight distribution worse, even if it works out that the carrier adds more drag than tire on roof. Safety is the first concern, mileage second.

That said, I know with my 4.0L, I was getting about 19mpg with the stock roof bars and no carrier. Once I added the carrier, I immediately lost 2mpg (which is exactly what I'm seeing gained when I remove the spare). The aerodynamics question to me, is secondary though to the COG.

Yes, spare on the roof is higher, but it's only the tire, and not all the supporting infrastructure required for the swing out. I'm trying to figure out which ultimately, is better. More mass on the rear, half way up the window, or about 70lbs less, but a full foot higher, and further toward the center of the vehicle. As I said, my ass is HEAVY, and I'm on 63" chevy leafs, so the rear really doesn't like the extra mass. I feel I may be doing my Jeep a favour by putting the tire on the roof as it'd be engaging all four corners of the suspension instead of just the rear.

Also, a Liberty doesn't get any better mileage than an XJ. If anything, they're actually worse. Sticker says 14/19mpg for the manual in 2004. Manual XJ was 16/22.

Current setup looks like this:

Yes, I'm aware the silly hood scoop isn't helping my fuel economy, but it IS helping in the whole "having a hood at all" thing, as well as dramatically helps keep the engine bay cool. I may block off the front of the scoop as I'm sure it isn't helping with the cooling anyway, and probably hurts the economy very slightly.



Unfortunately, that spare is brand new, while the other 4 are at around 50% so I'm only using it as a "get out of the trail" spare. Can't be run on the street due to lockers. *sigh*.

I figure the aerodynamics of having it on the roof could go either way. I feel intuitively, that it won't make any difference, which is GREAT if it helps my COG, but pointless if it doesn't.
I'm going to throw it up there to give it a mileage test, but it's difficult to quantify the COG "feel" without pushing it to the point where I'm ready to roll over

blistovmhz said:

Unfortunately, that spare is brand new, while the other 4 are at around 50% so I'm only using it as a "get out of the trail" spare. Can't be run on the street due to lockers. *sigh*.

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Huh????


Regarding the mileage - if I was getting 18 +/- 1 mph in my XJ I'd be thrilled. My modified XJ gets about 14.5. Even my 96 ZJ and my 04 WJ (both completely stock) only pull 16-17. All 3 of them with the same I6 4L.

Sounds to me like you want to use the mileage as an excuse to fiddle around in the garage - if it ain't broke don't fix it. That's what I say!

Yea, 18ish ain't horrible, but it's not "good". I've never understood why Jeep folk can worry so much about some issues, but never give economy a second thought. To me, if I know it can do 20mpg without sacrificing any of my on/offroad capability, it's worth investigating why I'm not hitting that mark. Of three best mileage XJ's, all were over 450,000km (one over 650k) and all got somewhere between 12 and 15mpg when I bought them. I got them all over 20mpg with some basic troubleshooting. Before the motor swap, my current Jeep was 5000lbs on 35's and still pulling an average 17mpg. That is MOUNTAINS better than 99% of other Jeeps, only because I bother making my junk run right .

With the V8, I've seen it hit 26mpg sustained. I took a bit of a hit with the most recent tune, but no matter what, that spare and the basket are hurting me on fuel economy, and combined, they're destroying my COG. If I had a regular XJ suspension it'd probably be acceptable, but then I wouldn't be articulating as nice as I do, and there's no way I could do the high speed stuff I smash through now. COG has become very important recently, especially when it occasionally holds me back from a line that I know I could do if my COG was an inch or two lower. That said, wheeling is almost boring now as there's not much in my area that'll put up much of a fight with my XJ. Half tempted to attack the trails with some 31's just cause. But, I don't like being tippy. I'm fine with getting stuck or high centered or just not able to make an approach, but tipping over on light side hilling just ain't my thing.

So yea, it's not "broke" but it ain't what it could be. Also, it "ain't broke" YET, but the COG will be the death of either myself or my Jeep . The mileage is really incidental to the question of what to do with the spare.


As for the tires, ever try driving a pair of locked axles on 35's down the highway, with mismatched tire circumference? 1. Scary, especially with the v8. Every time you touch the gas, you get LAUNCHED to the side. No amount of steering will stop it. 2. Destroys tires, probably lockers as well, and puts massive strain on everything in between. Mismatched tires and lockers + pavement = very bad.

Thank you! I am kinda tired of seeing the "it's a brick on wheels what do you expect?" answer. That's a non-answer. Why wouldn't you want to save money? Personally, I'd rather spend that money on upgrading my jeep rather than burning it out the tailpipe.

I am looking at a low-profile roof rack cage myself forn the very same reason. Regarding mileage, if you fab a rack that is tall enough to go over the tire, you will do yourself a favor with mileage. Getting rid of weight (especially in the ass end) will be the best thing. I'm looking at fabing bumpers from aluminum to help with weight.

The detractors are right, it's a brick. So make the brick lighter. Tucking things in like lights and the tire will help on the trail and at the pump.

I haven't seen very aerodynamic roof rack carriers that's why I'm gonna fab one myself. I don't like any of the styles out there anyway... And the old adage is true: want it right, do it yourself.

I hit the same wall with roof racks/baskets. Everything sucks. Mine is only 75lbs but can easily support 1000lbs (not that I'd ever put that much up there, except when I go camping with the GF because she packs for 10 people for 3 months for an overnight trip).

My rack is probably the worst I could've done for aerodynamics, which is why I'm thinking I'll move the light bar above the bumper, and maybe start thinking about an enclosed roof basket of some sort (something big enough for a full size spare though.. I unno). Either way, COG has to come first (safety first). If the tire on the roof is worse than the tire + carrier on the back, then I'll just re-work my carrier to angle the tire forward to keep it from getting hung up, and build a much lower profile basket/rack.

Another thought on the tire: I have a friend that did a skinny tire for his spare. It's the same size diameter rim and tire, just super skinny. If I had the measurements I'd give 'em to ya, but I think you get the idea. Reduce weight, reduce drag, potentially reduce cog as well. Worth thinking about at least.

Thought about skinny spare,but realistically, if I need a spare, there's a good chance I need it for a good reason.

Just to be clear... Not talking about the donut tire variety spare. It's a real tire, just like a 6" instead of 7" rim and 195 instead of 225 while still being a 15" rim and 75r height (just numbers of the top of my head, don't judge my math!).

Yea, the problem is that if I've wheeled into a situation that blew a 12.5, I won't make it out on something skinnier

Are you watching AF ratios?

Gotcha! Well... COG will be helped with moving the tire to the roof rack and removing the rear rack (over all weight reduction) but you'll increase drag. More savings in weight vs drag for mpg? Not sure. I'd say better mpg simply because of weight reduction and the whole aerodynamics of a brick thing.

One other thing to consider if you move it up there, what about security? I have my hilift jack under my back seat because I fear it growing legs. Something to consider.

blistovmhz said:

Thought about skinny spare,but realistically, if I need a spare, there's a good chance I need it for a good reason.

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Skinny spare may not work any better than different diameter/circumference tires when a locker comes into play. You get on it and the skinny tire has less traction, so it will get squirrelly just as bad.

I got the same issue, as right now, my spare is a 1 size smaller tire, so if I get a flat, I have to put the smaller tire on the front, and move a front tire to the back, which for me usually means moving 2 tires around just to get out.

I do carry a plug kit & air for such things, but the 1 time so far I have had to do it, I gashed a sidewall too bad to plug, so the tire olympics were performed.


Best bet for aero & weight dist would be inside, but that sucks up a lot of room.....there's just no good way to carry a full size spare. When you figure it out, lemme know, as I am about to have to address the same issue.

I remember seeing a few XJ's with a large spare mounted in the cargo area flat, with part of the hatch cut out so that the rear seat is still usable. Maybe something like that and a cargo rack in the cargo area? SInce you already have the tire carrier apparatus, maybe make the swing out into a small "cage" of sorts to keep the tire in place?

Johnnie Walker said:

I remember seeing a few XJ's with a large spare mounted in the cargo area flat, with part of the hatch cut out so that the rear seat is still usable. Maybe something like that and a cargo rack in the cargo area? SInce you already have the tire carrier apparatus, maybe make the swing out into a small "cage" of sorts to keep the tire in place?

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Neat idea, but may still hang on it on a drop-off....(probably not as bad as now tho).

That was one of his complaints

Dude that spare is out there and not all that high. I can see why you land on it. That mounting for the tire could deffinately go up and slat toward the window. Also mount closer to the window.
Look at crok4x4. I use there bumper and tire carrier. Not sayin its the best setup but it is uber high clearence. I have never landed on the tire.

DJTimyD said:

Gotcha! Well... COG will be helped with moving the tire to the roof rack and removing the rear rack (over all weight reduction) but you'll increase drag. More savings in weight vs drag for mpg? Not sure. I'd say better mpg simply because of weight reduction and the whole aerodynamics of a brick thing.

One other thing to consider if you move it up there, what about security? I have my hilift jack under my back seat because I fear it growing legs. Something to consider.

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Very few people look at my Jeep, in my yard and think "Yea... Stealing something from that scary ass Jeep seems like a good idea...". Thus far, despite being in the worst part of town, I've lost a small jerry can, and that's it. Jeep sits unlocked with a tablet on the dash. When I was still in Vancouver I had the spare strapped to the roof and never had a problem either. Not terribly concerned about it around here.

ftwelder said:

Are you watching AF ratios?

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Heh. I'm watching EVERYTHING!!!! I've been super anal retentive about numbers since the swap, as I'm really trying to push this motor to the absolute limit of efficiency without losing any power. AFR is good. FT's are usually deadly within +/-5. I'm not happy with my LTFT at idle, but I tuned for 2000rpm as that's where most of my driving is at, and really, I'm not idling very often. O2 frequency is good and flapping properly on both banks. The fuel economy dropped quite a bit when I did the last tune, so I'm sure the guy messed it up a bit and I just haven't had time to look over a new log. I suspect he set the injector ramp a little too steep and opened up the pulse width at idle to give the throttle a little more snap, but overall I feel it could all be MUCH better. I can't say I'm unimpressed with the guy yet as he's only done two tunes for me thus far and I just haven't had the time to dig into it, but at the same time, he seems to be quietly refusing to tell me what he's actually adjusting... I don't dig that. Should've said **** it and bought the package myself. Very seriously considering building a megasquirt for just this reason. I need ALL THE DATA!.
Either way, with the spare and basket removed, I see around 5-6mpg increase and I'm pretty sure about 2-3 of that is just the spare on the carrier. Will try to confirm this sometime this week. Just haven't been out wheeling enough lately as all the roomies trucks have been out of commission and I've been busy rebuilding their junk.

TRCM said:

Skinny spare may not work any better than different diameter/circumference tires when a locker comes into play. You get on it and the skinny tire has less traction, so it will get squirrelly just as bad.

I got the same issue, as right now, my spare is a 1 size smaller tire, so if I get a flat, I have to put the smaller tire on the front, and move a front tire to the back, which for me usually means moving 2 tires around just to get out.

I do carry a plug kit & air for such things, but the 1 time so far I have had to do it, I gashed a sidewall too bad to plug, so the tire olympics were performed.


Best bet for aero & weight dist would be inside, but that sucks up a lot of room.....there's just no good way to carry a full size spare. When you figure it out, lemme know, as I am about to have to address the same issue.

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Eyup. Hadn't thought of the skinny tire not really working either, but you're definitely correct. I could probably roll home on it, but it wouldn't be fun. Either way, if a fat tire got me in and failed, a skinny tire probably won't get me out. I'm more than happy to sacrifice a small amount of fuel economy for the additional insurance of a full size spare.

Johnnie Walker said:

I remember seeing a few XJ's with a large spare mounted in the cargo area flat, with part of the hatch cut out so that the rear seat is still usable. Maybe something like that and a cargo rack in the cargo area? SInce you already have the tire carrier apparatus, maybe make the swing out into a small "cage" of sorts to keep the tire in place?

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Been thinking this. NordaWheeler's sweet mutha****in' XJ (the inspiration that finally got me off my ass and into a V8 swap) has that setup. I think he's got a 44 laying back there. Not sure if he has a back seat, but either way I'm sure a 35 would fit with some custom "hatching". Doing this while still maintaining a decent water tight seal might be a bit of work, but I was thinking I could just box line the back and build a few inch lip up the back seat to keep water out. That actually may be the answer. Solves COG, aerodynamics, rear viewability, and moves weight distribution at least a couple feet forward. Not entirely sure I'd still be able to get my tools and gear in there, and I'd have to relocate my amp and coils up to the roof, but I think that really may be the best plan at this point. Will do some measuring tomorrow and see if NordaWheeler can provide some pictures of how he did his.

What's really unfortunate is that if I do get rid of the carrier, I either have to cut it off my bumper, or build myself a new bumper, cause my bumper also has my shackle mounts. Seems a waste to tear apart a perfectly saleable swing out. Maybe I'll just build another bumper to weld it onto and sell it.

Will see how things measure out. I really do like the idea of just hauling the tire back inside, and I've long wanted to cut the hatch off and make some bad decisions with it .

blistovmhz said:

I'm more than happy to sacrifice a small amount of fuel economy for the additional insurance of a full size spare.

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:dunce:

What is the point of this conversation?

Quit hauling around so much chit or deal with it. Should have dropped in a diesel if you were worried so much about economy.

GordoSmasho said:

Dude that spare is out there and not all that high. I can see why you land on it. That mounting for the tire could deffinately go up and slat toward the window. Also mount closer to the window.
Look at crok4x4. I use there bumper and tire carrier. Not sayin its the best setup but it is uber high clearence. I have never landed on the tire.

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Yea, when I built it, I built it with the rear shackle mounts in mind. I could've put the tire up higher, but that gets awfully wobbly way up there without a lot more metal to hold stuff stable. If I do end up retaining the carrier, I was definitely going to tip it forward and possibly move it up a bit. I keep holding off on this because I haven't decided if I'm going to cage. If I cage, the hatch comes right off and I build something new from the ground up, in which case I'll have more angle to the rear, and thus a good spot for the spare.