Tire choice quandary?????????

[Explorer 1]

OK, I've probably modified my Jeep Cherokee beyond what would be done for most expedition vehicles, still I seek a little better fuel mileage on paved roads and a ride with a bit less tire noise for those less challanging adventure type treks.

All my research has shown that by lowering the unsprung weight (tire and rim), I can not only increase my fuel economy but also the acceleration. My current tire and wheel choice is on the rather heavier side, 35/12.50/15 BFG KM2's mounted on Raceline bead lockers.

I'm looking for a set of lighter "all terrain" types which I can mount on a light weight rim. Perhaps I can decrease my wheel/tire weight by as much as 30 - 40%.

My problem is that the gearing and suspension on my XJ is set up for a 35" tire. I'm willing to buy larger rims to accommodate a lighter tire but am having a hard time finding a lighter "all terrain" tire with less width and a 35" diameter.

Perhaps someone out there in "expedition land" can help me.

Thanks,
Fred
Explorer 1

 

[CA-RJ]

According to Tire Rack, the BFG MT KM2 is 61 lbs. The BFG AT is 54 lbs. Doing that and going to a non-beadlocked alloy would certainly shed some weight. If you could find a forged wheel, that would even be lighter. What are you getting for fuel mileage now? What are you hoping to achieve?

 

[reece146]

I dropped from ~85 lbs per corner to ~55 lbs per corner. Made zero difference to mpg or acceleration.

I'm running GY Duratracs currently. Fab tire over the 1000kms or so that I've had them to date. Grippy, quiet, good winter traction - have yet to try them off-road as wheeling season is currently closed here (sleds rule the trails atm).

GY doesn't make a floatation 35" tire AFAIK, you'll have to switch to a metric 16 or 17 inch wheel to get the same diameter.

 

[Explorer 1]

According to Tire Rack, the BFG MT KM2 is 61 lbs. The BFG AT is 54 lbs. Doing that and going to a non-beadlocked alloy would certainly shed some weight. If you could find a forged wheel, that would even be lighter. What are you getting for fuel mileage now? What are you hoping to achieve?

Off road more difficult trails mileage can get down to 7-8 mpg, around town 10-12 mpg and best highway, maybe 14-15mpg.

On the highway if I could get into the higher teens, keeping my speed down to 65 mph, I would be most happy.

Thanks,
Fred
Explorer 1

 

[Explorer 1]

I dropped from ~85 lbs per corner to ~55 lbs per corner. Made zero difference to mpg or acceleration.

I'm running GY Duratracs currently. Fab tire over the 1000kms or so that I've had them to date. Grippy, quiet, good winter traction - have yet to try them off-road as wheeling season is currently closed here (sleds rule the trails atm).

GY doesn't make a floatation 35" tire AFAIK, you'll have to switch to a metric 16 or 17 inch wheel to get the same diameter.

I'm really surprised that it didn't make any difference, JP Magazine recently did two articles on this very idea, one was about a lighter tire and rim combo, the other article was on same tire weight but less aggressive tire tread. The tire tread alone made 1/2 mpg difference.

This maybe a dead end, I'm just trying to get more exploring miles out of a tank and trying to plan trips without so many extra fuel cans strapped on the rack.

Thanks,
Fred
Explorer 1

 

[reece146]

I don't trust magazine articles: they are written to sell product.

(Edit: switching from a bogger to a high mileage fuel economy tire I could see making a difference - but who wants to run either of those tires on purpose?)

I can't see off-road usage changing much due to tire size... most of it is spent barely tipped into the throttle at all, lots of idling, punctuated by the occasional burst of throttle. Neither aerodynamics or rolling inertia are much of an issue.

I track my mileage via fuelly.com. If there was a difference in mileage it wasn't measurable in the daily grind of driving a vehicle at road speeds on public highways.

If you haven't already, consider opening up the the breathing of the engine. This will help more IME.

 

[reece146]

Consider importing a long ranger auxiliary fuel tank from Oz? I'll probably do this eventually because I hate carrying gas cans. I'd rather have the fuel under the Jeep and out of the way than tripping over the things.

 

[eric1115]

Have you considered a 255/85/R16 like the Cooper Discoverer ST? Much milder tread, and the width makes a bigger difference on highway mileage than the weight. It is a little over 33" in diameter, so a little smaller. How are you geared?

 

[4x4junkie]

I'm really surprised that it didn't make any difference, JP Magazine recently did two articles on this very idea, one was about a lighter tire and rim combo, the other article was on same tire weight but less aggressive tire tread. The tire tread alone made 1/2 mpg difference.

This maybe a dead end, I'm just trying to get more exploring miles out of a tank and trying to plan trips without so many extra fuel cans strapped on the rack.

Thanks,
Fred
Explorer 1

I agree, magazine articles always have to be taken with a grain of salt.
I see it this way: It might take a bit more fuel to get a heavier mass up to speed, but once at that speed, it takes little or nothing additional to keep it rolling at that speed. The added fuel used to get it to speed is infinitesimal compared to the amounts used to overcome wind (aerodynamics) and rolling resistance of the tire.

Raising the air pressure in your tires can help a good amount in lowering rolling resistance (as can tire type), however your aerodynamic profile (combined with the fact most Jeeps aren't the greatest when it comes to MPG to begin with) is going to have a greater effect on your fuel economy. From what I've seen with many friends and their Jeeps, I think the best that can be hoped for is likely to be in the range of 16-18 MPG on the freeway.

 

[Explorer 1]

I don't trust magazine articles: they are written to sell product.

If you haven't already, consider opening up the the breathing of the engine. This will help more IME.

Thanks, Ive already added one of these that are supposed to help with the breathing at higher speeds along with a bored out throttle body.

I'm using a custom K& N canister filter:

Seems there isn't too much confidence in the JP article........maybe with the weight, height and engine mods, what I'm experiencing is just what to expect.

Thanks,
Fred
Explorer 1

 

[reece146]

That roof rack is the equivalent of sticking a big drag chute on your vehicle. Aerodynamic drag increases with the square of velocity - the faster you go the worse it gets.

I'm guessing your Jeep is well over 4000 lbs (as is mine). That doesn't help either.

I'm lucky to get 16-17 mpg currently (down from 22 when it was stock - went to 21 with just the ARB bar alone). I think there is something not quite right with the Jeep as it seems to be running too cool since I replaced the cylinder head (cracked 0331 casting). Also, the Jeep seems to be lugging even though its got metric 32s and 4.56 gears. I just haven't had time to look into it... Made too many changes at once (gears, lift, tires, armour/weight) so not sure what is going on.

My plan is to do a snorkel with a quality paper element, free exhaust with high flow cats, change out the cam... The stock exhaust on our Jeeps is a horror show - particularly on the 00-01s like mine. I want to have more "torque reserve" when highway cruising at 110 km/h so that I don't have to put my foot into it to deal with hills or have so much throttle applied. I'm hoping to achieve a bit of that with these mods. I also want to start watching the PCM in real time and possibly add a piggy back ECU for extra tuning if there is merit. I was going to wait to do a stroker rebuild at the same time (more torque) but the engine still has great compression so there is no point. Thank goodness I have multiple vehicles with the same drivetrain because this is starting to get to be ridiculous amount of fettling. LOL

Getting back to the tires, I've run Truxus MTs (agressive and heavy), Michelin ATX M/S (bald and not as heavy) and the current Duratracs (moderate) all on the same Jeep all in the same size and same wheels and there is no difference in MPGs.

 

[4x4junkie]

That roof rack is the equivalent of sticking a big drag chute on your vehicle. Aerodynamic drag increases with the square of velocity - the faster you go the worse it gets.

×1,000

I had a rack ¼ the profile of yours and it still cost me almost 2 MPG, even empty. If losing it isn't an option, maybe an enclosed pod carrier could work (would still ding your mileage some, but not as bad)

 

[Explorer 1]

That roof rack is the equivalent of sticking a big drag chute on your vehicle. Aerodynamic drag increases with the square of velocity - the faster you go the worse it gets.

I'm guessing your Jeep is well over 4000 lbs (as is mine). That doesn't help either.

Your right I tip the scales over 4000 when fully loaded. I started a weight loss program considering some of the armour among other things. With my stock 1990 4.0 I just really couldn't keep up at freeway speeds, I have 4.88 gears. So out came the 4.0 and in came a Golen Stroker, 24lb injectors and their recommended ignition system. Interesting enough the additional HP and torque actually improved the mpg at the higher speeds by about 2mpg.

Thanks,
Fred
Explorer 1

 

[reece146]

Interesting enough the additional HP and torque actuall improved the mpg at the higher speeds by about 2mph.

I'm not surprised by this and two mpg is a decent increase.

Fwiw, old_man at NAXJA says he gets low 20s with his stroker on 35s and heavy rolling. That's my target. I believe the tires are a secondary (tertiary?) order affector of MPG for our rigs. Keeping a low cruising speed (i.e. 65 mi/h or less) and optimizing the he|| out of everything should get us closer to 20 mpg at cruise. I want to experiment with a removable air dam under the front bar. I've noticed that stock KKs and similar are about the same height as my XJ but have stockish ground clearance. If they can get around 20 then I should be able to as well I figure if I have a similar front profile. The more air that flows under a vehicle the more drag created.

 

[GroupSe7en]

Think of it as rollerskating wearing a parachute. How much difference is a new pair of skates going to make?
The percentage change that you'll get in overall performance, even if you had wheels/tires with zero inertia / rolling resistance would be almost unmeasurable given everything else that's going on.

Like the wise man once said: Putting a suit on a dog, won't improve its table manners.

When an independent think tank did a study of vehicle mods that actually made a difference, they found that reducing the size of the wing mirrors was the thing that had the greatest overall effect. I believe Roger Penske did a similar study - have to look it up. Anyway, that rack you're sporting is pretty big...

Remember also that it will take forever to recoup the cost of expensive mods with tiny decreases in fuel costs.

Maybe you could look at smoothing out the airflow around that rack.

Good luck,
Mark