Effect of tires on mileage?

Hey guys,

It occurred to me that, assuming a common tire size, different tires may still get significantly different mileage from another make/model.

Does anybody know if:
a) The difference is truly significant?
and:
b) Assuming the difference is significant, what are we talking, 10%? 20%? More?

I also understand that putting larger/smaller tires will affect mileage, but that is not my question.

I am just trying to figure out why I seem stuck around 14-15MPG (combined). I have GY Wrangler Silent Armor 225/75/15s. They have a little more aggressive tread, but I cannot think what else could be killing my mileage.

Thanks!

Post the specs on your rig. Crappy mileage on a vehicle with over 100k can normally be traced to an O2 sensor. There are other things as well, but the O2 sensor is the most common.

If it helps, Ive got '93 4.0 HO, 135K on the ticker, 225 tires, and I get at least 17 mixed, 23+ highway. Post your specs for us, we'll try to help!

Andrew

Are your tires properly inflated? I inflate my tires to 35 psi for all on road driving. I check them regularly. 35 PSI makes for a firm ride, but the experts say it helps my mileage.

with my 35s i get 14

Tom-

Will a bad O2 sensor throw a code?

Steve

Rather than a straight failure, the O2 sensor's output starts to drift out of tolerance, it still works, but gives an inacurate reading. In this situation, you get no code.

A service guy at the dealership told me that a bad O2 sensor would even cause the check engine light to go on.

Is that true?

I hate to brag, but I have 176,XXX on the clock for my 97'. 30" BFG AT's and a highflow cat/flowmaster muffler, and I'm adding acetone to my gas. (Read up on it, its working wounders for me)

And I'm getting roughtly 20-23mpg, crazy huh?

Anybody going to answer his original question. It is a good one and something I am curious about as well.
BSD

Realistically, tire size and inflation pressure matter more than what particular tire is on.

And in the case of the original poster, there's more going on than just tires. He's at least 5mpg low.

O2 sensors: 96+ (OBD-II) XJs can throw a code for a bad O2 sensor, there's 2 for that particular purpose. Like Tom (old_man) said, an earlier O2 sensor can run out of tolerance for a long ways without throwing a code.

O.K. I guess I will take a stab on it. I would imagine that with regards to tire type, a less agressive tread with a low rolling resistance rating would help out. The former more than the later. A less agressive, street tread would more efficiently transmit the traction to a smooth driving surface. As for tire size, assuming the focus is on improving mileage where there is more highway driving, a taller and thinner tire would improve it. You have to increase the size whilel reducing the footprint of the tire. Your engine would turn lower rpm's at highway mileage. However, you would feel the impact in the low end. However, I do not know a formula for figuring the improvement and at what point it becomes counter productive. I would echo the sentiment of others to take care of general maintence first. I doubt you would recoup your costs if you bought tires just to improve mileage. The best bet would be to make the switch the next time you need tires.
BSD

Ok sorry, just finally got a chance to check the responses.


Ok. I have '01 4.0 AW4. DIY K&N, DIY IAT relocate to intake tube, Poweraid TBS (yes it does help), Borla cat-back, hollowed primary cat, 180F thermostat, DIY hood vents, GY Wrangler SilentArmor 225/75s on stock aluminum 15s, I keep them at 34-36PSI. Mobile 1 5w30 plus Synergen, synthetic rear diff, Mobil 1 TC plus Synergen, OE tranny & front diff. I typically run between 2 and 6 fluid ounces acetone/10 gallons gas. Experimenting to find Nirvana Around 65k miles. No engine codes, so all the O2s are telling the engine what it wants to hear. Bad bearings in the front; they make a lot of noise, but I can't see them affecting mileage that much.

So the way I figure it, I'm either getting terrible gas constantly (and I tend to stick to Mobil, Sunoco, or Noco, regular unleaded), or just have excessive leadfoot compared to the rest of you. I can't believe that, even as open as my exhaust is, I'd be losing THAT much mileage there.

Oh yeah, I'm in Buffalo, NY.

Thanks!

EDIT: Even now when it's finally starting to get cold (mid to upper 40s at night), I don't smell unburnt hydrocarbons, so I don't think she's running terribly rich, at least not much at ignition. Also, I can see the 180F thermostat decreasing mileage maybe .5-1MPG in hotter months, but definately not now when it's much cooler out.

Best thing for mileage is leaving things alone and running close to stock. We have had 5 cherokees and one wrangler in the family all with the 4.0 and left things pretty much as is minus putting 235 tires on a few over the years. the more you play around with things seems to hurt mileage on the 4.0.

Yeah but how......most of my mods have reduced/minimized parasitic losses.....I'm not saying you're wrong (and wonder if you're totally right), I just need to understand the problem before I accept the solution.

krakhedd said:

Bad bearings in the front; they make a lot of noise, but I can't see them affecting mileage that much.

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Anything that a bearing does that makes noise or heat is wasted energy. I suspect you're losing more than you realize there.

krakhedd said:

180F thermostat

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Why are you running a 180 tstat? This may be part of your problem, if you're continually running in open loop mode.

ChiXJeff said:

Anything that a bearing does that makes noise or heat is wasted energy. I suspect you're losing more than you realize there.


Why are you running a 180 tstat? This may be part of your problem, if you're continually running in open loop mode.

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Bingo! Give that man a prize.

I'm not sure on the t-stat, while it's certianly not helping effiency if it were causing you to stay in open loop I'd think it would throw a code. The bearings however are a HUGE factor. You have to consider that not only do you have the added resistance in the bearing but this will permit lateral runout and "scrubbing" of the front tires thus wasting even more energy. If your bearings are bad enough to make a lot of noise I suspect a close inspection of the tires will reveal some abnormal wear patterns, a sure sign that your wasting a significant amount of your engines power output. As for the original question, I should have been one of the first to jump on this, but I didn't see it until I was on my way to bed last night. The answer is yes, but the difference isn't really all that much. I work in a tire factory and one of our main products is high effiency tires for semi-trailers. By high effiency were talking a 2% fuel savings over a standard tire of the same size. Vehicle maintenance is defineately a much bigger factor. I've run bad bearings before and the difference in fuel consumptions after replacing them was very noticeable.

I think the things I would check would be:

- front end alignment (check toe-in, check steering damper)
- axle alignment (rear axle tracks square with front end)
- tire pressure (correct pressure for your tire)
(not all 235's are the same load rating...)
- check brakes (pads dragging?)
- check bearings (probably means new ones for you)

If everything is where it should be, I wouldn't expect tread pattern to really make that big of a difference. Just buy the best tire for your driving conditions. If you've done all the above and you're still getting low mileage, then I would suspect something on the engine side.

I have a 2000 Cherokee Sport with 255/70R15 Dunlop GT Qualifiers. I can set the cruise on 80 and still get 20-21mpg. If I go -really- easy in-town, I might get 15-16. It doesn't take too much "foot" to get that down to 12-13.

Craig said:

I think the things I would check would be:

- front end alignment (check toe-in, check steering damper)
- axle alignment (rear axle tracks square with front end)
- tire pressure (correct pressure for your tire)
(not all 235's are the same load rating...)
- check brakes (pads dragging?)
- check bearings (probably means new ones for you)

If everything is where it should be, I wouldn't expect tread pattern to really make that big of a difference. Just buy the best tire for your driving conditions. If you've done all the above and you're still getting low mileage, then I would suspect something on the engine side.

I have a 2000 Cherokee Sport with 255/70R15 Dunlop GT Qualifiers. I can set the cruise on 80 and still get 20-21mpg. If I go -really- easy in-town, I might get 15-16. It doesn't take too much "foot" to get that down to 12-13.

Click to expand...

Yeah, I have a REALLY hard time going "easy" in-town, which is primarily where most of my driving occurs, but I don't think I drive anywhere near hard, by any means.

Alignment is good. Tires look fine. I still have a hard time believing that bad front bearings would make a 25-33% difference in mileage, but I don't doubt they affect it maybe 5-10%.

Tire pressure good. I can't really run "street" tires because I can't afford to own 2 sets of tires, and there's enough rain/inclement weather here that I need something that works well in precipitous/snowy/icy conditions.

Brakes I'll check after my next class (10 minutes - shit, gotta go!)

Last time I had the thing aligned, I *believe* the front and rear axles were with .1* of each other. Well within tolerance, anyway. And I haven't run over any rice burners since then :d maybe smoked a few though.....

I wish I had cruise control!

Thanks guys, I hope I get instant email notification of future posts, which I had not past my last post......

Oh yeah, I forgot to mention that the coolant is definately not operating in "open-loop". I have hood vents that help to evacuate a great deal of heat, and even in ambient temps of around 75-80*F, I am able to maintain a constant (assuming I am in constant motion) 180*F on the thermostat. The "open-loop" is certainly not an issue, especially with high 30s this morning.

My main concern, however, is that my tires, basically an evolution of the Wrangler AT/S, may have had too much tread on them, or too aggressive a tread. I know the footprint is ~5-10% more than stock-size tires, and those are the crappy RT/S. 215/75R15 is stock; I have 225/75R15s.