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Unread 02-10-2014, 07:27 PM   #1
mschi772
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Choosing Tire Size for MPG/Economy

Facts:
  • Larger tires will increase distance per revolution and reduce RPM which in turn reduces gas consumed while cruising to a point--too few RPM is bad but irrelevant due to my maximum tire size choice not being excessively large.
  • Larger tires are heavier and harder to accelerate which hurts "city" mileage.
  • Larger tires are often wider with increased drag.
  • Smaller tires are lighter which eases acceleration and improves "city" mileage.
  • Smaller tires are often narrower and generate less drag which helps all around.

Other relevant info:
  • 4.0 engine
  • AW4 transmission
  • 231 transfer case
  • 3.55 gear ratio
  • 15x7" wheels

I'm pondering the pros/cons of all the tire sizes I could potentially use ranging from 215/75r15 (hell, 205/75 is even technically possible) up to 30x9.5. Please do not let aesthetics or peer pressure (all the "cool" Jeepers use big tires) play a large role in your advice to me. I know that more accurately describing the type of driving I do would be enormously helpful here, and as soon as I know how to accurately define it, I will, but that is a separate problem I've been contemplating: where should one draw the line between what constitutes "city" and "highway" driving?

If my driving was clearly focused on one type or the other, this would be a fairly easy choice, but my driving is very mixed. I'm not really sure the extent/magnitude to which all of the above factors play a role and am hoping to find a balance that is right for me.

When addressing this question, please note that no matter what size I choose, my speedo/odo will be calibrated to be accurate. Also note that my offroading is uncommon and limited to point A to B means-to-an-end offroading which can be accomplished by any potential tire size.

For the purposes of this discussion, assume that my Jeep will remain and stock height and that I won't use a tire larger than 30x9.5. Also for the purposes of this discussion, assume that I will be using AT tires.

If I could just try-out every tire size for awhile and collect MPG data to compare, I would, but that would be quite expensive.


EDIT: To describe the majority of my driving to the best of my ability...

Very rarely stop-and-go traffic. In-the-city driving at low speeds with lots of stops and turns is fairly uncommon. Around town (well-regulated traffic in 25-40 mph zones) is fairly common. Country roads ranging from 35-55 mph sparsely populated by lights or stop signs is very common. Highway cruising at 55+ mph is very common.

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Unread 02-10-2014, 07:57 PM   #2
ECL
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If your objective is better gas mileage and off-roading really isn't an issue, then lower profile tires (lower aspect ratio - 70,65...) should also help your aerodynamics by effectively reducing your frontal area. They should also improve handling as well.
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Unread 02-10-2014, 08:01 PM   #3
jimk403
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My current dd XJ is a '90 Sport 4.0, 5sp, 3.07 gears, Dynomax exhaust, stock intake, no roof rack, no winch, no driving lights. Even though it has 3" lift, it only has 235/75-15 Dunlop Rover AT tires. Front are at 30psi, rear at 28. I average about 20 mpg on the highway. I don't drive in the city much so no info there. I live at 3045ft elevation and drive to San Diego which is mostly sea level. So I drive down and up a 6% grade regularly.
Just food for thought.
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Unread 02-10-2014, 08:43 PM   #4
mschi772
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Quote:
Originally Posted by ECL View Post
If your objective is better gas mileage and off-roading really isn't an issue, then lower profile tires (lower aspect ratio - 70,65...) should also help your aerodynamics by effectively reducing your frontal area. They should also improve handling as well.
Smaller diameter tires have equally smaller sidewall heights as well as being narrower. I know intimately what the various numbers in a tire's size stand for, but a 70 or 65 profile doesn't really do much considering all of my options other than, usually, increase price and reduce availability.

No sense in me getting a 235/70 (for example) when I could pay less for a 225/75 and have a narrower tire of roughly the same height. The only example I can think of that would be beneficial to my particular set of requirements would be if I specifically wanted a standard load P 27-28" General Grabber AT2 since 225/75 seems to be rare and/or discontinued and 215/75 is a heavier LT load C. Since General AT2 is a contender, I suppose I should thank you for making me take notice of that little pricing/availability anomaly.

I know the aerodynamic benefits of smaller tires is there, but anecdotally, it seems that increased diameter benefits highway MPG more than aerodynamics would (since a larger diameter is obligated to have worse aerodynamics). At some point the increased drag (wind and road) as well as weight must surely outweigh otherwise guys on 35's would get fantastic highway MPG. I'm not sure what that point is, but if I had to guess, I'd guess that once you get much larger than 235 or 30's (which I can't), the diameter increases stop helping highway MPG as much due to the increasing drag and weight.



To describe the majority of my driving to the best of my ability...

Very rarely stop-and-go traffic. In-the-city driving at low speeds with lots of stops and turns is fairly uncommon. Around town (well-regulated traffic in 25-40 mph zones) is fairly common. Country roads ranging from 35-55 mph sparsely populated by lights or stop signs is very common. Highway cruising at 55+ mph is very common.
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Unread 02-10-2014, 09:43 PM   #5
ltrail
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One question to ask, if the overall diameter is to grow will the torque output and therefore economy to get up to speed negate the fact that once you are up to speed you'll turn a few less rpm?
Second thought about this is how will you monitor such as the tire diameter difference will slightly skew any measures since you are moving faster than speedometer reads. The odometer will also lose a bit of accuracy.
Curious as I know my economy is worse than once before since im running a stroker and 32's on my dd with stock gears. Normally I'd gear to match tire size for an overall factory ratio. axles are in the cards so not gearing the stock stuff.
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Unread 02-11-2014, 04:48 AM   #6
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Quote:
Originally Posted by ltrail View Post
One question to ask, if the overall diameter is to grow will the torque output and therefore economy to get up to speed negate the fact that once you are up to speed you'll turn a few less rpm?
In "city" driving, yes. "Highway" cruising has relatively little acceleration, so the benefits of the size prevail over the downsides of the weight and reduced power. Where "city" driving ends and "highway" driving begins is something I'm finding to be a tricky thing to define.

Quote:
Originally Posted by ltrail View Post
Second thought about this is how will you monitor such as the tire diameter difference will slightly skew any measures since you are moving faster than speedometer reads. The odometer will also lose a bit of accuracy.
Well, I did write in my OP that it should be assumed that I won't be neglecting speedometer calibration, so MPG calculations will be accurate. If you change tire size without recalibrating your speedo/odo, your MPG calculations WILL be wrong unless you compensate for the differing tire size mathematically or calibrate your speedo/odo. On YJ, TJ, XJ, ZJ, and I think WJ, this is simply a matter of loosening a little bolt and popping a gear off of the speedometer sensor in the transfer case and popping a correct one in (see here http://go.jeep-xj.info/HowtoSpeedoGears.htm). On newer models, it requires a computer reprogram--still pretty easy and can be done by a dealership quickly and cheaply.
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Unread 02-11-2014, 12:49 PM   #7
Charley3
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On my 96 XJ 235/75R15 all season tires gave me better gas mileage than stock 225/75R15 all season tires.

So in my experience 235 are as good as it gets for gas mileage.

I think 30x9.5R15 might be as good for gas mileage as 235, if the same all season tread, but that's only a guess because my 30s have all been AT.

The cool thing is 235 are decent off road, and 30 are good off road. So it's a free lunch.

BTW - I'm referring to highway gas mileage.

All season tires aren't as "good" in mud as all terrain tires, but AS are quite good on snow and ice IME. AS are as good on snow as AT IME. AS are BETTER on ice IME. Other than mud, AS tires do fine offroad.

Also, AS tires hold up fine offroad if they are load C tires. I did a lot of successful wheeling woth AS load C tires. They did great as long as I avoided mud.
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Unread 02-11-2014, 02:46 PM   #8
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I'm talking strictly about the pros/cons of size. Load rating, tread compound, and life are separate issues. For what it's worth, I'd be happy with P or C and don't need to care much about tread life since they'll almost surely get too old before they wear out anyway. I'll be using a 5-tire rotation and likely have a set of snow tires for winter.

How much time do you spend engaged in pure "city" driving? While I'm open to it, I'm very cautious about believing that there wouldn't be a "city" downside to 235 or 30. Also, was the comparison between smaller stock and 235 the same tire or a different model? I don't want advice on tire models, but I'm curious because if you switched models when you switched sizes, that would confound the comparison with extra variables.

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Unread 02-11-2014, 04:32 PM   #9
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I've been having a hard time wrapping my head around the fact that everywhere I look seems to put the 4.0's greatest efficiency at about 2100-2500 RPM. I'm already cruising on the highway at about 2050ish; 235's would put me about about 2000ish. I know that's not a HUGE difference, but I've heard a lot of how 235's and 30's improve highway MPG. That seems to contradict what I keep reading about the engine, though, by dropping engine speed below the ideal. Any help here?

Yep, I'm splitting hairs. That's what this thread is for.

Aerodynamic drag is an exponential function of velocity multiplied by some constants. The drag coefficient of an XJ is very large (0.52 vs most cars being 0.3-0.4 and a Prius's 0.25), so speed really is an enemy and achieving good MPG at high speeds may be a futile endeavor without significant aerodynamic improvements. People love to cite their "highway" MPG because it is the larger, more impressive number, but maybe it would actually be best to try to improve MPG at lower speeds and "city" type driving where aerodynamic drag is less brutal? I wonder if I could gain more MPG there than I'd lose on the highway netting an overall gain... Probably if I "city" drove enough, but do I? Questions questions...
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Unread 02-11-2014, 04:38 PM   #10
Charley3
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Quote:
Originally Posted by mschi772 View Post
I'm talking strictly about the pros/cons of size. Load rating, tread compound, and life are separate issues. For what it's worth, I'd be happy with P or C and don't need to care much about tread life since they'll almost surely get too old before they wear out anyway. I'll be using a 5-tire rotation and likely have a set of snow tires for winter.

How much time do you spend engaged in pure "city" driving? While I'm open to it, I'm very cautious about believing that there wouldn't be a "city" downside to 235 or 30. Also, was the comparison between smaller stock and 235 the same tire or a different model? I don't want advice on tire models, but I'm curious because if you switched models when you switched sizes, that would confound the comparison with extra variables.

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I was thinking of and referring to highway gas mileage.

However, I don't think there is a city downside to 235.

There probably is a slight city downside to 30s, but can't be much. My 30s are getting good enough mpg in city.

I was comparing all season 225 P rated to all season 235 C rated (cause those arexwhat I had). Different brands and load ratings, but similar tread.
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Unread 02-11-2014, 04:50 PM   #11
Charley3
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I think best efficiency is from 2000 to 2200.

Still good up to 2300 rpm.

OK at 2400 rpm.

Starting to guzzle gas at 2500 rpm.

Really guzzling at 2600 rpm and above.

I know that from personal experience with 3 different diff gearings I had on an LJ.

Keep in mind a lot of Internet info about this stuff is posted by guys with giant tires tryjng to convince themselves that gearing for 2500 rpm is efficient for their giant tires. Maybe they're correct for giant tires. Maybe that's the only way to turn giant tires without lugging the engine. Maybe that is the least inefficient way to turn 35 & 37" tires. Maybe it's the only way a 4L can deal with giant tires.

But that in no way means 2400 or 2500 rpm is efficient for smaller size tires. IMO
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Unread 02-11-2014, 08:46 PM   #12
ltrail
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reason I was interested in this is my stroker is only about 400 miles into the game. I am amassing you will be tested off regular 87ish octane fuel? I've always hear(never fully researched) that higher grade results in better economy. I can't say either way. I've run this premium since the beginning and after about 100mi more I'll hook up wideband and see how fuel does for me. I hear stories of pinging and don't like it.

Carry on!
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Unread 02-11-2014, 09:11 PM   #13
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My RPM numbers are not just guesses from random hopeful Jeepers. They're based on real math involving most efficient piston speed and volumetric effciency. I have little doubt that the engine is most efficient within that RPM range, but maybe the rest of the drivetrain, air/fuel delivery, or other real-world factors modify the theoretical ideal into a realistic, practical ideal which is slightly lower? Maybe the assumption of what is the "ideal" piston speed for an engine is wrong--that's a piece of data I just sort of had to roughly fact-check and take for granted since I don't have have the expertise to know something like that on my own.

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Unread 02-11-2014, 09:16 PM   #14
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Quote:
Originally Posted by ltrail
reason I was interested in this is my stroker is only about 400 miles into the game. I am amassing you will be tested off regular 87ish octane fuel? I've always hear(never fully researched) that higher grade results in better economy. I can't say either way. I've run this premium since the beginning and after about 100mi more I'll hook up wideband and see how fuel does for me. I hear stories of pinging and don't like it. Carry on!
This is off-topic, but the short response is that higher octane fuel can yield better performance ONLY if your engine is tuned for it (higher compression ratio and/or higher operating temperature). I'm frankly uninterested in going into detail on this since it is not the topic of this thread and because I'm still burnt-out after recently battling ignorance regarding octane ratings in another thread with no luck. Some people will just believe what they want to whether it's right or not.

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Unread 02-12-2014, 04:56 AM   #15
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Quote:
Originally Posted by Charley3 View Post
Now regarding rpm. A bit off topic if you're a stickler, but you brought up rpm. I know what rpm have been most efficient for me. I can't explain why, but they have been.
I don't think RPM is off-topic at all. Without a gear change, shifts in tire size directly impact it which is quite relevant to efficiency. I believe you when you say that's what has been most efficient for you, but its conflict with what should be true of the engine alone compels me to wonder why and wonder what else is at work to make the reality what it is.

Regearing...probably choosing the smallest tire size and regearing to like 3.07 or something would get great results since Jeep probably compromised a bit on the power/efficiency front when choosing the stock gear ratio, but regearing isn't something I'm interested in doing. Plus that would just evade the question of tire size's effects with my current setup without really answering it.

I'm a stickler about staying on topic because while you create rather broad topics that allow for a bit more intellectual wandering, I've created a very specific one. I don't need this thread to grow to enormous sizes to consider it productive/constructive.
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