Tire size vs. fuel mileage

[Jtrux]

I'm in the process of getting my MJ ready for DD duties and since the tires are needing replacement, I'm trying to determine which direction to go with them.  

 

While not everyone's focus, I do want to try and get somewhat reasonable mileage since this will be driven 50 miles round trip and 300 miles round trip to the beach and back when I go fishing so I'm leaning towards keeping the tire size small.

 

The size I have been considering is 215/75-15.  The size I see everyone running is 235/75-15.  My truck has a 4 cyl/5 speed and it is 4wd.

 

Has anyone gone from the small size I mentioned to a 235 and if so, what was the difference, if any, in mileage?

 

I have no problem going with 235's but if I can get better mileage with the 215's I'll go that route.

 

 

[Marine1Texas]

you would have to do some math here. Hills, speed and highways vs city.

If you put on 235's you increase 1.18 inchs per turn. At 65 mph your really doing 67.77.

So you will be increasing your top end gear and lowering your RPMs. So basically if you hit a hill you will notice it drops faster then normal going up the hill.

I my self don't have the 4 cyl, I have the 4.0. When I put the 235's on my stock mj that came with 225's, I noticed no difference. however from 215 to 235 I think you would notice.

If your not off-roading and your driving I would stick with the size that it came with for mpg. more to factor in is your 4wd, even in 2wd mode you require more power to go the same speed as a 2wd setup, more weight and a extra gear box the transfer case to turn.

So only in a pure math if your wanting mpg same size tires as factory your get best mpg.

[johnj92131]

Stay with the 215/75 x 15 tires.  Keep the tire pressure up. Hold your speed down to 60/65.  You will get the best mpg that way.

[benjy_26]

You can also pick up somr efficiency by going to lighter wheels if you're using steelies now.

[Megadan]

Wheel/tire weight will factor in very little for constant speed fuel econ, but will greatly impact acceleration (city driving).  Larger diameter tires have a greater moment of inertia, which means they will also take more energy to accelerate.  That's why cars designed for fuel econ use small diameter light weight wheels and tires (along with low resistance narrow tires).

As a general rule, the larger tires will cause your engine to turn fewer RPM's at the same speed.  In a perfect world this would equal better fuel economy, but if you drop out of your engines ideal operating zone and start to lug on the engine to get up hills or accelerate you can end up using more fuel.  Calculated out on my truck, a 215/75-15 and 235/75-15 and 4.10 gears only constitutes a 100rpm drop at 60mph (2400rpm for 215's and 2300 for 235).  I can't imagine this would cause the engine to really fall out of it's happy place going down the road, but I have never driven a 4cyl to know.

If you want to see the effects of different tire sizes you can mess around with this.
http://fitbodyshop.net/gear/index.php