Tire width/diameter and the MPG impact?

[Ed_Flecko]

It's tire replacement time.

My daily driver is a 2004 Toyota Tacoma 2wd Xtra Cab, and my current tire size that I'm running is 215/75-15. While I like the look of the tire because it's just a hair wider and taller ( https://www.tacomaworld.com/tirecalc?tires=215-75r15-205-75r15 ) than the OEM (205/75-15) size, I'm wondering which tire might actually be the best choice in terms of MPG?

Because the 215/75-15 is taller, that would actually raise the overall gear ratio (right?), but the 205/75-15 being narrower should (in theory, right?) have a lower rolling resistance. I tend to lean more toward the 205/75-15 only because I'm one of those people that thinks it's generally best to stick with what the engineers who designed the vehicle recommend for overall best performance, MPG, etc., etc., etc.

Either way, I'm sure I might be splitting hairs, but I'd be curious to hear your opinions?

Thank you,
Ed

 

[gathermewool]

One size up in diameter is cheaper for my Civic, so I went with it. I've noticed on one hill that I now up-shift twice instead of once due to the SUBSTANTIALLY lower final drive ratio. By 'substantially' I mean 2-3%, but still, I'd swear that the AT is shifting more frequently than before.

On the highway, I'm sure I'm getting slightly better gas mileage, but I'd bet it's inconsequential. Again, I went for the cheaper tire with minimal impact to form and function (3% change in speedo and no rubbing or fitment issues at all.)

 

[d00df00d]

If either tire makes a difference, I'm sure it won't be huge. I'd pick based on other factors, honestly.


However, if you're interested in the theoretical side, a few points:

- Yes, all else equal, a narrower tire should have lower rolling resistance.

- "All else equal" is the important part here. If you're looking at two different tire models, it's unlikely that all else is equal.

- The larger tire may also be heavier, and either way its mass will be farther from the wheel hub. So, it might give you better MPG when cruising, but worse MPG when accelerating.

 

[userfriendly]

I always go taller on a given wheel before wider to minimize side-wall flex. You can't do that because you already have 75 series tires.
The roll-out of your tires is 85.1 inches (27.1" od).

To save fuel, I believe that 16s on a wider rim would pay for themselves over the life of the tire.

The 16" equivalent to your 15s is 205/70/16 27.3" od, roll out 85.7" or 99.3% of your present set up.

Your truck would look even better with 17s, the tire 205/65/17 od 27.5" roll out of 86.3" or 98.6% of what you have now.

Wheels on Flea Bay 17X7, perhaps $100.00 or free.

 

[JHZR2]

One needs to also be sure that the speedometer/odometer is accurate. If not, any calculations and assessments will be off.

 

[901Memphis]

I use the 215/70/15 on my Buick Cuz it looks better imho. The oem size is 205/70/15 and look too "skinny" to me. I think my mileage is a little worse but it's worth it to me. Speedo is off by a couple mph not much. I checked it ob2 vs gps vs Speedo reading as well as the ones on the interstate that show you your speed.

 

[901Memphis]

Here's a nice calculator with lots of information


https://www.tacomaworld.com/tirecalc?tires=205-70r15-215-70r15

 

[HerrStig]

Originally Posted By: d00df00d
If either tire makes a difference, I'm sure it won't be huge. I'd pick based on other factors, honestly.


However, if you're interested in the theoretical side, a few points:

- Yes, all else equal, a narrower tire should have lower rolling resistance.

- "All else equal" is the important part here. If you're looking at two different tire models, it's unlikely that all else is equal.

- The larger tire may also be heavier, and either way its mass will be farther from the wheel hub. So, it might give you better MPG when cruising, but worse MPG when accelerating.
And degrade braking slightly.

 

[gathermewool]

Originally Posted By: JHZR2
One needs to also be sure that the speedometer/odometer is accurate. If not, any calculations and assessments will be off.


Right, at higher speeds I need to keep in mind the 3% difference if I'm driving more than 3-5 MPH over. A cop may pull me over at an actual speed 72 MPH, even though I'm only indicating 5 MPH over the speed limit.

I also multiply my MPG calc by 1.03 to get an accurate MPG figure.

 

[CapriRacer]

Originally Posted By: Ed_Flecko
It's tire replacement time.

My daily driver is a 2004 Toyota Tacoma 2wd Xtra Cab, and my current tire size that I'm running is 215/75-15. While I like the look of the tire because it's just a hair wider and taller ( https://www.tacomaworld.com/tirecalc?tires=215-75r15-205-75r15 ) than the OEM (205/75-15) size, I'm wondering which tire might actually be the best choice in terms of MPG?

Because the 215/75-15 is taller, that would actually raise the overall gear ratio (right?), but the 205/75-15 being narrower should (in theory, right?) have a lower rolling resistance. I tend to lean more toward the 205/75-15 only because I'm one of those people that thinks it's generally best to stick with what the engineers who designed the vehicle recommend for overall best performance, MPG, etc., etc., etc.

Either way, I'm sure I might be splitting hairs, but I'd be curious to hear your opinions?

Thank you,
Ed



First, you should read this:

Barry's Tire Tech - Rolling Resistance 2

It is a followup on an earlier discussion of tire rolling resistance.

Short version:

1) There can be as much as a 60% difference in RR between tires of the same size.

2) The difference in RR between tires of a slightly different size, but otherwise identical, is single digit percentages.

Ergo, you should spend your time looking at the make and model of tire - and not worry about tire size. Please note: I am discussing RR and not the effect a different diameter has on engine performance.

3) There is a formula I derived that says if you are going to change tire size, then rim diameter has the biggest effect on RR, while width and aspect ratio have smaller, but have nearly the same effect.

 

[ryanschillinger]

When I replaced the regular passenger car P225/70-15 tires on my old Frontier with larger P235/75-15 Goodyears with a more "truck" style tread, my mileage really went down. Same with my old CR-V, which went from 205/70-15 to 215/75-15. They definitely look better, and performed better in the dirt and sand, but the cost in MPG was very noticeable. Yes, I adjusted the math for the difference in odometer readings. If you want to keep good gas mileage, I would pick something like a Michelin Defender, or something with proven lower rolling resistance unless you actually go off road.