Tire Size vs Gas Mileage Debate

I've been thinking about the debate/concerns that a lot of folks have regarding the loss of gas mikeage when going to a larger tire size, so I sat down and did some math to see if there was a connection..... This model does not account for the actual rolling circumference of each tire. I know that is slightly different than the stated circumference. I have also rounded the numbers.

Here's my thoughts and the math to back it up. I used my 08 DC Sport as my model and The Tire Rack to get the tire specs:

My 08 Sport came with Dunlop AT20's, 265/65R17. This tire has a diameter of 30.6", so the tire circumference is 96.13".

I upgraded to Bridgstone AT's, 265/70R17. This tire has a diameter of 31.9", so the tire circumference is 100.22".

With the larger size, you are covering a slightly longer distance with each rotation of the tire, 4.09" to be exact in this model. So Unless you can recalibrate your speedo, or use a scanguage, using the odometer to calculate your fuel mileage will be incorrect.

Without boring everyone with the speadsheet I did to calculate the tire rotations per mile, let's just say in this model, the larger Bridgestone tire will cover an additional 12.75 miles over the Dunlop tire in 300 miles.

That's in additional 4.2% longer distance than the Dunlop tire.

I just checked my fuel mileage for the first time since getting my new tires, and in an average tank driving to and from work, it was 18.7 mpg (287 miles / 15.3 gallons). The last time I checked it with the Dunlop tires it was 19.5.

So I actually traveled 299 miles given the larger tire size. 287 x 104.2% = 299.05 miles.

Hmmmmmm....... According to my calculations I'm getting exactly the same mpg with my larger tires because I actually traveled 4.2% longer distance. 287 odometer miles x 104.2% = 299.05 miles / 15.3 gallons = 19.54 mpg.

If there are any flaws in my math here please let me know.

I know many folks here say there's additional rotational weight associated with larger tires that will hamper mpg, but with this small tire size increase I don't believe it's an issue.

 

I can't see adding another 30lbs of tire killing your MPG that much. A couple hundred sure, but the truck can haul 6500lbs. While any parasitic weight would be detrimental to mpg, I see it the equivalent of wearing a watch and carrying my wallet around with me.

 

I agree that it's important to account for speedo inaccuracy when having this conversation. A simpler shortcut might just be to use a GPS.

I suspect a much larger factor in fuel economy than tire diameter is tread pattern. Rolling resistance is related to the tire surface in ways I don't entirely understand, although you can kind of summarize to the point that smooth tires at higher pressure will roll better than low pressure knobbie tires.

In a general sense I think the conversation about tire size vs mileage is probably confused by the fact that people who are putting on large tires typically are going to put on an all terrain or offroad tire, which is probably designed to flex and grip to a degree that you really don't need on flat pavement.

 

I'm sure the rolling resistance of a large M&S tire would effect the mpg, but there seems to be a lot of discussion on this board about loosing mpg when only going from a road tire to an AT tire that is slightly larger tire than stock.

 

Tire - 265/65R17 >Search Tires Tire 1 - 265/70R17 >Search Tires
Section Width: 10.43 in 265 mm
Section Width: 10.43 in 265 mm

Rim Diameter: 17 in 431.8 mm
Rim Diameter: 17 in 431.8 mm

Rim Width Range: 7.5 - 9 in
Rim Width Range: 7 - 9 in

Overall Diameter: 30.56 in 776.22 mm
Overall Diameter: 31.60 in 802.64 mm

Sidewall Height: 6.78 in 172.21 mm
Sidewall Height: 7.30 in 185.42 mm

Radius: 15.28 in 388.11 mm
Radius: 15.80 in 401.32 mm

Circumference: 96.00 in 2438.4 mm
Circumference: 99.27 in 2521.4 mm

Revs per Mile: 680.6
Revs per Mile: 658.2

Actual Speed: 60 mph 100 km/h
Speedometer1: 58.0 mph 96.7 km/h

Speedometer Difference: - Speedometer Difference: 3.403% too slow
Diameter Difference: - Diameter Difference: 3.3%

 

Adding unsprung weight, such as tires, makes a much bigger difference than it seems some of you are giving it credit for.

 

I agree, but I'm only talking about going from a 265/65 to a 265/70 tire. Not really that much added weight.

 

Talk about thread revival!

This is the closest thread I can find related to what I'm wondering.

What makes a larger difference in MPG, Diameter or Weight?

I recently went from a 265/65/17 (Dunlop) to a 265/70/17 All Terrain. The all terrain that I put on is approx. 10lbs heavier than the stock dunlop. I huge oversight on my part, I know. I'm debating on whether to put 265/65 back on, or a lighter 265/70.

Everyone that I've asked has had no intelligent response. I would assume weight, but it seems like some folks believe diameter would have the larger impact.

Thoughts?

 

Rotational mass. The leverage it takes to spin up a larger diameter tire is greater, in my opinion, then the effect of weight on milage. More weight, will affect the unsprung weight aspect of your suspension action, more mass to dampen, and control with spring rate. The change in gear ratio with a larger tire is decreasing the revs at speed, making it harder for the engine to keep a certain speed or accelerate.

 

Great thread! The technical post and responses have me thinking about my new truck tires the same way I did about my old Dakota 2wd one’s... stick with the factory size and get a good agressive interlocking tread.

Thanks!
~Jes

 

Great thread! Thank you

 

Thread revival..

I know how to math and I still lost 20-25% of MPG per tank going from P245s to P265s. Good info here if someone hasn’t figured this out already, but definitely doesn’t refute mileage drop.

 

I am getting ready to change my factory tires to LT235/85R 16‘s. Give me your thoughts on that? The factory tires are 265/7016‘s but they are P rated. The 235s will be narrower and 1 inch taller I spend a lot of time off road.

 

You should have minimal differences, possibly gains but depends on a lot of other factors.

I went from P265s to P255s and had little change in economy, seemed to be a little peppier too as far as drivability was concerned.

 

That is what I am hoping for. I just started a thread under tires and wheels to get some perspective. This is my first forum to ever be a part of.

I guess I am in for a learning curve.

 

Welcome to TW.

 

Thank you

 

Done! See pics