Why do bigger tires cause worse mileage?

[HuntRunner]

We have all discussed the pros and cons to upsizing tires and one con being that upsizing can decrease fuel mileage. I am not sure this makes total sense though.....If upsizing changes the effective gear ratio to a lower number, causing lower rpm's at a given speed; then shouldn't that offset the extra weight or what ever it is that is causing the mpg decrease?
In theory, if I changed my gear ratio to a lower number I should get better mileage on the highway???
I am not arguing this, just trying to make sense out of it. Maybe some of you engineers can help explain this?

 

[JSB]

Well when I'm doing 65mph on the highway, my rpms stay around 2800 rpms (which sucks). I have to push up to 75 or drop down to like 55 for them to drop to around 2000 rpms. I run 32's.

 

[boston_kevin]

extra weight = more fuel used

Even if you change the gearing so that the engine is operating in the proper torque band to maintain the actual speed/mileage, the amount of fuel you burn to get to that point increase because you are pushing more weight. At speed, you are pushing more wind resistance because of the higher profile, so it takes more fuel to maintain speeds.

RPM speed does not always match the fuel being burned. 2000 RPMs in first gear should burn more fuel than 2000 RPMs in fifth gear because of increased rolling resistance.

There are formulas that most physicists can go into, but suffice to say if you are pushing more weight and more wind, the engine has to maintain those RPMs, and it can't do it without an increased burn rate.

 

[KMX3060]

Aside from the additional weight from the larger tires, the problem is with the profile they present for wind resistance.

 

[Koz]

When you change the tire diameter you also change the final gear ratio. The trans and rear gear are calulated for a specific size tire when you change to larger diameter tire the engine will work harder using more gas.

Koz

 

[bulldog-yota]

The things I can think of:
- Bigger tires = bigger footprint = more rolling resistance (Also people tend to got to AT or MT tires which have more resistance to street tires due to thread pattern)
- More rotational weight, takes more energy to get up to speed. This makes a big difference especially in town driving. Also affect energy to stop, affecting brake pad life.
- Lastly bigger tires makes the vehicle higher and is physically bigger, thus adding to wind resistance.
- Also remember with bigger tires your odo will read slightly less, thus your indicated mileage goes down from that. I found that going from 265/65/17 street to 265/70/17 AT your odo will underread by 3-5%, thus you will actually see less miles than what you have driven.

The V6 with the 4 spd is affected the worst by bigger tires, due to gearing and torque. The V8 is affected the least. I have no issues with shifting patterns on my V8 with bigger AT tires. This might come in with even bigger MT tires.

Normally regearing makes a big difference in performance and mileage if bigger tires are fitted (33" +).

 

[HuntRunner]

I have no issues with shifting patterns on my V8 with bigger AT tires.

Bulldog,
Do you mean that you do not notice more downshifting because of the larger tires? Your truck does not shift out of overdrive more often with the 70's?

When I did the math on this, the v8 effective gear ratio went from 3.73 to a 3.60 (going from a 65 to a 70 tire). I guess I thought that might help offset the other factors that decrease in mpg.

 

[Koz]

65s have a bigger footprint then 70s.

Koz

 

[HuntRunner]

65s have a bigger footprint then 70s.

Koz

Yes, but I can't see going from 10.5 width to 10.6 width (Revos) making that much of a difference. I guess every little bit can count at 75 mph.

 

[00328]

Keep in mind also, that sometimes going to taller tire may require going to a higher octane fuel.
I had a Jeep GC that I changed from 15" tires and wheels to 16" and the engine pinged like crazy. Went to a higher octane and all was good again.

 

[Northof49]

Keep in mind also, that sometimes going to taller tire may require going to a higher octane fuel.

I use 87oct pulling a 4500lbs boat in the middle of summer with 0 pinging......unless you plan on running Bigfoot sized tires on a 4rth gen, I highly doubt this will be a problem.

 

[Cleric]

I use 87oct pulling a 4500lbs boat in the middle of summer with 0 pinging......unless you plan on running Bigfoot sized tires on a 4rth gen, I highly doubt this will be a problem.

Not to start up this debate again, but I thought that our ecu would automatically retard the timing. This means that you could be affecting your performance and not know it because you won't hear any pinging.

Is that incorrect?

 

[Northof49]

Not to start up this debate again, but I thought that our ecu would automatically retard the timing. This means that you could be affecting your performance and not know it because you won't hear any pinging.

Is that incorrect?

That is 100% correct......but since I have not noticed decreased performance and don't drag race towing a boat, the power is more than sufficient, even if some timing is being pulled.

From all of the fuel economy threads on this board, it seems that barely anyone is using all 270 horses (or whichever engine they have) very often anyways.....so why throw away money on premium fuel. The ECU will keep you safe on 87 octane 100% of the time.

 

[Cleric]

I get your point. I also agree that it is very unlikely that increasing your tire size would be enough of a strain on our engine to cuase this.

I could, however, believe that this may happen with some vehicles.

 

[bulldog-yota]

Bulldog,
Do you mean that you do not notice more downshifting because of the larger tires? Your truck does not shift out of overdrive more often with the 70's?

When I did the math on this, the v8 effective gear ratio went from 3.73 to a 3.60 (going from a 65 to a 70 tire). I guess I thought that might help offset the other factors that decrease in mpg.

I have not noticed any difference in shifting patterns or power. Obviously if it was run against a stop watch there will be a difference.

Footprint is more than just the width of the tire. Diameter plays a very important role in foot print.

 

[Koz]

I have not noticed any difference in shifting patterns or power. Obviously if it was run against a stop watch there will be a difference.

Footprint is more than just the width of the tire. Diameter plays a very important role in foot print.

Let me clearify; the 65s have wider/more tread contact on the road.

Koz

 

[bulldog-yota]

Let me clearify; the 65s have wider/more tread contact on the road.

Koz

Not really. Footprint is very dependant on diameter of the tire as well, not just width.

 

[HuntRunner]

Not really. Footprint is very dependant on diameter of the tire as well, not just width.

This makes sense......That is why those big combine/tractor tires are so tall. On the big ones, every rotation of the tire is grabbing so much ground that it makes it very difficult to get stuck.
Is this what you are referring to?

 

[boston_kevin]

Footprint of the tire is the width and length of the tread touching the ground. Since tires are not perfectly round when loaded and on the truck, the contact patch (or footprint) has a length associated with it as well. Ever see a NHRA race in frame-by-frame? The take off for those big tires depends on a large contact patch to launch that car out of the hole and the sidewall actually buckles slightly due to the increased friction of the larger contact patch against the pavement. Same thing applies here (without the hole shot and sidewall buckling).... Perfectly round tires would have a contact patch of an inch or so, but since these are loaded and not perfectly round, stock tires would have a contact patch of maybe 6 inches... Larger tires, maybe 8 inches or more?

 

[Koz]

Not really. Footprint is very dependant on diameter of the tire as well, not just width.

Why are wider tires put on to improve traction and handling? The only advantage the 70 series tires have over 65s is ground clearence and price, PERIOD. The height difference is less then 5/8". Unless you off-road frequently the 65s are better for your 4Runner.

Koz