tire diameter

[1.5Ldave]

Originally Posted by guitarterry

This question is kind of for metrompg but would like anybodys input. Has anybody checked out the difference in fuel economy in say the 12, 13 and 14, maybe even bigger ties all at the SAME RPM, for example, 12 inch tire at 3000 rpm and 55 mph versus 14 in tire at 3000 rpm and 55 mph, especially since the 12 inch should be alot lighter and narrower, even less sidewall to flex. ( assuming we arnt going into expensive low profiles.)

The 12 inch would have the most sidewall if your keeping tires of the same diameter for each rim size. The weight of the rim depends largely on the material and style. If the engine is going the same rpm with all of them then there should be a very small difference between their efficiencies. The biggest reason to change tires to a taller size is the gearing change it provides. The reason for skinnier tires is to reduce frontal area.

If your not changing the tires to reduce frontal area or improve gearing, then there is no point in the change except for looks, which obviously dont help with FE.

I think this is very informational. I drive a metro cause i put alot of over the road miles on my car. so the RRC figures according to yourself dont affect me as much. I have yet to have one person come up with a reason the 12's wouldnt be the best. This could be a good opportunity. it cost 1k to make one gear cause of r&d. what if somebody made 100 top gears. instead of using a tranny with a bad first gear to get a good top gear, drop in a new top gear and get the best of all worlds.

People have given reasons for the 12s not being the best, you just choose not to listen. The biggest reasons are availability since virtually no vehicles run them anymore and lack of choices when you do find them. No demand means no products.

Same reason no one is going to R&D and mass produce a new top gear for the metro. Its not worth the investment if its going to take the rest of your life to sell the 100 gears you had made. You could probly get a 1 off made, but like others have said, thats not cheap.

wow so u know the 14 inch out performs the 13 or 12. even if the engine effiencies are taken OUT of the equation thru gearing. (please, why are u going on a gearing tangent, the idea is the engine will be turning the same.) I thought u just said that aero was more important than LRR at speeds over 20 mph. Plus as right of right now i dont know the LRR of the 12 or 13.

Ok, in case you havent realized it yet, tires ARE gears. If you change the tires you change the gears, get it? If the engine is turning the same speed, the only changes will be marginally better for a skinnier tire (like the 145/82/12) or a LRR tire, like the bridgestone mentioned above. I would venture to guess that the wider 14" tire would perform almost identical to the skinnier 12 inch tire, since it makes up for its RR with less frontal area and vice versa for the 14.

[CapriRacer]

May be it is time I step into this discussion:

RRC: By definition it is the rolling resistance divided by the load. However, when it is quoted as a value, RRC means the value obtained in a test divided by the test load. Typically, the test load is the rated load (the one written on the sidewall) - which is hardly ever the load the tire sees in service.

BUT..... you can't determine the RR by taking RRC and multiplying by the actual load on the tire. RR is not proportional to the load, and while it is also not linear, it is pretty close. (I'll have to look up some typical response surfaces and post them later if folks are interested.)

This brings us back to the tire size problem. If a particular tire has a particular RRC, if you were to measure the same identical tire in a different size, you would find it has a different RRC. In general, the RRC value is most closely related to the load carrying capacity - and as a general rule, larger rim diameters have larger load carrying capacities.

It's been found that even if you keep everything the same, larger capacity tires are "More Efficient", but their RR values are higher (meaning not proportionally so.)

So beware of tires that quote RRC by itself. You need more information if you are to figure out what a particular tire is going to do in a different size.

Which brings me to the originally posted question:

IF the tires were the same diameter, but the rim diameters were larger, AND the load carrying capacity was the same, AND everything else were exactly the same: the tire would be wider in cross section (the first number in the tire size), the tire would be lower in aspect ration (the second number in the tire size) and as stated in the initial conditions, the rim diameter would be larger (the 3rd number in the tire size) - something like this: 175/70R13 -> 185/65R14 -> 195/60R15 (Sharp eyed readers will note that is rule of thumb doesn't quite work, but generally, this is how it works.)

With that in mind: Going larger in rim diameter results in more RR, because the tread width would be wider: more tread = more RR.

[modmonster]

Tire Size Calculator - tire & wheel plus sizing

this is a good calculator for judging spedometer difference with new wheels

[guitarterry]

I will restate my question just for u guys. If you had a 12 inch tire with a total diameter of 20 inchs and a 14 inch tire with a total diameter of 24 inches and the car with 12 inch tires had a rpm of 3000 and the 14 inch tire had a rpm of 3200, which would get better milage. everything else being the same. How about if both cars engines turned at the same rpms

[dcb]

The question is too ambiguous. What is the effect on drag with the taller tires? What is the effect on rolling resistance? How fast is the car going? What does the bsfc chart of the engine look like and how much torque is the engine making in both cases?

[Andyman]

I think I can answer the question of what tire size is best. I've read that rolling resistance is inversely proportional to tire diameter. This means that a larger diameter tire will require less power and will improve gas mileage, assuming that engine RPM stays the same. So, it is probably best to install the largest diameter tires that will fit the car. The aerodynamic drag may go up because larger tires makes the car higher, so the fuel economy benefit may be reduced at high speeds. This could be adjusted by changing the springs. The reduction in engine RPM with larger tires is also likely to improve fuel economy on the highway. It is probably best to increase the rim size instead of the tire height for best handling. I'm not sure if there is a best tire aspect ratio but some low drag tires have an aspect ratio of 65%.

[Blue Bomber Man]

I think everything Ive posted above clearly answers your question about tires size and efficiency. RRC is way more important than size (and size's effect on gearing). Generally speaking 15-16" tires have the best average RRC, however the lowest one I could find researching was mentioned earlier, it was a 14" tire. Now if you could find a 15-16 tire that only had slightly worse RRC, then perhaps the 15" tire would be worth it for the gearing benefit.

[CapriRacer]

Originally Posted by Andyman

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I've read that rolling resistance is inversely proportional to tire diameter.

.....

Unfortunately, that is not true. At best you could say this is directional.

It is true that that tires with larger load carrying capacity generally have lower RRC (Rolling Resistance Coefficient) - and that should translate to lower RR for the same load (this is far from clear to me.) - and that larger capacity tires are generally larger in overall diameter. But this chain of connections means that there are too many variables to be able to state this as a proportion.

It is also not clear from the literature available that at a given load, RR goes down as tire diameter goes up. This seems to be right, but getting my arms around this has been elusive.

Originally Posted by Andyman

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This means that a larger diameter tire will require less power and will improve gas mileage, assuming that engine RPM stays the same.

?????? How can the engine rpm stay the same if you change tire diameter - unless you change gearing?

Originally Posted by Andyman

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So, it is probably best to install the largest diameter tires that will fit the car. The aerodynamic drag may go up because larger tires makes the car higher, so the fuel economy benefit may be reduced at high speeds. This could be adjusted by changing the springs. The reduction in engine RPM with larger tires is also likely to improve fuel economy on the highway. It is probably best to increase the rim size instead of the tire height for best handling. I'm not sure if there is a best tire aspect ratio but some low drag tires have an aspect ratio of 65%.

From what I have been able to grab, the larger the load carrying capacity, the lower the RRC - and while I agree that probably larger diameter tires would have lower RR, I think that high aspect ratios (70, 75, etc.) will probably result in better fuel economy compared to low aspect ratio/same diameter tires.

So I am going to directly contradict what Andyman said. Do not go larger in rim diameter if you want to improve fuel economy (BTW, we haven't yet addressed the larger moment of inertia of a larger diameter rim and that alone is reason enough not to go there.)


So to try to answer Guitar's question:

I'm hoping this was a mistake: 12" rim / 20 inch tire vs 14" rim /24" tire. That means the tire had to go up in aspect ratio.

AND:

The rpm of 3000 for the 20" tire would result in an rpm of 2500 for a 24" tire. (not the 3200 stated in the question)

You should get better fuel economy with the larger diameter tire based on the gearing change alone. The tire's RR will probably be better, too!

[tasdrouille]

Capri,

Keeping everything else the same, but just changing tread width, would you agree that the contact patch will keep the same area, but change shape becoming shorter and off course wider?

Most of the rolling resistance in a tire comes from the flexing of the sidewalls (~80%). When you shorten the contact patch by going to a wider tire, you will flex less sidewall but more tread. The impact of the reduction of sidewall flexing will outweigh the penalty taken on the tread flexing.

In real life though, everything else rarely is the same.

Choosing a rim/tire combination is always a matter of compromise. It would be easy to make a recommendations if you were always driving down the highway at 85 mph (skinny and tall), or cruising along country roads at 40 mph without ever stopping (midsize and tall), or driving 100% city with stop and go all the time (short and wide). But that's generally never the case.

Also, I would like to comment on pressure and rolling resistance. Higher pressure reduces hysteresis, which is the main variable determining rolling resistance. There's hysteresis between pressure and RR.

[CapriRacer]

Originally Posted by tasdrouille

Capri,

Keeping everything else the same, but just changing tread width, would you agree that the contact patch will keep the same area, but change shape becoming shorter and off course wider?

........

No. My understanding of footprint length is that at a given load/pressure the length is essentially independent of the width - everything else being equal.

Originally Posted by tasdrouille

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Most of the rolling resistance in a tire comes from the flexing of the sidewalls (~80%).

.......

My understanding of RR is that most RR comes from the deflection of the tread and that the flexing of the sidewalls is coincidental. Put another way, if I take 2 identical tires, but one is pretty much worn out and the other tire is new - and then use the same load / pressure, not only would the footprint be the same size, but the worn tire will have substantially lower RR.

Originally Posted by tasdrouille

.........

When you shorten the contact patch by going to a wider tire, you will flex less sidewall but more tread. The impact of the reduction of sidewall flexing will outweigh the penalty taken on the tread flexing.

In real life though, everything else rarely is the same.

Choosing a rim/tire combination is always a matter of compromise. It would be easy to make a recommendations if you were always driving down the highway at 85 mph (skinny and tall), or cruising along country roads at 40 mph without ever stopping (midsize and tall), or driving 100% city with stop and go all the time (short and wide). But that's generally never the case.

Also, I would like to comment on pressure and rolling resistance. Higher pressure reduces hysteresis, which is the main variable determining rolling resistance. There's hysteresis between pressure and RR.

My understanding is that the thing that creates the most hysteresis (heat generation) is the thing that has the most mass - the tread rubber.