tire diameter

[bgd73]

The comment about 15 inch or 16 inch upgrade from as little as 13 is indeed true. given the original car from oem is the standard they built. Power to weights are proprtionate for every generation of automobile in the past several decades ..that magical diameter turns out to be 24 inches or even greater if you have an engine based on work more than a ricer (ie a boxer engine, or a volvo 240 inline, or most cars today it is no problems at all in the 2+ liters). the older inlines and first transverse were very bad proprietary socilaist trgaedies that originally took over my country. The metro may be a total loser going away from thier design like a honda. They made very very bad and tiny engines be economical and that means alot of dainty things like 12 inch tires...

Here is some very useful websites:

Tire size calculator

and after getting measurements go here:

4Lo.com :: Final Gear Ratio, Crawl Ratio, Tire Size Calculators

The old subes with 1781s love the big treads. A 1.8 or less in an inline config is very very sensitive. A metro owner could help alot more with thier own experience.

[Blue Bomber Man]

I really think that the analysis of whats more efficient needs to broke down into two catagories:

1) Engine Efficiency

2) Tire efficiency

So with 1) yes a larger tire means lower RPMs and thus is likely to be more efficient at the engine. However we also know that increasing the diameter would increase the rotational momemtum at the tire. But something that is being neglected is that conversly you are LOWERING the rotational momentum of the engine, transmission, and power train. I would speculate that the larger tire mass is not worse than the gains in the rest of the car.

Now of course as MetroMPG has done you could change your gearing and thus gain the advantage at the engine without having the negative effect at the tire....

But it has been shown by a survey of a 150+ tire models that on average size 15-16" tires have better RRC (rolling resistance coefficient). So if you compare say a car with 12" tires with a higher RRC to the same car with 15" tires that have a lower RRC, and equalize the gearing so that the RPMs match for any given speed, the car with the lower RRC run more fuel efficiency.


What effects RRC?

1) Tire pressure, as we have mostly all accepted raising your tire pressure significantly improves gas mileage

2) Tread depth, the less tread a tire has, the lower its RRC. A tire's RRC value actually improves with age, so suck the life out of those tires as long as can safely be done.

3) Tire width, narrow tires will out perform wider tires in general

4) The remaining properties that effect a tires RRC include: composition, sidewall strength, sidewall width, and road conditions.

[guitarterry]

a question on RRC. is a 16 inch compareable to a 12 inch. my point being is could a 12 with a bad RRC be better than a 16 with good RRC. An example would be an extremly economical big block V8 would still be worse than an extremly bad 1.0 3 banger. AS another example. CD ratings are also misleading a train has a real real low cd. but most of us couldnt afford the fuel to put in it. where as alot of hatchbacks like the metro's are not that good on CD, but they are still economical cause what really counts is CDA, Again to rephrase (cause we all know what will happen) is RRC what count or maybe something like RRCA

[Blue Bomber Man]

From my understanding it works something like this:

RRC x weight = total energy loss.

so reduction of weight or RRC have similair effects, however RRC is easier to reduce provided that you can find reliable RRC values. I think the best tire on the market right now is actually a Bridgestone 14" tire that has a RRC of .0065, where as most tires have around an average of .0100

RRC is an independent variable of tire diameter, this is no direct relationship. However based on the averages of different sizes the study found that overall the large 15-16" wheels had lower RRC.

To answer your question: If a 12" and a 15" tire have the same RRC, then they will both yield the same fuel economy if there was no other effects at play (larger mass of the 15" tire, or the gearing disadvantage of the 12")

RRC values will dominate any effects of gearing or weight in general. In fact at lower speeds (probably sub 20mph) RRC will dominate wind resistance.

RRC is less significant at higher speeds compared to aerodynamics because drag works at the cube of speed.

[guitarterry]

I just read the 5 most fuel efficient tire page, The smallest tire on the list was a 14, and it was the best. also listed was 15,16, and 17 inch tires. again i dont know about comparing just LRR facts. That is why i thought it would a good test for real world results. Where the engine rpms stay equal, but tires sizes change. (wow, that was the first post, and we still havnt got around to this question, we are still on tangents.)

[Blue Bomber Man]

Ya, there is the Bridgestone 381 that has the lowest RRC of any tire I've seen so far, its a 14" tire. I believe it was rated at .0065 or about 35% lower then the average. Doesnt fit my rims though, so I need to find a size for my rims and compare the RRC and see whether I find it worth it to get some nice light rims for the 381 or to find a low RRC for my size.

[guitarterry]

Originally Posted by Blue Bomber Man

RRC is less significant at higher speeds compared to aerodynamics because drag works at the cube of speed.

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.

[Blue Bomber Man]

Because only about 20% of a vehicles energy actually makes it to the wheels at highway speeds a lower RRC is still more valuable than improved gearing.

A 10% increase (in efficiency) at the engine only results in a 2% mpg gain.

However, like I said, the best tire I've seen so far is in fact a 14" wheel, so you would do well to work with that if you car's rims can take it, and then modifiy your transmission or final gear to gain any engine efficiencies through rpms

[guitarterry]

Originally Posted by Blue Bomber Man

Because only about 20% of a vehicles energy actually makes it to the wheels at highway speeds a lower RRC is still more valuable than improved gearing.

A 10% increase (in efficiency) at the engine only results in a 2% mpg gain.

However, like I said, the best tire I've seen so far is in fact a 14" wheel, so you would do well to work with that if you car's rims can take it, and then modifiy your transmission or final gear to gain any engine efficiencies through rpms

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.

[Blue Bomber Man]

Just because aerodynamics/drag becomes the main source of loss energy, does not mean you are losing any less at the tires.

Think of the energy loss as a pie, and say 80% is loss at the engine (typical value, I can cite sources later if you want), at urban speeds, tire losses will count for around 7% and aero is 6% and the rest is lost in drive train and suspension.

When you reach highway speeds, the energy loss pie gets bigger, the amount of energy loss by the tires, engine and drive train remain the same (or similair), but aero losses increase.

So you still get the same reduction of energy waste at higher speeds, its just less noticeable compared to loss due to aerodynamics.