Size of the tire - are smaller (narrower) tires really better?

[CapriRacer]

Originally Posted by CargoBoatTails

From my experience yes, less weight and size means less rolling resistance............

Cargo,

You should read this thread from the beginning. Some of the information in the earlier posts contradicts your experience - and there are explanations why your experience might be different.

Originally Posted by CargoBoatTails

....... A simple formula is the max load of the tire should be equal to or greater than half of the GAWR front or rear on the Vehicle Certification Label, usually the rear is less on a fwd car.......

First, that is REQUIRED by the Federal Government. Put another way, the regulations governing vehicles requires that the tire be capable of carrying the max load the vehicle was designed to weigh by axle. If the vehicle manufacturer doesn't do that, then that is a "defect" (I don't like using that word in that context, but the Feds do.) and subject to a recall.

Second, most passenger cars come with tires 2 or 3 sizes larger than the minimum - and I think there are 2 reasons for that:

1) Using larger load carrying capacity tires results in a more predictable behavior, because the handling parameters are more linear.

2) Larger tires give better fuel economy.

[CargoBoatTails]

This is a really annoying argument that defies the laws of physics. Rather than make my own independent claims I'll use an elementary example of a currently mass produced econobox that comes in different trims, ie the Ford Fiesta HB manual. If you get the 205/40/17s the epa rates it at 26/35mpg, if you get the 195/50/16s the epa rates it at 28/36. Now go argue with the epa that the bigger tire should have gotten better mpg and stop telling naive people bigger tires get them better mpg, ridiculous.

And anyone involved in racing knows you need to make less pit stops with a lighter tire.

[niky]

The 205/40R17 tire is shorter than the 195/50R16.

Using the good old tire calculator at miata.net/garage/tirecalc.html... it comes out to around 1% shorter than the bigger tire on the 16" wheel. Remember, we're talking outer tire diameter here, not rim diameter. Obviously, we want the lightest rims possible, but not at the expense of overall wheel diameter.

Taller is better, but narrower is also better. Which is a double whammy for the 205 on the 17" wheels. You can't ignore the effect of tire width on rolling resistance. This is why the BMW i3 electric has 155/70R19 wheels.

And also why Bridgestone is developing tall and narrow tires for high-economy cars.

[CargoBoatTails]

How ironic because that same Fiesta has a Super High Efficiency model that Ford chose 15 inch wheels for. I imagine BMW has an image to uphold.

[RedDevil]

One of thiese wheels is yielding this kid great MPG...



but which?

[niky]

Originally Posted by CargoBoatTails

How ironic because that same Fiesta has a Super High Efficiency model that Ford chose 15 inch wheels for. I imagine BMW has an image to uphold.

Again... bigger rims =/= bigger tires

17" rims - 205/40R17 tires - 23.5" tall* - shortest overall - lowest MPG
16" rims - 195/50R16 tires - 23.7" tall - taller overall - higher MPG
15" rims - 195/60R15 tires - 24.2" tall - tallest overall - Super Efficiency package

This just proves the point being put across in this thread... larger wheels with a narrower contact patch... better fuel economy.

And the lighter wheel is a bonus.

You could achieve similar results on the highway by putting a narrower and taller 185/50R17 tires on the car, but then you'd lose the economy benefits of the lighter wheel package in city driving, unless you had special 17" wheels that were extra narrow. (like the wheels the i3 uses)



*theoretically, of course.

[Frank Lee]

Originally Posted by CapriRacer

First, that is REQUIRED by the Federal Government. Put another way, the regulations governing vehicles requires that the tire be capable of carrying the max load the vehicle was designed to weigh by axle. If the vehicle manufacturer doesn't do that, then that is a "defect" (I don't like using that word in that context, but the Feds do.) and subject to a recall.

We aren't manufacturers and can put on nearly any tire we please.

If you know you aren't ever going to load your vehicle to max capacity then it should benefit Crr to have smaller, lighter, fewer belted, lower capacity tires.

[Cobb]

Thats my experience. You want to look at outer diameter of the tire, not the first number or third, but the 2nd one. thicker tires also give a better ride and feel of the road and keeps you from cringing when you go over a pot hole.

Originally Posted by niky

Again... bigger rims =/= bigger tires

17" rims - 205/40R17 tires - 23.5" tall* - shortest overall - lowest MPG
16" rims - 195/50R16 tires - 23.7" tall - taller overall - higher MPG
15" rims - 195/60R15 tires - 24.2" tall - tallest overall - Super Efficiency package

[niky]

Well, actually, you have to put all three together... 205 x 0.4 x 2 (sidewall counts twice in height) then divide by 25.4 and add rim size (17") for the height 23.45+"... but yeah, when comparing similar tires with similar widths, the middle number is what really counts.

My guess as to why Ford is okay with the big discrepancy (3%)... having the shortest tire on the 17s helps improve gearing for acceleration, partially negating the effects of wheel weight on straight-line performance and making the "sporty" Fiesta feel even more so... and it's going to be doing nearly 100 rpm more than the "high economy" variant on the highway at speed.

[RedDevil]

Simplified the formula for overall wheel height in inches is
(A * B / 1270) + C
where A, B and C are the width in mm, relative sidewall height% and rim size.
You can just fill in the numbers as listed on the tire: the (A/B/C) formula.

So (205 / 40 / 17) makes
(205 * 40 / 1270) + 17
= (205 * 4 / 127) + 17
= (820 /127) + 17
= 6.4567 +17 = 23.4567 inches.
(funny coincidence, all ascending digits)

In centimeter it would be (A * B / 500) + (C * 2.54).
The example makes (205 * 40 / 500) + 17 * 2.54) = 16.4 + 43.18 = 59.58 cm.