Is anyone running less than stock diameter tires?

[DaveNagy]

If you are, I'd be interested in hearing what the experiance is like.

Do they look "small"?
Do you notice any difference in the "gearing"?

(The stock and recommended tires for the MINI are all right around 24"-24.1" in diameter. Different size tires have different diameters, some "taller", some "shorter".)

The ones I'm considering are:

205/40/17  23.5" diameter, 2% less than stock
205/45/16  23.2" diameter, 3.3% less than stock

Anybody else have either of these sizes, or am I venturing into uncharted territory?

-Dave

[Bilbo Baggins]

I have been looking at these two tyre sizes also. Hope you get some responses.
I have even seen some 205/40R16 at 22.5" dia. that look interesting.

[DaveNagy]

Wow, 22.5". That is pretty dinky! On my "wheelweight spreadsheet" thread some fella (olddad) pointed out that 225/40/16s are right about 23" in diameter. I think those might be a bit wide however.

-Dave

[MINIAC]

Quote:

Originally posted by Bilbo Baggins

I have even seen some 205/40R16 at 22.5" dia. that look interesting.

I hope you're not planning on using these for the street

No more than ±3% variation is recommended for street cars in order to maintain accurate speed data going into the computer, thus keeping odometer and speedometer changes negligible. The 22.46" diameter of 205/40-16 tires is 6-8% less than than stock sizes [23.96" for 175/65-15 and 24.44" for 195/55-16]. Besides your speedo being adversely effected, you knows how much all the "alpha-bet soup" wizardry on the car will be messed up.

[DaveNagy]

The points that MINIAC made are good ones, and need to be considered, but let me play the devil's advocate for a moment.

Speedometer error:

Smaller tires will make the speedo read more optimistically, that's clear. Putting a 23" tire on the MINI would make the speedometer read 2.1 MPH too high at 60 MPH. You'd really only be going 58 MPH. A worst case with 22.5" tires would be a 3.75 MPH error at 60.

This just doesn't seem like a terribly serious problem. More like an inconvenience. BMW speedos are always optimistic by at least this amount anyway. It's just unfortunate that smaller tires make the problem worse, rather than "correcting" it. I wonder if there's any way to recalibrate the speedo to remove both the BMW and tire related innaccuracies...

I plan to test my car's speed with a GPS unit in order to get a feel for the amount of "mental speedo correction" I'll need to employ. (No matter which tires I end up with.)

ABS system confusion:

Here I must speculate wildly. I really have no idea how the various braking systems work. That said:

A smaller rolling diameter seems harmless, if not actually beneficial. It would guess that the various brake systems work by measuring and comparing the speed at each wheel. (Along with other measurements that shouldn't be affected by tire size.) If one or more of the wheels slows down, the system needs to act. If one of the wheels actually stops (skids), it again needs to act. I don't think having all four wheels spin slightly faster at any given car-speed would affect these measurements.

But perhaps the system includes logic of the type:

"If a particular thing happens at 61 MPH, do this. If it happens at 60 MPH, do this slightly different thing."

If that's the case, then the braking system might get "overly stimulated" because it thinks you're going 60, when you're really going 58. It might be ever-so-slightly more aggressive in detecting and reacting to problems. But could you actually detect this change in behavior? I doubt it. Is the behavior dangerous? On the contrary, it might make the car slightly more "protective" than usual.

Smaller diameter tires means less rotational inertia, all else being equal. That should make it (very slightly) easier to modulate the brakes. But perhaps the ABS system relies on there being heavy wheels to "smooth out" its pulsed braking impulses. Putting light wheels on might make the ABS effect "more jittery".

I guess I'm already resigned to that potential effect, since I'm definitely putting light weight tires/wheels on. That change will overwhelm any further changes due to tire diameter. IMO.

-Dave

[Mugami]

Ok I have the MCS DS/W and Ive changed the stock rims and tires to a set of 17inch Koyto rims and 205/40/17 Khumos tires. Can I tell a Diff. hmm lets see. the rim and Tire combo is 8-10 lbs per Wheel lighter plus the small diamter. Yes with just the wieght diff you could tell the diff. As for Time wise I could see a .4-.6 Quicker in the 1/4 with the tire combo. Since I did both at the same time I cant really say if just lowing the diamter on how much just that improved the car. well that my 2cw . Have a Peachy day

[DaveNagy]

Thanks, Mugami.

How's the look? Is it even possible to tell that the tire diameter is (in your case) a half inch less than stock? The car must ride 1/4" lower. Can you tell? Do you notice any additional visual gap between the tire and fender arches?

I'm just trying to avoid the "low rider" look!

That's really impressive if you gained .5 sec in the quarter mile. When you say 8-10 pounds lighter per corner, is that compared to the 17" S-Spokes, or to the 16" V-Spokes (X-Lites)?

-Dave

[Mugami]

They are compared to the 17" S-lites and yes I can tell the difference in the wheel arches. I do have a set of Coil overs to lower the car to my liking (on order) .. not quite the Low rider look but enough to bring the arches back to stock looking gap. you can see a quick site I made showing the car and wheels here

[DaveNagy]

Thanks so much for the pics. I'll make myself a local copy of that page, in case you need to take it down again. Those aren't the exact wheels I'm leaning towards, but both the color and the tire diameter is very close, so they really help.

I find it encouraging that you were able to see the gains you did in 1/4 times with "just" a 10 lb reduction per corner. I'm probably going to see a reduction of around 20 lbs, so I'll be anxious to measure my gains.

As far as the "low rider" look goes, I'm wasn't talking about the sort of lowering you're doing. Adjustable coilovers are very nice. The look I'm hoping to avoid is the one you get when you put tiny little Honda wheels on a Monte Carlo. (With whitewalls of course!)

Not my cup of tea.

-Dave