Dan Martin

I switched over to a set of 205/50/17 snow tires about a week ago and I've been pretty happy with them. The funny thing that I wasn't expecting is the car actually feels a little faster now. Not so much off the line but more for lane changes on the highway where grip isn't really a factor.

My theory is that the smaller tires (about 2% according to the spec sheet) are effectively lowering the gear ratio hence increasing acceleration but reducing top end speed. Not like I would be doing high speed runs on snows anyways, but this could be interesting for people considering summer tires in the 225/45/17 or 225/40/18 size.

I know the speedometer will also read a little faster because of the reduction in diameter so that's probably magnifying the effect but it still feels quicker.

Any thoughts, or have I been :blaze:?

CGTSX2004

iTrader: (1)

You're right about the gear ratio thing (I think...) but you could also have been doing a little too much :blaze:.

biker

I think it's just the different feel of the tires that makes you think it's quicker.

dom

I was wondering the same thing with my 205/60/16 snow's on the lighter steel rims. Each wheel/tire weigh 5lbs less than stock but there's also less grip so I'm not sure it makes a difference. You'd need numbers to back it up.

dzuy

i think it is a combo of both weight savings and your speedo being off

sauceman

Lowering the gearing to the ground would be equivalent to increasig your torque to the ground. Add to it the decresaed weight of the tire, and also the reduction in the contact patch, effectively meaning a reduction in friction, and improvement in aerodynamic drag.

Add it all up (granted, they are minor details) and you get a perceivable increase in performance IMO.

Dan Martin

That's more or less what I'm thinking.

Dan Martin

Ah, but the reason the speedometer is off is because the ratio has changed.

Dan Martin

It would be tough to get numbers because there's not as much grip off the line so 0-60 wouldn't be a fair test. I don't think my g-meter can do a 50-70 run but that would be the best test I can think of for this scenario.

TSXDude

I believe the 205 width has contributed to your feel of increase acceleration as well. Eventhough the width on each tire is 10 mm thinner, it will somewhat increase the acceleration due to less friction surface on each tire. I read this in one of those Japanese tuner magazine. Of course, with reduced width comes reduced handling capabilities. That's why, according to the Japanese, they will never really specified anything outrageous over stock sizing.

sho-one

so if you get 18" rims, your speedometer will read slower than you are actually going and also make your gear ratios taller? hmmm I was thinking about geting some rims in a larger size and width...now I might change my mind. Is there a way to recalibrate the speedometer?

Dan Martin

The overall diameter is mostly atributed to tire size. A 235/40/18 has nearly exactly the same diameter as the stock tire.

A 225/40/18 actually makes for a shorter ratio so the speedometer will read slightly faster than what you're actually going. A taller ratio would make for a slower speedometer. There are plenty of threads on this in this section.

There's really no reason to recalibrate the speedometer because the difference is so small. Mine is off by 3kmh at 120kmh = 2.5% +/- 1%. Besides, I think the speedometer sender is mechanically linked to one of the rear wheels and there would be a lot customizing to correct for the ratios.

iTimmy

Your also sucking colder air and cars always pull harder with cold dry air.

xizor

Since the smaller diameter does change the effective gearing, does that increase rpms at cruising speeds? And if so, does MPG go down?

Dan Martin

Yes the RPMs are higher and I imagine it does affect fuel economy. I've only had to do one fill since I got my tires on but I didn't really notice any difference. The biggest difference would be on highway driving. I'll have to go somewhere and test it out.