tiltmode43

iTrader: (15)

I'm reluctant to post a topic but am truely curious. Would a wider than stock (say 9-9.5 rim with wide tire) significally effect the 8's acceleration/mileage? I ask because there is more contact/friction to the ground which will need more energy to move at the same rate as the 8 setup. Sorry if it's a silly question. Assume the 8 and 9.5 setups have the same diameters and weights to remove other variables.

alnielsen

iTrader: (4)

Sounds about right. An increase of tire section width will increase rolling friction and increase the wind resistance of the car. This means slower rolling start times and a reduced top speed of the car. In the early 80's, Formula Ford cars in club racing went to narrower tires for that reason.

swoope

iTrader: (5)

first of all the dia might be the same, but the weight will not... and the na motor is not powerful enough to overwelm the stock tires...

mileage. more tread more friction less mpg...

beers

Umbra

Yes it is. I've had the back end swaying back and forth with no traction at a stop sign in dry summer conditions. I've also lost the back end on corners quite often with the DSC/TSC off.

The Mighty Red

I'll agree that the stock 225s are adequate for the amount of straight-line torque the engine provides.

However, in agreement with Umbra, a wider tire (of good quality) will give the car better lateral grip - ie sticker in the turns.

dsmdriver

iTrader: (3)

Rotational inertia is important for acceleration. If the wheel tire combo has more inertia, it will reduce your acceleration. It's also important for MPG in the city since you stop and start the wheels a lot.

Rolling friction and wind resistance is important for MPG on the highway. Wider will be worse here, but it might be so small you don't notice.

Red Devil

I've averaged 17.5mpg with both the stock 8" 225 Dunlops and with my 9.5" 275 KumhoMXs. The car definitely brakes and corners better.

swoope

iTrader: (5)

thx for explaining the weight part of the deal...

beers

tiltmode43

iTrader: (15)

The weight could be near the same, stock wheel Vs light weight weel with wider diameter. I guess the tire would add a tad more for the overall weight. Anyways, its as I thought, improved handling with slightly diminished straight line performance and mpg (I drive mostly city mostly hills)

Now the question for myself: better looks+better braking+better grip>worse mileage+slower acceleration

Thanks for the input guys...

dmp

BUT - wider tires COULD help fuel economy. Most of our relative economy is lost through acceleration (getting back up to speed). More grip can help maintain a constant speed through corners.

:D

The Mighty Red

Food for thought..

For racing or performance driving, you could use lightweight 17" (or 16" if you can fit them) wheels and you'll have the best of both worlds. Smaller diameter means less rotational resistance - and this relationship is linear. Thus, it takes approximately 6% less energy to spin a 17" and approximately 11% less energy to spin a 16" wheel to the same speed - all things being equal.

Plus, the lesser overall weight of the smaller wheel will further lessen rotational resistance and give the beneficial side affect of reduced unsprung weight which further improves handeling, accelerating and braking.

Bottom line, with smaller lighter wheels you'll accelerate faster, turn better, and brake quicker. More info - https://www.rx8club.com/series-i-wheels-tires-brakes-suspension-55/effects-wheel-size-weight-performance-51865/

On the down side, smaller wheels have a higher rolling resistance than larger wheels. In other words, once you're up to speed the larger wheels require less energy to maintain their speed (for various reasons). Also, the larger wheels move the car a further distance for each rotation so there are MPG advantages.

In conclusion, smaller wheels are great for racing, short sprints, and autocross, etc. But regular street driving, the higher level of comfort and reduced rolling resistance of larger wheels wins.

tiltmode43

iTrader: (15)

The problem with smaller in diameter is the tires will be higher profile. Technically this gives more grip in turns because there is greater flex but this is arguable. However, the overall diameter of most wheel + tire on 17 and 18 setups are about the same (higher profile for the 17's) thus disregarding the whole smaller wheel/better gearing argument.

Thanks for the help but my initial question was more regarding width rather than diameter.

DOMINION

^Right

The Mighty Red

What a bummer. Why can't they just keep the sidewall the same height so that a smaller wheel equals a smaller outer diameter?

Now that I look at it... going to a 17" Kumho V710 actually ends you up with a larger outer diameter than the 18".

dtorre

Noticed that with wider tires I get noticabally worse gas mileage...like a 1/12 of a tank extra per my normal half tank =(....also top speed comes into play aroun d the top of 3rd gear...after that wide tires kill our cars!!

Xantium

Wider tires only increase your friction circle to a certain extent... for example if you had 11 inch wide tires the weight on a square inch of the tire would be much lower, therfore giving a smaller contact patch. Just food for thought and I think theres a whole book about this (something to do with maitas)...

mikeschaefer

what?

sidewall height is determined by the ratio (second number in the size, ie for 245/40 the ratio is 40) so yeah it can be adjusted

btw why would you want a smaller overall diameter??

tiltmode43

iTrader: (15)

smaller overall diameter = better "gearing" Tons of topics on it...

Clavius

Hmm meh like this post might have to show it to a friend who drives a Escalade. He still believes some falsehoods of larger rims/tires. Great to see this thread not turn into a flame war.

Razz1

You guys really talk n generalities.
It's eay to get 17" Wheels with same or lower profile.

This will change the gear ratio.

The Mighty Red

I thought I was being fairly specific when I was talking about Kumho V170. From what I've seen, it's not that easy to go to a lower profile particularly in racing applications (which would really be the only reason for going to an overall smaller outer diameter).

For example, these are ALL the possible sizes for the V710
http://www.kumhousa.com/Products/Pri...=1&PtnID=EV710

If you want to run the V710 and you're limited to a tire width of 245 and have 18" wheels your only choice is the 245/35R18. If you go to a 17" wheel and keep the same width, your only choice is the 245/45R17 -- that's the only profile they offer in that size.

If you did the math or physically measured, the 17" tire actually has a larger outer diameter than the 18". You can verify this here http://www.miata.net/garage/tirecalc.html

You could go with the Hoosier 245/40ZR17 but that would be the exact same outer diameter as the above mentioned 18" wheel. No gearing advantage there.

If it is so easy, find an r-compund tire for a 17" wheel that can get me an outer diameter smaller than 24" and I'll be impressed (as well as grateful). I can't seem to find one.

peloponisios

While at this, I 've checked 17's rear with 235/45/17 tire and the car was noticeably faster(I could chase an s2k until some kms). Take into account that I drive the 5mt euro version with the low powered motor.
The thing is that while I could install 17s and have an advantage, the looks trouble me.
So here is a comparison. My Volks arrived and are 18*8 front and 18*9 rear so the tire would be 225/45/18 front and 245/40/18 rear.
The other combo would be 17*8 front and 17*9 rear with 225/50/17 front and 245/45/17 rear.
With the 18's will I lose significantly in acceleration(even if Volks are much lighter)?
With the 17's at that diameters, what will be the actual gain?

The Mighty Red

A few observations.

1) You can easily fit 9" wheels and 245s all around which will give you better grip at all four corners. If you put 245 on the rear and 225 on the front you will be invoking understeer - the tendency for the car to push through turns.

2) Also, running the staggered setup will result in one set of tires having a slightly smaller outer diameter than the other. As a result, one set of wheels will turn at a different speed than the other set causing the DSC to think the car is constantly losing traction. This may cause the car to do strange things (DSC puts the brakes on) from time to time particularly when you take sharp turns.

3) a 245/40-18 will have he exact same outer diameter as a 245/45-17 and will thusly have minimial advantages if any - other than the fact that there may be slight weight and cost savings.

Overall 18s will be better for street driving for several reasons but smaller wheels and tires will usually yield better results in racing due to weight and gearing advantages -- though they aren't as nice to look at. But whatever way you go, it's best to keep them the same size all around to retain the car's balanced handeling.

peloponisios

Thnx for the help.

I never thought of it this way and it's amazing because it's simple common sense.
Staggering might confuse electronics to a point of being dangerous.

So I am making my mind up and I have these in mind:

Either Enkeis RPF-1 17*8,5 +40 front rear
Or else Volks CE28N 17*8 +38 f/r or 17*8,5 +40 f/r

Tire 235/45/17

As I wrote above, I tried this combo in the rear with an ancient heavyish rim and I was impressed. A different car both accelerating and braking.

Will I benefit from shortening the gearing with these? I believe yes. But any info would be appreciated.

Again thank you.

The Mighty Red

Stock 225/45-18 to 235/45-17

You'll go from a 26" OD to a 25.3" OD
It defiantely changes the gearing -- from 777 revolutions per mile to 796. Nothing extreme though. http://www.miata.net/garage/tirecalc.html

On the down side, your speedometer will read 2.5% too high - you'll be going a tad slower than the readout says. But just by a little bit.

It should sprint a tad quicker but your long range MPG might suffer.

Perhaps someone with a bit more knowledge on the matter wishes to add something???