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Discussion Starter · #1 · (Edited)
Ok, so I've decided to lose 150kms a year on my odometer and go with a 225/35-18 front tires to add pep to the car. Question is, shift points changes and rpms and speedometer changes especially on 6th gear on an AT. 811 rev/mile stock vs 833 rev/mile new tires.

Any takers on this one?
 

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Your wtire package is out of spec by nearly 4%

What exactly do you want to know?

You've lost half an inch in ride height and a full inch in tire diameter.
It should make sense, if not, let me know.

You're asking a smaller tire to do the same work that a larger tire is supposed to be doing.
 

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Discussion Starter · #3 · (Edited)
Your wtire package is out of spec by nearly 4%

What exactly do you want to know?

You've lost half an inch in ride height and a full inch in tire diameter.
It should make sense, if not, let me know.

You're asking a smaller tire to do the same work that a larger tire is supposed to be doing.
Actually it's -2.7% and 0.7 inch diameter from stock according to this site. I forgot to mention that I own a skyactiv 2012 with 205/55-16 stock tires. I installed 225/40-18 on RX-8 rims and it's now 0.8% change with 0.2 inch diameter change.

http://www.miata.net/garage/tirecalc.html

I found a neat site for final gear ratios but it's mostly for changing the gear itself. What I was wondering is what RPM the engine would be at going 65mph for example vs stock. The rest I realized later that the auto trans would just do it's thing and shift when it's time. I'll gain some dsplacement on the pave with less effort from the engine but lose on MPG on the high gear. Ijust wonder how high the RPM will rev on 6th gear.
 

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Discussion Starter · #6 · (Edited)
Oh... well, your OP stated 225/35 FYI which is hugely different than 225/40.
I now realize what you are saying. Just to make sure I've explained myself well.

I originally had 205/55-16 tires, I changed them to 225/40-18 and now wish to change the front ones to 225/35/18. Although I'm contemplating taking a small loss and buying all 4 and selling my 225/40's. I would break even because of the deal I get on tires and my current ones are used for only one season. Where I do lose is installing and balancing them. $60 for the set. pretty cheap. Changing only the front ones would keep some ride comfort with taller walls on the back tires and give the car a slight downward angle towards the front. Wind friction may be a concern here thought. May be minimal but I have no idea how much.

So the -2.7% change and .7 inch difference is from stock 205/55-16 to 225/35-18.

My initial post was too vague and missing some details.

2570rpm is not too shabby. I could deal with that. It's wind friction at higher speed that causes a major loss usually. WHo knows, with smaller tires reducing some engine load I might actually gain some mpg on lower speeds and at lower rpms on the AT since it shifts to a higher gear as fast as it can.
 

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The front tires are what take the brunt of road impacts, not the back. Especially when braking when the weight shift forward. Be prepared for a harsh ride with 225/35's. You'll feel every bit of those low profile tires running them in front.
 

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Discussion Starter · #8 · (Edited)
The front tires are what take the brunt of road impacts, not the back. Especially when braking when the weight shift forward. Be prepared for a harsh ride with 225/35's. You'll feel every bit of those low profile tires running them in front.
even when running lower air in them? I see that you owned a few 3's over the last few years. Are you speaking from experience?
 

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It's where ~63% of the weight of the whole car is.
Doesn't matter, nor should you lower the air pressure either. Tire pressure makes a huge difference in evertything, as all things are tested and rated to run at a specific pressure. Deviate from that and you won't experience anything positive.

On my Rays I had 215/40/17s and the ride was absolutely harsh on them compared to spec, even now on spec 18s the ride is much more soft (but also a softer sidewall tire).

I doubt you'll see a "worthwhile" change in gas mileage, as even the guy that replaced his final gear, spent what he could have saved with a (2mpg worse out of 40mpg) barely worse stock transmission, in ... oh, probably 4 years.

It's all relative; if you want to spend it, go for it, but there is no secret or magic answer that will say you'll gain X amount of MPG and save Y dollars each year. Because chances are (at least 99% of the time), that primary purchase, will cost more than any possible savings gained which kind of nullify the purpose.
 

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Discussion Starter · #10 ·
MPG's wasn't really my goal. Just wondered about it's affects. My goal was to give the car a bit more get up and go and reduce some load off the engine. Wheel size and gears adding performance. I find that our 3 being an AT (even with stock wheels) has a small first gear and lots of Tip-in. Floor it and you have to wait until it reaches over 3500 for it to bolt. Knowing that the fuel mapping is robbing some power in the lower band and having such a small gear on first is a nuisance for take offs especially with a clutch lock that waits to operate until the engine is warmed up. The first weeks I bought it I almost got T-boned trying to jump in traffic. I never expected a $20k car to resist like that. I had no idea about the mechanical adaptations done to this car to save fuel nor should I have to but now I know why the car acts this way. I'm not about to mess with the fuel mapping because the car is still on warranty.

The RX-8 rims I installed are heavier + tires. I do realize that this will affect torque at the pave. I'll tell you though, it now drives like a comfort cruiser. It made it feel like a bigger car which is planted down to the pave. The stock setup made the car feel bouncy and too light to hold itself level on bumpy roads. I love the look and drive comfort of these wheels. They fit the car extremely well. Like they were made for it. I just wanted to free up some load. I also thought of getting some material machined off the rims at strategic place so that the rim's integrity would not be affected much. Such as removing some quarter sized material behind the spokes but keep the front of the spoke intact or even drill hole through the spokes completely in a fashionable and reasonable look for the rim although I rather go with the first option. But before I would go that "route" I was going to try to downsize the tires. I did this in the past with another car and I still remember how awesome that car was and how it just came alive for such a small mechanical change.

I guess it's a tricky one when it comes to keeping the ride comfort.
 
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