Tire size/gear ratio and width theory

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sachilles
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Tire size/gear ratio and width theory

Post by sachilles »

Long story short, my non turbo subie currently has 225/50 16 toyo RA1's. Love them......LOVE THEM. They are at least five year old, and still give me plenty of stick. However, I'd guess this will be there last season. Overall diameter is 24.9 inches.
At Ascutney and Burke I don't get beyond 3rd gear, and there are points where 3rd gear is barely holding speed. GPS data says 71mph is as fast as I got at burke (right before check 3 and right before check 7). Calculating the gear ratio, at 6500 rpm, the car should be at 81 mph. It really looses any steam by 6000 rpm(or lower).
At Burke for example, I'm in third gear in the large sweeper before check two. I'm at 69-70 going into that sweeper, I loose 1 or 2 mph in the sweeper with the pedal on the floor. At the point where I hit the brakes before check 3 and down shift, I'm back to 69 or 70. I pretty much am only able to hold my speed there. There were other points where I held 2nd gear pinging the rev limiter rather than up shift to 3rd, since I'd have to downshift immediately after, and felt it wasn't worth the lost time in shifting.

I am contemplating switching to a 15 inch rim which is lighter, and going with a 205/50 15in toyo RA1. That drops the diameter to 23.1 inches(compared to 24.9).
At 6500 rpm in third gear, speed would drop from 81 to 75.

Advantages I perceive:
Reduced weight in both tire and rim(4-5lbs per corner). I'd have to go to a smaller diameter rotor setup, which is also slightly lighter. 205/50 15 is spec miata tire size, take offs are readily available and dirt cheap(less than $200 for 4). Tough to find used 225/50 16's used, new are ~$170 per tire.

Disadvantages:
Tire is narrower. However, I don't feel like I've exceeded the corner holding ability of the 16 inch setup, given my power levels.
Must put on a smaller brake rotor(caliper stays the same). This would be the oem size for the drivetrain. I have the parts for all of this, so the cost to do it is nill(other than time).

My thoughts are that this car is not a high horsepower car, so I must preserve momentum through corners when possible. Taking tire width away may harm that. I also feel like I need to get more power getting out of the corners, I think this might help. I think this will help acceleration on the steeper parts of the hills.

What do folks think? I'm leaning towards it just from a cost savings perspective, if at worst there is no performance gain. 15 inch allows for many option in tires at reduced prices, including rallycross tires.
Smaller brakes should be a non issue other than one venue(sccv track event at slmp). I can still change brakes easy enough.
I know Terry runs a smaller diameter tire. Unsure if you still have the stock gear box(4.111 final) or not.
Sachilles
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honda#72
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Re: Tire size/gear ratio and width theory

Post by honda#72 »

I think the 15 inch is the way to go in your car. I had 16" then went to a 15" on my 98 prelude, and I shaved 12 seconds off at Ascutney. Now some of that time was due to a slightly stickier compound, and perhaps me learning the hill more. However, the car did really pick up acceleration. I had essentially changed my finally drive from a 4.26 to aroung a 4.56. It made the car actually pull in 3rd gear, not just stay at a rpm and speed when floored up hill and it had some grunt under the vtec curve. I stayed with the same width, but if you feel you are not exploiting the width of your current size, you should be fine.I think you will find the gear change help more than the small changed in contact patch size. Plus as you said, 15's are much easier to find.


Chris
Last edited by honda#72 on Thu Jun 28, 2012 2:54 pm, edited 1 time in total.
1997 cc + Vtec + 9,200 rpm fuel cut = Fun !
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sachilles
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Re: Tire size/gear ratio and width theory

Post by sachilles »

I'm thinking mostly 205/50 15 as they are so common with it being spec miata size. A set of 4 can be had on ebay for $175 plus shipping with 80% tread left. RA1's wear very well for me.
Can't beat that price, and the RA1 don't heat out cycle as fast as the kumhos and hoosiers.
They do make a 225/50 15. Which is still shorter than what I run now, but taller than the 205/50. Those aren't as common, but if I find the 205's don't cut it I can go to those...though they'll be more expensive(but still cheaper than 16 inch ra1's).
I'd also like to pick up some take off rally tires for an occasional rallyx, and you need 15 inch rims for that.

Need to start stocking up on 15 inch rims I guess. If anybody has any subie 15 inch alloys, ugly but functional, let me know. I prefer cheap and ugly, over expensive and pretty.
Sachilles
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Re: Tire size/gear ratio and width theory

Post by STI NICK »

by going with a slightly narrower tire you may also reduce the rolling resistance. If you arent finding the limits of your current tire perhaps you are using to wide of a tire for that car/power level. Wider is not always better. just a thought.
Nick
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Re: Tire size/gear ratio and width theory

Post by KevinGale »

STI NICK wrote:Wider is not always better.
Listen not to this heretic! :evil:
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sachilles
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Re: Tire size/gear ratio and width theory

Post by sachilles »

I'm willing to bet you have more horsepower going to one of your wheels than I have going to all four of mine. :o :lol: :(
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Re: Tire size/gear ratio and width theory

Post by STI NICK »

KevinGale wrote:
STI NICK wrote:Wider is not always better.
Listen not to this heretic! :evil:

:lol:
Nick
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Re: Tire size/gear ratio and width theory

Post by KevinGale »

sachilles wrote:I'm willing to bet you have more horsepower going to one of your wheels than I have going to all four of mine. :o :lol: :(
I wonder if anyone has put a smallblock chevy in a subaru?
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sdwarf36
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Re: Tire size/gear ratio and width theory

Post by sdwarf36 »

KevinGale wrote:
sachilles wrote:I'm willing to bet you have more horsepower going to one of your wheels than I have going to all four of mine. :o :lol: :(
I wonder if anyone has put a smallblock chevy in a subaru?
It certiantly would improve the sound. :lol:
Translating road racing to hillclimbing:
Proper tire selection== nothing hooks up on moss or wet leaves.
Staying on the racing line==anything paved is considered good.
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Re: Tire size/gear ratio and width theory

Post by Mopar 151w2 »

Skooky told me a long time ago :"A small-block on a skateboard is about right!"
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