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Old 07-17-2009, 10:33 PM   #1
Roland Saul
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Join Date: Mar 2008
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2002 TS Wagon
Sparkly BLue

Default Tire Basics Questions/Doubts

Greetings all... I have a couple of questions regarding my next set of tires on My 2002 Impreza TS. I currently have 15/225/70 size tires with what appears to be about 60% tread left, on "standard" or stock steel wheels.
The tires are Yokohama Avid 4's, which seem to be wicked tires to me, I'll buy another set I guess.

My gripe with the Impreza has always been that it feels too "geared down", like 1st gear is literally useless after 2 feet of forward motion, and 2nd,3rd, and 4th are about the same. I feel like the motor revs up too quick for the way I want the car to behave, so.... If I go to a tire with a larger overall diameter, will that change the ratio of engine speed to forward motion at all?? I mean even a few percentage points of rev reduction would save me a hundred rpm's maybe, right?? I'd like the car to run at 2200 rpm's at 55 rather than at 2400 if you catch my drift.

I would gladly put a different ratio final drive in the car if I could, but that won't happen anyway, so I'm looking to eke out every last possible way of lowering the engine speed per foot travelled, there's plenty of torque for this IMHO.

Also, I don't need heavy cornering ability, I drive like a granny, so would there be any advantage in getting a tire with less width? Would it lower the rolling resistance and maybe give me some added gas mileage? The tires just seem a little wide for my driving style, and again I'm hoping for a little free mileage with a thinner tire. Is this nonsense, or a negligible difference?

Thirdly, would the ride be a little cushier with a higher "profile" tire, I mean the higher distance from rim to tread is the profile, right? I am prepared, again, to sacrifice a little unused cornering ability if it gives me a nicer handling of ordinary road bumps.

Lastly, do I gain any appreciable handling ability with mags over stock rims? Are there rigidity bonuses to some mags, or are they really just for styling?
I'm talking identical widths for comparison i guess.

Thanks for reading this guys, I know it's a little long, but I can't seem to find any other direct answers after combing through about a million previous posts..... The car is fantastic, but actually seems to be tuned about 10% more for hard driving than I realistically do, so if I can get a little lower engine speed, a little better ride, and a little better mileage with a larger,higher profile, and narrower tire, I'll so it.

Thanks again.
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Old 07-18-2009, 03:33 AM   #2
mosc
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Sounds like what you want is a grand touring tire.

not sure you got your size right. Your car is designed for ~24.2" tires. 225/70/15 is very tall, basically an SUV tire.

Anyway, you could go 195/65/15 probably if you really wanted to (your stock size is 195/60/15). Fender wells should clear for a 25" tire no problem even on soft springs. Lots of good grand touring choices there too like the Bridgestone Turanza Serenity.

Last edited by mosc; 07-18-2009 at 03:42 AM.
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Old 07-18-2009, 05:00 PM   #3
yzer
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The stock tire on your 2002 TS is 195/60/15. Check the driver's side door sill for the silver sticker. This sticker includes the OEM tire size. The 195/60/15 has the familiar Impreza tire diameter of 25.0".

The 225/70/15 is just wrong for the 2002 TS. The diameter is 2.4" too large. This kills acceleration by making the wheel/tire gearing too tall and making the speedometer read almost 10% too slow. Suspension geometery is compromised as well.

I realize you don't want performance (you drive like a granny). Then why have all of that extra 225 width? You are not going to use it. The extra width and weight of the tires is costing you fuel.

If you want the fuel savings with lower RPMs at highway speed you should have bought a car with overdrive (5th gear) instead of trying to lower RPM with tires that are too large. And what about all that useless weight you are hauling around with 4 oversized 225/70/15 tires?

Just don't hit the accellerator so hard and if you have cruise control for the highway stints, use it. This will save fuel. I'm sure you have heard it before, but you can save fuel by keeping tires properly inflated.

Given the kind of driving you do I'd highly recommend the 195/60/15 Kumho Solus KH16. Look at the test results at Tire Rack. Some of the Kuhmo tires can be an excellent value. I just finished putting a new set of Solus KH21s on the minivan today and so far they are just great.

http://www.tirerack.com/tires/tests/...lay.jsp?ttid=5
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Old 07-18-2009, 06:16 PM   #4
Roland Saul
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You're absolutely right Yzer, sorry I don't know why I thought that. I had those numbers in my head for some reason.... Anyway, they're actually 195/60/15. Thanks, I didn't mean to waste your time guys...

And I should have been clearer, it is a 5 speed but 5th doesn't really feel undergeared as much, it's a fair highway gear with barely adequate passing torque, but at my normal highway speeds if I'm passing anyone it isn't usually at high speed, more that they are doing 40 on the highway. [/i]

I'd like to be able to use 4th more as a punchy passing gear, but the way this car is geared takes it to close to 4 grand for me to like to use it much at anywhere near highway speed. I like to it at between 2200 and 2600 on the highway, which gets me from 50 mph to about 100kph in 5th. Fits my needs perfect for cruising, so 5th isn't a problem, but it would be nice to take a couple hundred off it without getting into transmission fabrication.

THIS is the main reason why I'm looking into slightly higher profiles because when I do the high school math that I never thought I'd use, lol... it appears to me that with a higher profile I put more physical distance down on the ground per rev of the motor. Is is worth slightly heading this way with a larger profile, ESPECIALLY if there is ride and mileage benefits? I definitely could lean towards a skiddier tire, it would have little impact on my real driving needs. It's rare that I squeal or chirp the tires in my hardest cornering.

I'm just wondering really if anyone has noticed any appreciable benefits of going taller and narrower, it's almost always the other way round. I don't need any more acceleration, in fact I would gladly trade 10 percent of the acceleration speed for a more realistic powerband. I mean this car is up to 28 million rpm in first before it makes it across the intersection. I've never driven a car geared down like this, and frankly I wish it was more sedate in the lower 4 gears. It's got enough torque to still feel strong in the low and mid 2's, ideally I'd like to be able to use that range as wide as possible instead of going through the whole gearbox in half a city block.

Would I really be able to feel it through the pedal, Yzer , if I went to 205/60/16?? Would it be dramatic do you think?
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Old 07-18-2009, 07:30 PM   #5
yzer
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I'm not sure if the gearing is the same on your TS as it is with my 2003 WRX. I actualy like the gearing and often take the car from a traffic metering standstill from 1st to 3rd (65 MPH) and pop it into 5th. Actually, the car gets it's fastest 0-60 just using 1st through 2nd.

Still, even with the WRX turbo engine I have no problem following a loaded semi out of a red light. The car isn't going to go any faster than I want it to.

Regarding your idea of oversized profile tires affecting RPM, etc. Yes if you oversize the profile and have a tire with increased overall diameter then the tire is going to travel more ground with less RPM from the engine. You can do the same thing by selecting a higher gear though. It's not a good idea to lug the engine more than needed when it's asked to work under increased load, like accelleration or climbing a grade. That's why shorter gearing is considered easier on the engine's lower end under higher loads. The WRX actually gets worse MPG climbing stiff grades in 5th (overdrive) and lower RPM than in 4th with higher RPM because here an engine lugging at 3000+ RPM is more likely to open the wastegate and engage the turbo.

What is a realistic powerband? These are 4-cylinder cars, not 6 or 8-cylinder and generally run higher engine RPMs for a given speed.

If you want lower RPM and don't mind less performance, shift gears earlier and use the highest gear practical for a given speed. Trying to do the same thing with an oversized diameter tire may work but it also adds the big handling and MPG disadvantage of higher unsprung weight.

Generally, the lower the profile the harder the ride. You can counter this trend by selecting an easy riding tire. Generally, you get widest performance advantages to using larger wheels and lower profile tires if the wheel/tire combination is lighter or close to the same in weight as stock and offers the same or larger tread width. You probably won't notice any performance advantages in turns with a 10mm increase in width. You will notice a difference with 20mm. Are you going to feel any difference in the pedal between 195/60/15 and 205/60/16? If both wheel/tire combinations weigh the same, probably not. Your speedometer will read 2% too slow, though.

Last edited by yzer; 07-18-2009 at 07:47 PM.
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Old 07-18-2009, 07:45 PM   #6
Roland Saul
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Wow dude thanks for the insight. Maybe I'm looking for something that just doesn't exist realistically.

Huge difference in the WRX by the sounds of it. The TS runs up to about 4000 rpm WAY too much for me unless I'm in 5th gear almost all the time, 1-4 are strictly accelleration. I guess what I'm trying to do is get the car to idle through life and last forever haha. I very rarely will notice it's lugging, I'll always go to 4th gear when it drops to about 2k, and it SEEMS to be running smoothly.. I hope I'm not overtorquing the tranny parts because it seems to hum perfectly between 2k and about 3500, then I feel as if I'm getting less bang per rev. It'll do 4k very comfortably but the mileage seems to go to hell after that, and I'm hoping to baby the car, because it's the best all round car I've ever had.

I fear that if I buy another one in a couple of years, Subaru will have gone and distorted the design somehow and it will be harder to find another later model with the same feel as the 02. I read a few complaints here about the move from 2.2 liter motors, and the "old school" impreza's, I guess I'll never know the feeling of the 2.2 hehe unless I find an older one around here someday. Anyways thanks for the info man, greatly appreciated as I know very little about tire geometry vs. performance. I keep them as new and clean as I can and check the pressure all the time, that's all I know.
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Old 07-18-2009, 08:18 PM   #7
yzer
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The power band on the 2.0 liter turbo starts falling off around 5700 RPM and redlines at 7K. Don't worry about higher engine RPMs and engine longetivity on these Subaru flat-fours. They are well-designed engines.

I had a 1990 Mitsu Eclipse GS Turbo for 13 years and put 274K miles on it. The car was in good shape when I sold it. That was another well-designed 200HP four-banger (the same Mitsu turbo is used on the WRX.) Change the oil and filter every 3-4K miles and they last a long time.
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