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914World.com _ 914World Garage _ Illustrated Lecture on Tire Size

Posted by: Porsche Rescue Mar 4 2005, 11:10 PM

Members are often asking tire size questions and I usually respond by advising that they stay with something very close to the original equipment size. Unless the car is going to be frequently autocrossed or tracked I believe it is the best way to go. You don't change the gearing, create an inaccurate speedometer, reduce fuel economy or have to worry about clearance issues.
Today I replaced the tires on my 914-6 recently acquired from Eric Shea. He sent along a set of OE option 5.5x14" Fuchs ( beautifully polished by Eric). The car came to me with 6x15 Fuchs with 195/50x15 Dunlops.
Since the car will be a Sunday driver, cruiser (1000 miles to WCC '05 I hope), and since I bought the car for its originality, I wanted as close to stock tire size as possible. Sixes with the 14" Fuchs had 185x14 tires (same wheel/tire combo available on 911's as the "comfort"option). They are pretty much NLA. Their diameter was 25.6" with a cross section of 7.4". I bought a set of Michelin X 195/75X14's at Costco (used on Toyota Camrys and mini-vans!), diameter 25.4" and cross section 7.7". Close enough.
Here is a pic of the 195/50x15's beside the 195/75x14's. The 50 series, even though on a 1 inch taller wheel are nearly 3" shorter with a diameter of 22.7". No wonder gear ratios are changed (maybe a good thing for autocross) speedo is way off and fuel economy suffers.
Also, much less cushion and thus harsher ride (but more precise cornering) with the low profile.
All of the above applies to 4's as well. They came with 165x15 (as did the six with standard 15" steel wheels) which is exactly the same diameter as my 195/75's. The best modern size which is close is 195/65x15 (25" tall).

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Posted by: Porsche Rescue Mar 4 2005, 11:11 PM

Also, I wanted the 1970 look, wheel wells full of rubber. The front is the old 50 series 15, rear is the new 75 series 14.

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Posted by: redshift Mar 5 2005, 12:22 AM

Excellent illustration! I agree totally.

If I lived on a mountain, I might think it over.

M

Posted by: rick 918-S Mar 5 2005, 12:46 AM

See, I like the wheel wells full of rubber look too!

Posted by: MarkV Mar 5 2005, 12:58 AM

50 series tires are extremely short. I am running 205/60-15's on stock 5.5" Fuchs. They are .71" shorter than the stock tires at 24.68" & they run 2.8% faster.

rocking nana.gif

Posted by: Rhodes71/914 Mar 5 2005, 10:30 AM

I willl be putting on a set of Falken Ziex Ze-512's, size 105/60-15, next week. Should be a lot better than the mis-matched used ones I put on just so I could roll the car around.

Posted by: Porsche Rescue Mar 5 2005, 11:24 AM

Your 205/60's will be about .75" smaller in diameter. You will be a little lower geared and your speedo will read about 3% fast. No big deal in trade for more crisp handling and 40mm more tread on the road than stock. I have never had trouble fitting 205's but some cars require massaging the fender lip on one side or the other.

Posted by: Eric_Shea Mar 5 2005, 05:54 PM

That looks gay... I should have never sold you that car

Seriously, looks REALLY good Jim!

I can't wait to go for a ride. I'll bet it feels a ton different.

The car was a drive around town car so with the 195/50's it was actually a ton of fun. I love that profile for a more modern look. It think it's the perfect tire to fit under a stock wheel well for a teener. You do throw off the speedo (but you don't get in trouble with the law) It was reading about 7mph over best I could judge

HOWEVER... those 14's look great on there now. Way ta go.

Posted by: Aaron Cox Mar 5 2005, 05:56 PM

i like my 205/50's myself.... 10% lower gearing.... great for whoelshots and cornering...... not optimal on the freeway though

but....my sidewalls dont tuck under in corners!

Posted by: Rhodes71/914 Mar 5 2005, 07:19 PM

QUOTE (Rhodes71/914 @ Mar 5 2005, 08:30 AM)
I willl be putting on a set of Falken Ziex Ze-512's, size 105/60-15, next week. Should be a lot better than the mis-matched used ones I put on just so I could roll the car around.

I meant 195/60, I think this should be a good all around size.

Posted by: Eric_Shea Mar 5 2005, 10:38 PM

205's didn't fit on that car.

That's why I endore the 195's. I'd bet they're sure to fit "any" teener. They do have all those nasty side effects that Jim talked about though... speedo off (I think you can get that recalibrated). Fuel and gearing issues...

I used to use 205/60x15's all around on my stock bodied 71, 911. Almost a perfect match for the stock tire heigth. I tried those on the 914-6 and they rubbed on the rear passengers side.

Posted by: MattR Mar 5 2005, 10:43 PM

QUOTE (Eric_Shea @ Mar 5 2005, 08:38 PM)
205's didn't fit on that car.

Someone get the baseball bat, we're gonna roll some fenders

Nice pics! I have 205 50s on the 914, but Ive never driven it, so I cant tell you how they perform... Ive got some 205 60 15s on their way for the 912, but again... on their way, so I cant compare. Im not very useful, am I

Posted by: markb Mar 6 2005, 01:15 AM

I've got 205 60 15s on the Blue car, but I'm thinking of 205 70 15s for the Black car. Think they'd rub?

Posted by: Porsche Rescue Mar 6 2005, 10:16 AM

If the 60's have a little clearance on both sides you should be ok with 70's. The width is the same but the "fat" part of the tire will be a little higher because the tire is taller. Could be close.
Diameter will be same as stock so speedo and gearing will be correct.

Posted by: Rhodes71/914 Mar 6 2005, 10:29 AM

I have some 205/70's on the back of my 71 and they are real close to the fender, like I can barely get my finger in between. I think they also look big in there.

Posted by: Aaron Cox Mar 6 2005, 10:31 AM

im telling you....low profile tires TREMENDOUSLY help handling....

if you are worried about rolling diameter, go to a 16" wheel with a 50 or 55 series.....really wakes the car up!

i went the tall tire route.... never again!

Posted by: DuckRyder Mar 6 2005, 10:46 AM

Some common ones:

Specification Sidewall Radius Diameter Circumference Revolutions Speedometer Odometer Difference

165/80-15 5.2" 12.7" 25.4" 79.8" 794/mi 60MPH 10000mi N/A
185/65-15 4.7" 12.2" 24.5" 76.9" 824/mi 62MPH 10378mi -3.6%
205/60-15 4.8" 12.3" 24.7" 77.6" 817/mi 62MPH 10287mi -2.8%
205/50-15 4.0" 11.5" 23.1" 72.5" 874/mi 66MPH 11007mi -9.1%
195/75-14 5.8" 12.8" 25.5" 80.2" 790/mi 60MPH 9952mi 0.5%

From: http://www.powerdog.com/tiresize.cgi

I personally kinda like the 185/65/15 on a street car.

Posted by: Porsche Rescue Mar 6 2005, 11:01 AM

16's on a 5-lug car are great. You can have best of both worlds. Where do you find sixteens for a 4-lug car?

Posted by: Aaron Cox Mar 6 2005, 11:03 AM

QUOTE (Porsche Rescue @ Mar 6 2005, 10:01 AM)
16's on a 5-lug car are great. You can have best of both worlds. Where do you find sixteens for a 4-lug car?

real cars have 5 lugs

Posted by: Eric_Shea Mar 6 2005, 05:49 PM

QUOTE
real cars have 5 lugs

Buy them here

Posted by: Aaron Cox Mar 6 2005, 06:44 PM

QUOTE (Eric_Shea @ Mar 6 2005, 04:49 PM)

QUOTE
real cars have 5 lugs

Buy them here

L O effin L!!