Tyre size discrepancy - Pontiac GTO Forum
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post #1 of 12 (permalink) Old 08-19-2013, 04:03 PM Thread Starter
 
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Tyre size discrepancy

I have a 225/70R14 tyre on American racing rim, but if I measure the diameter it's about 25" with wheel off the ground yet the specs suggest it should be 26.4". Wheres that 1.4 inches gone, or am I being stupid?! The tyre is worn more in the centre also.

Reason I care is I'm moving from a 3.73 to either a 3.08 or 3.23 rear diff, as currently it's running about 60mph at 3000rpm and not pleasant in the slightest to drive on freeway, and quite jumpy around town (455ci). It's a 4 speed hurst short shift.

Thanks for any guidance.


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post #2 of 12 (permalink) Old 08-19-2013, 09:16 PM
 
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Yes, the 225-70/14 should be about 26.4" diameter.
Switching to either the 3.08 or the 3.23 will make freeway cruising much more enjoyable and you'll still have fun at the green lights.

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post #3 of 12 (permalink) Old 08-19-2013, 09:33 PM
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At 60mph I'm doing 2400rpm's with a 3.55's and 295/50/15s and the 4-speed.
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post #4 of 12 (permalink) Old 08-20-2013, 06:44 AM Thread Starter
 
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Thanks guys. I can't figure out why my 225/70R14 tyre measures 25", but it absolutely does, unless there's a US/European difference in tyre dimensions. FYI I'm in the uk. Tyres are cooper cobra radial gt. If I plug 25" and 3.73 gear ratio into the rpm calculator it's bang on what my engine at 60mph is doing, so 25" is right, but I'm confused why its not 26.4"?!

What is the measured diameter of your 295 15" rear wheel? Could you pm me a pic? I may go for 3.23 gear and bigger diameter and fatter tyre as these cooper tyres are a bit thin looking and im concerned theres a dinension not quite right if its only measuring 25", so 3.08 and larger / correct diameter tyres may make green lights dull.


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post #5 of 12 (permalink) Old 08-20-2013, 08:02 AM Thread Starter
 
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Could it be the tyre is / has been over inflated which had caused side wall to expand, and overall wheel diameter to reduce? Tyres are worn in the centre.


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post #6 of 12 (permalink) Old 08-20-2013, 11:21 AM
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My tires are about 26" tall.
Wish they still had that much tread left!!
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post #7 of 12 (permalink) Old 08-20-2013, 11:48 AM Thread Starter
 
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They look the biz, yet in keeping with the car. What is the rim make/brand called?


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post #8 of 12 (permalink) Old 08-20-2013, 12:05 PM
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They are called Cragar SS. Bought them from Summit Racing.
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post #9 of 12 (permalink) Old 08-21-2013, 02:17 AM Thread Starter
 
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Thanks Rukee. I'm going to check if I can get American racing torq thrust wheels bigger at the rear and swap current 225 rears to front. Would you advise running a 15" wheel on rear and 14 on front, with approx 1" overall diameter difference? Can't see an issue myself and don't want steering to get heavier as its borderline as is! (Unsure if there's a quick ratio steering box option).
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post #10 of 12 (permalink) Old 08-21-2013, 12:31 PM
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That's what I'm running, 14" on front and 15" on the rear. Gives the car that instant Muscle Car stance!!
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