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Author
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Topic: 14" Wheels vs 15" Wheels?
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-21-2006 08:20 AM
 
I took my Pony from 14 inch wheels to 15 inch wheels. In regards to the speedometer, if it indicates 5o mph, how fast do you think I'm really traveling?
Anyone have any thoughts?
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Blacksmith
Gearhead Posts: 604
From: Front Royal, Va., USA
Registered: Feb 2006
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posted 11-21-2006 09:01 AM
Depends on outside tire diameter, rear end gear ratio & # of teeth on the speedo. cable trans. gear.Mine with 225/60R15's, 3.00:1 rear & 17 tooth gear is running about 5mph fast: I have this "cheat sheet" taped to inside of the windshield by tach.
RPM MPH
1200 = 25
1400 = 30
1600 = 35
1800 = 40
2000 = 45
2200 = 50
2400 = 55
2600 = 60
2800 = 65
3000 = 70
3200 = 75
3400 = 80
3600 = 85
3800 = 90 Check this site out, it might help you:
http://www.4lo.com/4LoCalc.htm
[This message has been edited by Blacksmith (edited 11-21-2006).]
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-21-2006 09:13 AM
Blacksmith, Thanks! I'll check this out.
I'm curious to know, that when my speedo was indicating 120 mph, how fast I really was going.
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68 Coop
Gearhead Posts: 5847
From: Mesquite, NV. 89027
Registered: Oct 2004
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posted 11-21-2006 09:15 AM
I don't think it affected it by more tha a couple MPH. The gear change in my car changed it a lot, I went from 279's to 355's, and now the speedo is off by about 10 mph now.------------------
William
M&M# 4256
MCA# 64831 68 Coupe
289
C4
3:55's/Trac-Lok
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-21-2006 09:21 AM
I'm going to have to get my tire measurements once I get back home and work with the program Blacksmith gave me. Looks like a great tool.
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indyphil
Gearhead Posts: 3394
From: Senoia, G.A. USA
Registered: Jul 2002
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posted 11-21-2006 09:27 AM
Like he said by switching to 15 inch wheels you may not have changed the tire diameter. In most cases when people "plus size" the wheels the outside diameter changes very little.Its possible to calculate "nominal" tire diameter by reading your tires. Using the modern metric notation:
225/60R15 means 225mm wide, aspect ratio of 45, on a 15 inch wheel. The aspect ratio compares the sidewall with the width. so with a 225mm tire a 60 ratio means a 225x0.6 = 135mm sidewall. 135mm = 5.3 inches. 15 inch wheel + 5.3 + 5.3 (you must add the sidewall twice to get total diameter) = 25.6 inch tires. to know if your speedometer is honest you must compare with the standard 185/75/R14 tires. which were about 25 inches tall. BUT the main thing to think about is your rear end ratio. My 2.79 was replaced with a 3.00, my speedo reads about 5mph high at 70mph. if your rear end ratio is original (matches the little drive gear in the transmission for your speedo) then your speed is pretty close to honest. I have heard others complain that even with the matching gears in their rear end and transmission the old speedos read a little fast compared with actual speed. I have not found this to be the case with mine - mine seems honest with the exception of the adjustment for the gear ratio.
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Blacksmith
Gearhead Posts: 604
From: Front Royal, Va., USA
Registered: Feb 2006
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posted 11-21-2006 11:47 AM
quote:
Originally posted by rca:
when my speedo was indicating 120 mph, how fast I really was going.
WAY TOO FAST!!! Here's another site to help calculate what speedo. cable gear you need.
http://www.bgsoflex.com/speedo1.html
[This message has been edited by Blacksmith (edited 11-21-2006).]
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-21-2006 02:25 PM
Thanks.....and you're right, it was way too fast. ------------------
Step On It Until You Smell Stink!
www.randrews.mustangsandmore.ws
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-21-2006 05:14 PM
Here's a formula.
15 divided by 14 =, times speed.
Example:
15 divided by 14 = 1.07 x 120 mph = 128.4 mph.
This is a formula a coworker swears by.------------------
Step On It Until You Smell Stink!
www.randrews.mustangsandmore.ws
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-21-2006 06:42 PM
The OD of the 15" tires are the same as the 14" tires. So the speed should be the same? ------------------
Step On It Until You Smell Stink!
www.randrews.mustangsandmore.ws
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3154tm
Journeyman Posts: 37
From: Pensacola, Fl
Registered: Feb 2006
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posted 11-21-2006 07:07 PM
here's another calculator
http://www.geocities.com/z_design_studio/
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68 S-code GT
Gearhead Posts: 3835
From: Sayreville, NJ, US
Registered: Mar 2000
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posted 11-21-2006 07:20 PM
quote:
Originally posted by rca:
The OD of the 15" tires are the same as the 14" tires. So the speed should be the same?
Yup!
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RogueS
Gearhead Posts: 120
From: Wichita, KS, USA
Registered: Oct 2004
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posted 11-21-2006 07:28 PM
quote:
Originally posted by rca:
Here's a formula.
15 divided by 14 =, times speed.
Example:
15 divided by 14 = 1.07 x 120 mph = 128.4 mph.
This is a formula a coworker swears by.
That's more flawed than a SBC in a Ford.
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indyphil
Gearhead Posts: 3394
From: Senoia, G.A. USA
Registered: Jul 2002
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posted 11-22-2006 08:05 AM
quote:
Originally posted by RogueS:
That's more flawed than a SBC in a Ford.
Agreed dont listen to that co-worker about any car related problems again...
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-22-2006 10:07 AM
I'm going to get it the hwy tomorrow and have my son beside me in the truck and go different speeds. I have no idea what the gear ratio is. It's a '66 pony coupe, originally 289 2 brl. w/C4 automatic transmission. I rebuilt the engine and trans and put a 4 brl. on it. Does anyone know the rear end I might have in it?
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68 Coop
Gearhead Posts: 5847
From: Mesquite, NV. 89027
Registered: Oct 2004
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posted 11-22-2006 10:20 AM
I would lean towards the 2:79's, but there's a simple way to tell for sure. Mark the driveshaft and one of the tires with tape, spin the tire 1 revolution and count how many times the tape on the driveshaft spins.------------------
William
M&M# 4256
MCA# 64831 68 Coupe
289
C4
3:55's/Trac-Lok
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-22-2006 11:23 AM
68 Coop, I'll check that out. Right now, it's more of a highway car. We need to find some time to mess with the timing a bit to see what we can come up with as far as being quicker. We wanted to run the engine some to loosen it up. It's different than the day we got it running for sure. I'll let yall know what we come up with.
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rca
Journeyman Posts: 45
From: Dallas, Texas, USA
Registered: Nov 2006
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posted 11-25-2006 08:08 AM
Speedo is still calibrated correctly with the new wheels. I'm glad to know that when I was going 120 mph, I was just going 120 mph. Dang........I would had hated going any faster.------------------
Step On It Until You Smell Stink!
www.randrews.mustangsandmore.ws
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Ryan Wilke
Gearhead Posts: 3237
From: Stanton, Michigan, zip 48888
Registered: Oct 2000
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posted 11-25-2006 08:28 AM
quote:
Originally posted by rca:
Dang........I would had hated going any faster.
Yeah, RIGHT!!!  We all HATE that!
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