How far off is speedo with Camaro 20s? - Pontiac G8 Forum: G8 Forums - G8Board.com
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post #1 of 23 (permalink) Old 07-22-2010, 09:36 AM Thread Starter
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How far off is speedo with Camaro 20s?

A take it a tune is the only way to correct the speedo?

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post #2 of 23 (permalink) Old 07-22-2010, 09:43 AM
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Are they a taller/shorter diameter than stock?


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post #3 of 23 (permalink) Old 07-22-2010, 10:34 AM
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Type in your numbers, bud.

http://www.miata.net/garage/tirecalc.html

To make the camaro rims "fit" a g8 (without rubbing), the standard camaro tires are replaced by lower profile tires.

Thus, it's closer to a 19" stock g8 tire, generally speaking.

Plug in your stock numbers. For example, I have 245-45-18's which are still my stock RS-A's on stock rims. Now find what others are wrapping their camaro rims with, 275-30-20's ring a bell, not sure though.

Using that formula... compared to "stock", your aftermarket tire&rim is causing your speedo to be reading 0.7% too fast. thus, when you're going 60 mph according to your speedo, you're really going 59.4mph.


If you chose to run 275-35-20's on the camaro rims (slightly larger sidewall than the 30-series tire) then it goes the other way. Since the overall tire height is higher with that combination, your speedo is now reading 3.4% too slow. Thus, when you're going 60mph according to your speedo, you're really going 62.0mph. Be careful speeding with this size tire...

Hope this helps. You can use that miata.net calculator for any combination of tires to compare, it's really the best one out there!

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Last edited by zepcom; 07-22-2010 at 11:25 AM. Reason: color code to seperate two comparisons
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post #4 of 23 (permalink) Old 07-22-2010, 10:51 AM
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Thanks man! I was just trying to figure this out yesterday!
Appreciate it!

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post #5 of 23 (permalink) Old 07-22-2010, 10:44 PM
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Yup, as long as you get the right tires. Any size rim will make the speedo stay correct.

Truthfully, its no big deal for it to be 1-2mph off. My GTP is and never had issues.

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post #6 of 23 (permalink) Old 07-22-2010, 10:51 PM
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My 275-35-20's on my Camaro wheels make it about 2 mph slow at 65 by running side by side with my dad (I wanted to test this when I got them). Zep's info appears spot-on.
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post #7 of 23 (permalink) Old 07-22-2010, 10:57 PM
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Quote:
Originally Posted by cgbr4d View Post
My 275-35-20's on my Camaro wheels make it about 2 mph slow at 65 by running side by side with my dad (I wanted to test this when I got them). Zep's info appears spot-on.
ofcourse man. Thats why its a wheel/tire calculator. It will never be wrong. It will tell you correct answer no matter what brand, size, width tire/wheel you have.

I like this one too.
http://www.rimsntires.com/rt_specs.jsp

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post #8 of 23 (permalink) Old 07-22-2010, 11:13 PM
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I was not doubting it at all. I was just chiming in with my own personal experience.

The MPH difference is minor but something to keep in mind just in case.
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post #9 of 23 (permalink) Old 08-24-2010, 05:18 PM
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Due to our sue-happy culture, most factory speedos read a little fast anyway (if you found you were going 1mph faster than the speedo said it would be "unsafe", and you could sue the car maker, but the other way around is "safe"), so going to a rear tire 2% larger may actually CORRECT your speedo. That is exactly what I found when I changed the tires/wheels on my S2000. The slightly larger rear tires just about exactly corrected the speedo. I tested it using GPS before and after.

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post #10 of 23 (permalink) Old 08-24-2010, 06:17 PM
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Wouldn't replacing the tire pressure sensor when getting the 20" Camaro rims correct the problem?


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post #11 of 23 (permalink) Old 08-25-2010, 12:36 AM
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Quote:
Originally Posted by Mike M. View Post
Wouldn't replacing the tire pressure sensor when getting the 20" Camaro rims correct the problem?
Completely unrelated.

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post #12 of 23 (permalink) Old 12-15-2010, 10:41 PM
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Originally Posted by Ridin ON Chrome View Post
ofcourse man. Thats why its a wheel/tire calculator. It will never be wrong. It will tell you correct answer no matter what brand, size, width tire/wheel you have.

I like this one too.
http://www.rimsntires.com/rt_specs.jsp
Awesome link man!
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post #13 of 23 (permalink) Old 12-15-2010, 11:25 PM
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Originally Posted by Mike M. View Post
Wouldn't replacing the tire pressure sensor when getting the 20" Camaro rims correct the problem?
I'll bite...why? I'm guessing because the TPMS system maybe measures the RPM of the sensor and gets MPH from that??? That's the only thing I can think of....really not being a jerk, just interested in the thought process

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post #14 of 23 (permalink) Old 12-16-2010, 08:50 AM
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Originally Posted by 2dbatcv View Post
I'll bite...why? I'm guessing because the TPMS system maybe measures the RPM of the sensor and gets MPH from that??? That's the only thing I can think of....really not being a jerk, just interested in the thought process
No, the sensors just "sense" pressure, and relay that info to your car. You could build a pressure chamber out of 6" PVC pipe, put all 4 sensors inside, seal it up, and pressurize it to 35psi, put it in your trunk, and everything would be good.


Just bothers me when the OP never returns back to a thread and answers questions or thanks others for helping them.


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post #15 of 23 (permalink) Old 12-30-2010, 05:40 AM
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With 245/35's up front and 275/35's on the rear, and according to my iphone speedometer, I have found it to be only about 2 mph faster @ 75 mph!

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post #16 of 23 (permalink) Old 12-30-2010, 03:18 PM
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With 245/35's up front and 275/35's on the rear, and according to my iphone speedometer, I have found it to be only about 2 mph faster @ 75 mph!
Overall diameter changes from 26.7" to 27.6" on that size rear tire. Speed difference SHOULD calculate out to 3.3%, or 62MPH at 60MPH actual.

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post #17 of 23 (permalink) Old 12-31-2010, 09:44 AM
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Just bothers me when the OP never returns back to a thread and answers questions or thanks others for helping them.
He's a zookeeper and the animals are escaping! (They're running for office).
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post #18 of 23 (permalink) Old 02-26-2011, 03:31 PM
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Nice calclulators! So I could actually run a 225.60.16 tire for winter tires. This would be a little cheaper than buying 18's and maybe open the door to using steel wheels. The only question that just came to mind is will 16" wheels clear the brakes?
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post #19 of 23 (permalink) Old 02-26-2011, 08:04 PM
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Nice calclulators! So I could actually run a 225.60.16 tire for winter tires. This would be a little cheaper than buying 18's and maybe open the door to using steel wheels. The only question that just came to mind is will 16" wheels clear the brakes?
No, they won't. Smallest you can go is a 17" wheel.

I run a 225/55/17 during winter on 17x8" wheels

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post #20 of 23 (permalink) Old 03-03-2011, 08:19 AM
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THANK YOU! So one thing ruled out on why my car got slower once the wheels and exhaust.
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