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How far off is speedo with Camaro 20s?

24K views 29 replies 22 participants last post by  Trenton Man 
#1 ·
A take it a tune is the only way to correct the speedo?
 
#2 ·
Are they a taller/shorter diameter than stock?
 
#3 · (Edited)
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... :judge:

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!

--zep
 
#4 ·
Zep
Thanks man! I was just trying to figure this out yesterday!
Appreciate it!
 
#6 ·
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.
 
#7 ·
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
 
#8 ·
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.
 
#9 ·
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.
 
#10 ·
Wouldn't replacing the tire pressure sensor when getting the 20" Camaro rims correct the problem?
 
#15 ·
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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#16 ·
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.
 
#18 ·
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?
 
#19 ·
No, they won't. Smallest you can go is a 17" wheel.

I run a 225/55/17 during winter on 17x8" wheels
 
#24 ·
For Camaro 20's (I am shopping for them), I keep seeing 275/30-20 as the choice. 285/30-20 is an exact match to the OEM 19's at 26.7." Why isn't this size the one being used? Is there a rubbing problem due to the offset?
 
#25 ·
9" wide wheel, recommended treadwidth on most all tire manufacturers for a 9" wide wheel is 275mm wide tire.
 
#27 ·
I understand that (the ride).
 
#28 ·
Got a set of 'maro 20s with 245-35/275-30 Nittos G2. No rubbing on stock suspension. However, the speedo is off by 3mph on average at highway speed. I feel like the biggest issue is not even the weight of these rims, but rather the fact that 20s gear the whole car up and with the stock 2.92s it's just slow off the line. Needs 3.45 swap badly. The rims look dope tho...
 
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