What do 18's do to the speedometer?
TECH Addict
Joined: Mar 2002
Posts: 2,189
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From: Glendale, CA
Re: What do 18's do to the speedometer? It depends...
I was going from a 245/50/16 tire to a 275/35/18 and it is off by a VERY small amount - totally negigible. I used this calculator to figure it out:
http://www.superford.org/garage/calculators/
--> Scroll down to Tire Conversion
If it's off by a big amount, say more than 4mph (use the calculator underneath the converter), go get the speedo recalibrated at teh dealership, or LS1Edit, or even an HPP3.
Hope that helps!
I was going from a 245/50/16 tire to a 275/35/18 and it is off by a VERY small amount - totally negigible. I used this calculator to figure it out:
http://www.superford.org/garage/calculators/
--> Scroll down to Tire Conversion
If it's off by a big amount, say more than 4mph (use the calculator underneath the converter), go get the speedo recalibrated at teh dealership, or LS1Edit, or even an HPP3.
Hope that helps!
Re: What do 18's do to the speedometer? a 275/35ZR18 tire is the exact same size overall diameter as the 275/40ZR17 tire factory on an SS.
I am running 18" all the way around, 18" x9.5" w/ 275/35ZR18 front and 18" x 10.5" w/ 295/35ZR18 rear. The od of the rear tire is a fraction of an inch taller than OEM, so the ODO reads kust a touch slow, like 1 mph slow when reading 60 mph.
I am running 18" all the way around, 18" x9.5" w/ 275/35ZR18 front and 18" x 10.5" w/ 295/35ZR18 rear. The od of the rear tire is a fraction of an inch taller than OEM, so the ODO reads kust a touch slow, like 1 mph slow when reading 60 mph.
Re: What do 18's do to the speedometer? Here are tire size diameters:
245/50/16 = 25.64" tall. OEM F-body size
275/40/17 = 25.66" tall. OEM F-body size
275/35/18 = 25.58" tall. "Plus one" size
275/40/18 = 26.66" tall OEM C5 size
295/35/18 = 26.12 tall
Keep in mind with a .50" taller tire, you not only change the change the rear ratio slightly, but combined with a heavier weight tire and wheel
you will add rotating mass which will cost some hp. With that said I run 18"s on the street and switch to ET streets for track.
245/50/16 = 25.64" tall. OEM F-body size
275/40/17 = 25.66" tall. OEM F-body size
275/35/18 = 25.58" tall. "Plus one" size
275/40/18 = 26.66" tall OEM C5 size
295/35/18 = 26.12 tall
Keep in mind with a .50" taller tire, you not only change the change the rear ratio slightly, but combined with a heavier weight tire and wheel
you will add rotating mass which will cost some hp. With that said I run 18"s on the street and switch to ET streets for track.
