Go Back   Corvette Forum > C4 Corvettes, 1984 - 1996 > C4 Tech/Performance
Sign in using an external account
Register Forgot Password?
Register Search Today's Posts Mark Forums Read FAQ Vendor Directory
Search
C4 Tech/Performance
L98 Corvette and LT1 Corvette Technical Info, Internal Engine, External Engine Sponsored by
J&D Corvette

Welcome to Corvetteforum.com!
Welcome to Corvetteforum.com.

You are currently viewing our forum as a guest, which gives you limited access to view most discussions and access our other features. By joining our community, at no cost, you will have access to post topics, communicate privately with other members (PM), respond to polls, upload content and access many other special features. Registration is free, fast and simple, join Corvetteforum.com today!


Corvette Store
 
 
C7 Parts & Accessories
C6 Parts & Accessories
C5 Parts & Accessories
C4 Parts & Accessories
C3 Parts & Accessories
C2 Parts & Accessories
C1 Parts & Accessories
Wheels & Tires
Sponsored Ads
 
 
Vendor Directory
  
Reply
 
 
 
 
Thread Tools Search this Thread
Old 12-05-2012, 05:52 PM   #1
C4in mesa
CF Senior Member
 
C4in mesa's Avatar
 
Member Since: Dec 2011
Location: mesa az
Default 1988 C4 Wheel size ? calibrated for the speedo

I know my car was sold with 16" wheels and I am pretty sure Chevy put 17" wheels before the production year was over. My question is how much if any difference is there running 17" wheels versus 16" wheels regarding the speedometer ?
C4in mesa is offline   Reply With Quote
Old 12-05-2012, 06:03 PM   #2
c4cruiser
CF Senior Member
NCM Sinkhole Donor
Support Corvetteforum!
 
c4cruiser's Avatar
 
Member Since: Dec 1999
Location: Lacey, Washington RVN '68-'69
Default

The 88 was the only year to offer two wheel sizes; the 16" diameter wheel was stock and the 17" wheel was optional. The OEM size tires (255/50-16 and the optional 275/40-17) were very close in diameter so there really wasn't a lot of difference in the speedo readout. And the two tires has very close to the same treadwidth.
c4cruiser is offline   Reply With Quote
Old 12-05-2012, 06:12 PM   #3
leesvet
CF Senior Member
 
leesvet's Avatar
 
Member Since: Apr 2011
huh uh-huh
Default

yep...

the biggest difference was in the size of the hole in the middle of the tire...not the circumference of the outside diameter.

Think of it like this....the more extreme the easier it is to visualize.

a tire thats a given size, can have a rim of much different size and NOT change the distance traveled in 1 revolution. It can have a 5" rim or a 17" rim...as long as that outside stays about the same. Its so little, so minor that its not noticable. You have tread wear and that takes 1/4" off each side or radius, so about 1/2" of diameter is gone by the time the tire wears out but it has almost no effect on the speedo..
leesvet is online now   Reply With Quote
Old 12-05-2012, 06:39 PM   #4
RollaMo-LT4
CF Senior Member
St. Jude Donor '10-'11-'12-'13-'14
 
RollaMo-LT4's Avatar
 
Member Since: Oct 2005
Location: Rolla Missouri
Default

As noted, the wheel size really isn't what matters.
It is the overall diameter of the tire.

255/50-16 = approx. 26.04" in diameter
275/40-17 = approx. 25.66" in diameter

That is only 0.38" difference.
With the speedometer reading 60mph, you will really only be going 59.1mph with the 274/40-17 tires.
__________________
1996 Corvette - LT4
2011 Camaro SS Convertible
2011 Harley Davidson FLHX Street Glide
RollaMo-LT4 is online now   Reply With Quote
Old 12-05-2012, 10:15 PM   #5
James93LT1
CF Senior Member
 
James93LT1's Avatar
 
Member Since: Oct 2004
Location: Mililani Hawaii
Default

http://www.corvetteforum.net/c4/vett...Gearshtm.shtml
James93LT1 is offline   Reply With Quote
Old 12-05-2012, 10:30 PM   #6
vetteoz
CF Senior Member
 
vetteoz's Avatar
 
Member Since: Jan 2004
Default

Quote:
Originally Posted by James93LT1 View Post


The factory thought there was enough of a difference to install different speedo gears for 16" or 17" tires

Tire Ratio Drive gear Driven gear
P255/50ZR16 Tires 2.59 Red 17 Orange 35
P275/40ZR17 Tires 2.59 Red 17 White 36
P255/50ZR16 Tires 2.73 Red 17 Red 37
P275/40ZR17 Tires 2.73 Red 17 Blue 38
vetteoz is offline   Reply With Quote
Old 12-06-2012, 09:07 AM   #7
c4cruiser
CF Senior Member
NCM Sinkhole Donor
Support Corvetteforum!
 
c4cruiser's Avatar
 
Member Since: Dec 1999
Location: Lacey, Washington RVN '68-'69
Default

But according to that page, the 4+3 trans and 3.07 ratio got the same speedo gear (Green) regardless of the wheel/tire size.

And the table shows 16" wheels listed up through 1990
c4cruiser is offline   Reply With Quote
Old 12-06-2012, 09:31 AM   #8
Keystring
CF Senior Member
 
Keystring's Avatar
 
Member Since: Aug 2006
Location: Canandaigua New York
Default

Here is a calculator to help you determine tire/wheel size in relation to speed..

Just put in the old size on the left, and the new size on the right.. speed difference is below.

http://www.miata.net/garage/tirecalc.html
Keystring is offline   Reply With Quote
Old 12-06-2012, 11:08 AM   #9
WVZR-1
CF Senior Member
 
Member Since: Sep 2003
Location: Charles Town WV
Default

Quote:
Originally Posted by c4cruiser View Post
But according to that page, the 4+3 trans and 3.07 ratio got the same speedo gear (Green) regardless of the wheel/tire size.

And the table shows 16" wheels listed up through 1990
The GM engineering formula for odometer/speedometer calibration and the catalog applications through the years have had some "conflicts" - that's the first time that particular one has been mentioned that I've noticed. I believe I've got the formula (or so I've been told). It's likely that in the GM engineering formula when using the 3.07 ratio there was a marginal difference so one tire size or the other was rounded up or down. There's no way of confirming without "rolling radius" or "rolling circumference". The side wall support/stiffness may have effected those dimensions.

The "chart/table" is a compilation of the individual and there's no GM validation.

Quote:
Originally Posted by Keystring View Post
Here is a calculator to help you determine tire/wheel size in relation to speed..

Just put in the old size on the left, and the new size on the right.. speed difference is below.

http://www.miata.net/garage/tirecalc.html
That calculator and "MOST" used advertised RPM & diameters. Neither of those are used when calibrating/calculating speedometer/odometer.
Speedometer/odometer is done using a "rolling radius" - or a "rolling circumference".

OP - Your only concern is the tire dimensions. If you're using a "factory spec" tire size then you should be able to rely on the information in GM Charts. They need to be confirmed by you. If it's a "non-spec" tire size then to accurately do the math you need "rolling radius" or the "rolling circumference"!

rolling radius - center of rear axle to ground dimension

rolling circumference - chalk tire at 6 o'clock and mark floor "roll" 1 revolution back to 6 o'clock mark floor and measure.

Last edited by WVZR-1; 12-06-2012 at 11:22 AM.
WVZR-1 is offline   Reply With Quote
Old 12-07-2012, 07:52 PM   #10
Keystring
CF Senior Member
 
Keystring's Avatar
 
Member Since: Aug 2006
Location: Canandaigua New York
Default

Quote:
Originally Posted by WVZR-1 View Post
The GM engineering formula for odometer/speedometer calibration and the catalog applications through the years have had some "conflicts" - that's the first time that particular one has been mentioned that I've noticed. I believe I've got the formula (or so I've been told). It's likely that in the GM engineering formula when using the 3.07 ratio there was a marginal difference so one tire size or the other was rounded up or down. There's no way of confirming without "rolling radius" or "rolling circumference". The side wall support/stiffness may have effected those dimensions.

The "chart/table" is a compilation of the individual and there's no GM validation.



That calculator and "MOST" used advertised RPM & diameters. Neither of those are used when calibrating/calculating speedometer/odometer.
Speedometer/odometer is done using a "rolling radius" - or a "rolling circumference".

OP - Your only concern is the tire dimensions. If you're using a "factory spec" tire size then you should be able to rely on the information in GM Charts. They need to be confirmed by you. If it's a "non-spec" tire size then to accurately do the math you need "rolling radius" or the "rolling circumference"!

rolling radius - center of rear axle to ground dimension

rolling circumference - chalk tire at 6 o'clock and mark floor "roll" 1 revolution back to 6 o'clock mark floor and measure.
Maybe so.

But most people aren't that picky.

I have yet to find a speedometer that is that accurate to begin with.

And the only 'speed detector' that counts, is the one being held by the cop as you drive by.

If your car is within a 'resonable' speed as you pass, that's all that counts on the street.

On the strip might be a different story, but for street, driving 1 or 2 miles per hour off won't make a bit of difference.
Keystring is offline   Reply With Quote
Old 12-08-2012, 07:17 AM   #11
PAINTER MAN
CF Senior Member
 
Member Since: May 2004
Location: Athens Ohio
Send a message via ICQ to PAINTER MAN Send a message via AIM to PAINTER MAN
Default

My 86 came with 16" wheels which I replace with 17'' wheels. According to my GPS the speedo is right on.
PAINTER MAN is offline   Reply With Quote
Old 12-08-2012, 08:09 AM   #12
RollaMo-LT4
CF Senior Member
St. Jude Donor '10-'11-'12-'13-'14
 
RollaMo-LT4's Avatar
 
Member Since: Oct 2005
Location: Rolla Missouri
Default

Quote:
Originally Posted by PAINTER MAN View Post
My 86 came with 16" wheels which I replace with 17'' wheels. According to my GPS the speedo is right on.
What was the tire size on the 16" wheels?
What is the tire size on your new 17" wheels?

Tire size is what matters, not wheel size.

If the tire on your new 17" wheel has the same overall diameter (and rolling circumference) as the tire on your old 16" wheel, your speedometer would be exactly the same.

__________________
1996 Corvette - LT4
2011 Camaro SS Convertible
2011 Harley Davidson FLHX Street Glide
RollaMo-LT4 is online now   Reply With Quote
Old 12-08-2012, 03:39 PM   #13
QCVette
CF Senior Member
 
QCVette's Avatar
 
Member Since: Jun 2009
Location: Mclean IL
Default

Quote:
Originally Posted by RollaMo-LT4 View Post
As noted, the wheel size really isn't what matters.
It is the overall diameter of the tire.

255/50-16 = approx. 26.04" in diameter
275/40-17 = approx. 25.66" in diameter

That is only 0.38" difference.
With the speedometer reading 60mph, you will really only be going 59.1mph with the 274/40-17 tires.


Starting in '93 the base 17" wheels used a 285/40/17 tire = 26.0" in diameter, so very close to the same size and almost no change in the speedometer

On my '88 with the base 16" wheels I went to 17" wheels with 275/40/17 and later 285/40/17 and they worked fine.
QCVette is offline   Reply With Quote
Old 12-09-2012, 06:16 AM   #14
gerardvg
CF Senior Member
 
gerardvg's Avatar
 
Member Since: Aug 2009
Location: Melbourne, Australia Victoria
Default

Quote:
Originally Posted by PAINTER MAN View Post
My 86 came with 16" wheels which I replace with 17'' wheels. According to my GPS the speedo is right on.


Same here went from 255 50 16 to 275 40 17 and my speedo is spot on with my GPS
(T700R4 with 3.07 rear)

gerardvg is offline   Reply With Quote
Old 12-09-2012, 06:16 AM
 
Go Back   Corvette Forum > C4 Corvettes, 1984 - 1996 > C4 Tech/Performance
Reload this Page 1988 C4 Wheel size ? calibrated for the speedo
 
 
 
Reply


Thread Tools Search this Thread
Search this Thread:

Click for Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
looking for dimensions for hub adapters for 17" wheels on a 85 85corvettec4 C4 Tech/Performance 13 10-30-2013 10:25 PM
16x9.5 wheels for autocrossing? Biancone C4 Tech/Performance 15 09-23-2012 08:57 PM
17 Inch BBS Wheels Brian308gts C4 General Discussion 10 10-02-2011 09:36 AM
C4 with Rally Wheels azrocketman C4 General Discussion 1 05-26-2010 05:53 PM
17 inch wheels, what size tire?? 66stingrayrestorer C1 & C2 Corvettes 10 02-15-2009 07:11 PM


All times are GMT -4. The time now is 08:48 AM.


Emails & Password Backup