Speedometer calibration needed?
Thread Starter
Melting Slicks
Joined: Sep 2003
Posts: 2,103
Likes: 1
From: Kingsport TN
Speedometer calibration needed?
I'm gonna put 275/40/17's all the way around my '96 so that I can rotate them. They will take place of the stock sized 255's in the front, and the 285's in the rear.(yes I'm gonna get two more 9.5" sawblades or a set of ZR1 repo's in 9.5" inch if anyone is wondering about the 8.5" front wheels 
)
Anyway, does the speedometer need to be recalibrated? Did the 285/40/17 rear tire C4's have different speedo parts than the 275/40/17 rear tire ones?
I did a search and didn't find anything on this subject. All the posts I saw were about large changes in tire size or different rear end gear ratios.
Thanks,
Randall
) Anyway, does the speedometer need to be recalibrated? Did the 285/40/17 rear tire C4's have different speedo parts than the 275/40/17 rear tire ones?
I did a search and didn't find anything on this subject. All the posts I saw were about large changes in tire size or different rear end gear ratios.
Thanks,
Randall
Thread Starter
Melting Slicks
Joined: Sep 2003
Posts: 2,103
Likes: 1
From: Kingsport TN
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
They change the formula used to calibrate road speed based on tire size and gear ratio. I believe Hypertech makes a DIY version if you prefer, sold by several Corvette specialty stores.
Thread Starter
Melting Slicks
Joined: Sep 2003
Posts: 2,103
Likes: 1
From: Kingsport TN
I didn't know that cars today could have their speedometers recalibrated without ever changing the speedo gears. 
Safety Car
Joined: Jun 2006
Posts: 3,629
Likes: 28
From: FL
Looking back at my statement it should have said the data inputs to the formula for calculation, not the formula but see you understood the point.
You are right that it is better in some ways than older vehicles but the programmer or software to make the changes is more expensive than a couple of plastic gears.
The up side is that with those tools you can also change fan temp calibration and some other nice mods.
You are right that it is better in some ways than older vehicles but the programmer or software to make the changes is more expensive than a couple of plastic gears.
The up side is that with those tools you can also change fan temp calibration and some other nice mods.
Race Director
Joined: Oct 2005
Posts: 11,769
Likes: 175
From: Rolla Missouri
2021 C4 of the Year - Unmodified Finalist
St. Jude Donor '10 thru '25
The difference in those two sizes is pretty small.
You can find tire size calculators on the net, that will tell you exactly what your speedometer difference will be with different size tires.
In your case here, I found this:
Your speedometer reading with the 275/40 tires will be 1.2% too fast.
When your speedo reads 60 mph, you are actually traveling 59.3 mph.
You can see, not much difference. And it is in the right direction to keep from getting a ticket for speeding too.
You can find tire size calculators on the net, that will tell you exactly what your speedometer difference will be with different size tires.
In your case here, I found this:
Your speedometer reading with the 275/40 tires will be 1.2% too fast.
When your speedo reads 60 mph, you are actually traveling 59.3 mph.
You can see, not much difference. And it is in the right direction to keep from getting a ticket for speeding too.
Race Director
Joined: Sep 2000
Posts: 17,298
Likes: 33
From: The Top of Utah
Originally Posted by LT4fun
Thanks again. I didn't know that cars today could have their speedometers recalibrated without ever changing the speedo gears.
I didn't know that cars today could have their speedometers recalibrated without ever changing the speedo gears. RACE ON!!!
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Top 10 Corvette Engines RANKED by Peak Torque (70+ Years of Muscle!)
Joe Kucinski
Corvette ZR1X Will Be Pacing the Indy 500, And Could Probably Race, Too!
Verdad Gallardo
Top 10 Corvettes Coming to Mecum Indy 2026!
Brett Foote
Top 10 C9 Corvette MUST-HAVES to Fix These C8 Generation Flaws!
Michael S. Palmer
10 Revolutionary 'Corvette Firsts' Most People Don't Know
Joe Kucinski
5 Reasons to Upgrade to an LS6-Powered Corvette; 5 Reasons to Stay LT2
Michael S. Palmer
2027 Corvette vs The World: Every C8 vs Its Closest Competitor
Joe Kucinski
10 Most Common Corvette Problems of the Last 20 Years!
Joe Kucinski
Team Owner
Joined: Dec 1999
Posts: 34,873
Likes: 487
From: Lacey WA RVN 68-69
NCM Sinkhole Donor
The '96 Z51 cars came with 275/40-17 tires and 9.5" wide wheels at all 4 corners. You should not see any appreciable difference.
Also, not all tires by brand or model will have the same diameter even though the marked size is the same. Go to the Tire Rack website and check out the specifications for a number of tire models within brands in the 275/40 sizes. You will find differences in tread width too.
Also, not all tires by brand or model will have the same diameter even though the marked size is the same. Go to the Tire Rack website and check out the specifications for a number of tire models within brands in the 275/40 sizes. You will find differences in tread width too.
Safety Car
Joined: Apr 2003
Posts: 3,793
Likes: 9
From: VA
If that's your only reason to get the PCM programmed, it's a waste of time and money.
Why not use 285 all around?
Thread Starter
Melting Slicks
Joined: Sep 2003
Posts: 2,103
Likes: 1
From: Kingsport TN
I see there are some Hypertech III's on ebay for around $220 or so, so it may not be worth it to correct a 1.23% speedo error; but if it bugs me enough at least it's not a terribly huge amount of money.
Race Director
Joined: Oct 2005
Posts: 11,769
Likes: 175
From: Rolla Missouri
2021 C4 of the Year - Unmodified Finalist
St. Jude Donor '10 thru '25
04, 17, 1984, c4, calibrate, calibrated, calibration, computer, corvette, kingsport, rear, speedodmeter, speedometer, tire, tires, tn
