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

 

96 cars require no parts to calibrate the speedometer but you will need a tuner to make a change in the PCM. See the supporting vendors list on the left hand side bar.

 

Thanks for the info. Now just what exactly do the tuners change in the PCM and what does it do?

 

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.

 

Thanks again. I didn't know that cars today could have their speedometers recalibrated without ever changing the speedo gears.

 

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.

 

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.

 

No speedometer gears to be changed. Just a number of electrical pulses from the wheel, like with the ABS. The computer counts those pulses to determine the vehicle speed. The computer has to be properly programmed, like using the proper gears, to determine the correct speed.

RACE ON!!!

 

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.

 

You're talking about a 1mph difference at 90mph. The difference between worn and new tires is larger. The difference between two brands of tires is larger. The factory calibration is off by more than that much to start with.

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?

 

There are more 275 tires available than 285's and most of the time they are cheaper. 275's would also look stock as the Z51 option had them at all four corners while 285's in the front would look stock to nobody.

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.

 

Be careful about buying a used Hypertech. If the previous owner has not returned their car to the factory tune, the programmer will not work on another car.