I've noticed that my displayed speed is 5-10 MPH over what it should be. Any tips on recalibrating? Is it better to take it to a shop or buy the $230 tool from summitracing.com? Any experiences with this?

Have you departed from the stock tire diameter and/or 3.42:1 rear axle ratio? By “displayed” do you mean on the speedo meter and/or viewed with a scanner from the PCM?

Has your car had a change to the rear end gear ratio?

Numbr44coldcuts,

Sounds okay to me. Lets you go speeding a bit, without getting a ticket. Seriously, others here have raised good considerations, already. I'd want to be sure that I'd finished making any more drive-line changes, before calibrating the speedometer.

If you are just concerned to know precisely how fast you are going, no matter about the tires, wheels, and gear ratios, then go GPS. Amazon sells several digital GPS speedometers that only need a USB cable, nothing else, to run. They read out big digits in miles or kilometers, elapsed time at the end of the trip, et c. Typically they cost less than US$30.00 but you can spend more if you like. I use the cheapest one on my sneaky-fast motorcycle; that bike will be doing sixty in town when you think you're doing forty. The GPS unit always agrees exactly with those roadside radar advisory signs that they put up on small residential streets and near schools. HTH.
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Everyone, I'm not positive about the gear ratio change. Nothing had really been modified when I purchased the car, so I would say its stock. That type of modification seems like it would go alongside other performance upgrades. Is it possible its the tires or can a speedometer lose calibration due to mileage? The GPS speedometer sounds like a good solution, but I do like the traditional gauge cluster.

The speedo gets it's input from the PCM. The PCM gets its input from the VSS sensor on the tailshaft of the transmission. The PCM has to have the correct tire size and rear axle ratio programmed into it.

Did this problem come with the car when you bought it, or did it develop recently? Did the speedo ever read correctly? Is it always off by 5-10 MPH, or is it off by a percentage of the actual speed? If its off by a fixed amount, could be someone changed the overlay on the instrument panel, and screwed up the removal and replacement of the speedo needle. Or the speedo itself is faulty.

A tire would take a fairly large size change to affect the speedo by 5-10 MPH, but the actual impact is a percentage change, not a fixed value change. Stock tires are either 235/55-16 = 26.18" diameter, or 245/50-16 = 25.65". What size tires do you have on the REAR? But to have the speedo read higher than actual, the tire diameter would have to be smaller than stock, and that's unusual.

On the other hand, putting a set of 3.73 gears on a T56 with stock 3.42 gears will cause the speedo to read 9% higher than actual speed, or e.g. 65 MPH indicated at a true 60 MPH. A 4.10 gear set increases the error to almost 20% or 72 MPH indicated at a true 60 MPH.

Its always been this way. I suppose its possible the gears were changed and that is what it sounds like happened. I'll have to get it programmed correctly when I get a tune. Tires are standard size with stock wheels. Are the stock rear ends on these cars limited slip or just the one wheel drive?

All V8 models have the RPO “G80” limited slip differential. It was optional on V6 models. There should be a service parts label in the glove box or end of driver’s door that shows what the car had when it left the factory. All T56 cars from 1994-2002 had rhe “GU6” 3.42:1 rear axle.

I've confirmed GU6 in the glove compartment which translates to 3.42 gear ratio from the factory. So, before Solomon pcm tune, the car was at 5.3% over on the speedo. Now, I've confirmed the pcm is programmed for 3.42 gears and no change to the speedo. Does this mean for sure that the gears were changed? Could it be anything else? Why would you lower the ratio?

Assuming tires are stock overall diameter and everything else close to stock, then a gear change is a reasonable assumption. Not an uncommon modification to see 4.10's on a 6 speed car, although the percentage difference is more likely to equate to 3.73's. Only way to know for sure is to jack the car up and count the revolutions of a rear wheel compared to the driveshaft.

A 5.3% high difference would correspond to a 3.60 ratio rear axle. Or, if the wrong number (3.24) was programmed in the PCM, when the car has 3.42 gears.

Did you determine the % difference at multiple values of MPH? Not aware of a 3.60 being available for a 7.625” 10-bolt. Any chance the car has a different rear axle assembly - GM 12-bolt, Strange/Dana 60, Ford 8.8” or 9-inch, all of which are available for the 4th Gen. Expensive changes and unlikely unless the vehicle has been highly modified.

As noted earlier, change from 3.42 to

3.73 = 9.1%

3.90 = 14.0%

4.10 = 19.9%

To check whether it is a VSS/PCM problem, or a speedometer problem, you need to have a real-time scanner with the ability to read MPH from the PCM, and compare it to the speedometer reading. A dealer Tech-2 can do that. Maybe some of the cheap OBD-2 scanners or apps as well.

You could also check the odometer reading. Compare the miles driven per the odometer to the 0.1 mile markers on an interstate, over a distance of 5 or 10 miles. Sometimes you can find a “measured mile” set up by county or state officials. With a digital odometer used in later model year 97’s, interpolating between 1/10ths of a mile can be difficult, unlike an analog odometer.

As an outside possibility would be some sort of high voltage inducing extra pulses in the signal between the VSS and the speedometer. Unlikely.

Thanks everyone for your input. I'm thinking that the percentages of error are closer to 10%, its displaying 5-6mph on the dash over the speed recorded on a GPS speedometer and I tested it on one of those police department speed signs too. So, its looking more likely that 3.73 gears were installed or the whole rear end was replaced. Am I getting any significant changes to the torque with leaving these vs. changing to 3.42 gears? My goal with this car is to take it to the road track and rip it around the corners and down the straights. Would changing gear ratios make a big difference in the enjoyment the drive if this was my intended way to drive the car?

We run 4:10's in our HPDE/autocross car so 3:73's would be fine. These cars make great track cars - have fun with it!

If it's always 5-6 MPH over the true speed, no matter whether it’s high speed or low speed, it’s not a percentage error. 6 MPH over at true 30 MPH is 20%. 6 MPH over at true 60 MPH is 10%. Different rear gears would not cause a constant 5-6 error.

Another vote for 4.10’s with a T56.

One quick note to add: even if you do not use a cheap GPS speedometer forever in your car, you can make an accurate "conversion chart" to say that 50 mph on the speedo is (maybe) a real (GPS) 46 mph, for future reference. You can make a number of such speed checks on your "speed chart," for whatever may be the case for your Camaro. My small GPS unit lives nicely under the dash cowl, above the stock speedometer. I find the stock speedometer more difficult to read than the GPS, especially in bad light or sun glare, when the instrument lights don't help much.
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