my speedo is approx. 10% faster than my gps. which is right? if it is the vette then i have 6800 more miles on my odo than on the car. (and thats not fair). how to recalibrate?
also keep in mind that this is an export car. 300kph speedo

 

If everything is stock, I'd vote for your speedo as correct.
If they have those roadside radar speed displays in your area, compare with that.
Or look for a speedo shop to check it.

 

the car is stock exc. for the exhaust.
also gps agrees with my suburban but disagrees with my v-max. although bikes are notorious for inaccurate speedos.
my wife says she has compared to radar and claims its the speedo at fault.
speedo shop or stealership?

 

The GPS is correct.

 

my gps and speedo always read the same.

 

unless it has something to do with being an export, i don't know why it should be off.
is there a tuner or programmer that can calibrate the speedo? this would kill 2 birds.

 

Same here.

Non-stock size tires can cause speedo error. But, 10% is a lot.

 

The speedo in both Vettes and our Silverado are within 1 mph of my Garmin so your speedo must be off

 

OK,
Here's the deal. The only way to 100% acurately gage your speedo is to have a dyno tune. No miscalculation there. Another close way is to put it in cruise control and time yourself between known distances, such as mile markers on a highway. The greater the distance, the better.

GPS is pretty accurate. However, it is only accurate in certain circumstances. The road has to be flat, with little to no elevation changes.

All else equal, I'd wager that the GPS is more accurate.

I had a similar problem with my TransAm. Someone messed with the calibration during gear changes, but did it wrong. Later, after another gear swap, I found out that it was still off. Speed calculators, the infamous 75mph in 3rd gear trick, etc are all inaccurate. Little things like tire pressure and tread wear also contribute to your speedo being a couple percent off.
I used a combination of methods, and the GPS, being paced, measured distances all agree'd that my speedometer was off.

I ended up correcting it with a Diablosport, and then later refining it more with HPTuners.

The ONLY thing that should affect your speedometer in a vette is the tire size. Replace them lately?

 

You can check your speedometer accuracy with a stopwatch and a straight uncrowded Interstate Highway with mile markers. Set your cruise on 60 mile per hour. Clock your time between several sets of mile markers. If it averages out to 60 seconds between each set of mile markers then your speedometer is accurate at 60 mph. Speed = distance x time. 60 mph is one mile a minute.

 

 

My Vette and GPS read the same.

 

Before Changing C5 Coupe tires to Z06 Sizes (295/35/18) my GPS and speedo were EXACTLY same.

After tire swap the speedo was 2 MPH faster.

Recal'd tire sizes with Predator and they are EXACTLY same again.

 

I wouldn't bet my license on that. The GPS system is sending a radio signal which is then averaged in a computer chip in your unit. It will always lag in some degree, your actual speed. You might want to do a software download to your GPS or find out if there are any updates for it that aren't installed.

 

I have the 300 mph speedo and it reads the same as the GPS.

 

I would have to go with the GPS. On a flat surface it is very accurate.

Rick

 

Just a FYI, you cannot fix it via a Tech 2.

Im waiting til I buy HPTuners to fix mine myself!

 

Gps!

 

not quite.
GPS is an approximation. The speedo is a direct measurement; and that's as accurate as it gets.
(this is assuming there isn't a failure somewhere in the OP's speedometer system)

 

Subscribed.

-Alex