I would think that MPH display, and miles accumulated, go hand and hand. But if you can have MPH displayed, and Kilometers displayed for distance covered, this could be it. On the conversion table for "miles to kilometers" it said, 1700 miles equals 2735 kilometers. If those are the numbers that you have calculated for the trip, maybe that's it. Maybe you better look at the DIC real quick, to see if it says MI or KLM. Or something like that.

 

How sure are you that it only had 25K miles on it when you left?

Are your tires the original size?

You can verify your odometer accuracy with a GPS, either your own or a borrowed one from a friend.

The IPC calculates the distance travelled based on the GMLAN vehicle speed information from the ECM

 

The movie Ferris Beuler's day off comes to mind. You have to drive lots in reverse. Someone has been joyriding in your ride.

 

hahaha that gave me a good laugh

 

Update: For the last couple of weeks I've been driving with a GPS synced with the trip odometer.
At first they were both even but after a few miles I noticed the trip odometer was clocking slightly faster than the GPS unit. The more I drove the bigger the gap got. So far I've driven 130 miles and at this point the trip odometer is tracking just about 1 mile ahead of the GPS. The difference is getting greater the more I drive. Its too early to try and calulate the differences. I need to drive more to see if the rate of increase is constant.

I've double checked the setting on the G8 and everything is set to "English" which shows miles.

The tires are the original factory new Goodyear Eagle RS-As and I have the factory 18" G8 GT wheels.

Any thoughts on this?

 

Bring the car to a dealer let them look at it for any tampering

 

1 out of 130 is a 0.76% error. Probably considered acceptable. To put it in perspective, tire diameters can vary about that much.

 

I've noticed my speedo seems to be 2mph off at 70. GPS says I'm running 68, but I have the same issue on all 3 of my vehicles. 2 different GPS units, one is a Garmin and the other is a TomTom.

 

My speedo is WAY off. Once, a cop clocked me at 96 but I KNOW I was only going 35.

 

Nice. Lol

 

My problem is is that mine reads over 50k miles...
Miles rack up too quick !

 

Auto manufacturers almost all do this to help the car to warranty end quicker ;-)

 

Of course you know this already, so I agree with others, and recommend you take it to a place that can test the speedometer calibration. Maybe if it's off 4mph, it's ticking quicker by, than real map mileage denotes.

Read the tire sidewall and tire pressures and report back.

Verified P245-45-r18's all around, and 35psi front, 39 psi rear?

If you're running low or high or might have purchased "new Eagle RS-A's" in a similar size (but not the correct size) then every rotation of the tire will result from the factory-calibrated speedo to be off from "real mileage".


There's also a tire size calculator on miata dot net... to figure out the difference between "stock" and the real tire size.

I don't think I caught, did you buy this car used (unknown history of mods?) or are you an original owner?

Since all of this is electronic, (and assuming you bought the car USED) are you sure no one messed with your rear diff? Sometimes racecar drivers put in different "Rear ends" ... like a Camaro 3.27 Differential is beefier and with other supporting mods, could be the cause of a discrepency. I think the stock gearing was 2.92 in the G8 GT. What the "engine and tranny" thinks is the speed, with a different rear gearset, would be off by so much per revolution. Just a guess, here, but worth considering.



Good luck, need to take a look at any / all the above to figure out what the problem is.

Once again, I think you need to hide the keys better, I think there is someone driving your car without your consent. Who can blame them!?

--zepcom

 

I am using google maps, I figured the round trip and daily driving during our trip and came up to around 1700 miles total for the trip. Only problem is my odometer is showing around 2700 miles driven! That is a huge difference.

Update: For the last couple of weeks I've been driving with a GPS synced with the trip odometer.
At first they were both even but after a few miles I noticed the trip odometer was clocking slightly faster than the GPS unit. The more I drove the bigger the gap got. So far I've driven 130 miles and at this point the trip odometer is tracking just about 1 mile ahead of the GPS. The difference is getting greater the more I drive. Its too early to try and calulate the differences. I need to drive more to see if the rate of increase is constant.

1st: You need to do a nice longish highway drive, instead of what appears to be piecework around town.

2nd: 1 mile off in 130 = 13 miles off in 1690.....not 1000 miles in 1700 as originally stated...