Hi all! Two questions:

1. My speedometer seems to be about 2MPH fast across the board. I've had three different GPS systems in my car and they all show me going about 2MPH slower than my speedo says I am. Is there any way to fix this? I am running stock wheels, tires and everything. I've seen stuff like the Yellow Box, but I don't really want to "hack" the car. I'm just wondering if this is something I can fix or have my dealer fix.

2. What's the best service manual I can get for my 8? I am going to be tearing apart the dash soon to do some carputer install stuff and I'd like some guidance so I don't break things. I'd really like exploded diagrams of everything. Anything like that exist?

Thanks all!

 

speedos are all abotu 2 mph fast- set that way from factory on purpose. no way to fix it.

buy the wiring diagrams and shop manual from mazda

 

i went past one last night, and it was dead on. crested a hill and it showed my initial speed at 37mph, looked down, yup. coasting down the hill, each time the speed increased on the radar display, i looked down and my speedo matched exactly...well, between 37 and 42 anways. outside of that speed range, i can't say for sure

 

Every single car I've driven, the speedo is approximately 2mph faster than the actual speed. Not much you can do about that..

 

So WHY?

 

I've checked my speedometer dozens of times against those stationary radar billboards and it's been 2 MPH optimistic everytime. I can live with that.

1.3L

 

Thanks for all the great responses. My reason is the same as Cool's. I like accurate data. I've had GPS in my cars for a long time but I've never had a car with a digital speedo before, so I've never noticed. I guess now that I know, I know, so it's not a big deal.

 

Several countries have certification rules requiring that the manufacturer must guarantee that the speedometer
shows under no circumstances less than the actual speed. I guess that is the reason for the general higher readout.
The error of a digital speedometer can however kept lower than that of an old traditional analog speedometer,
which had to take non-linear characterisitcs into account.

Q-chan

 

No, I definitely meant why do they do it. I suppose it's what Q-chan said. But it's still pretty lame.

 

This would not actually be a fix, but slightly larger tires, like 245/45X18 could get it closer.

 

But, 245/40x 18s (like I have on my car actually make it worse - their rolling circumference is a good 5 cm less than the stock tires - my speedo is close to 4 mph off.

 

Not 245/40 (which are a hair smaller), but 245/45.

 

I've compared the speedo readout against a few GPS units, and it does not seem to be a fixed MPH difference from true speed. Rather, it appears to read a constant percentage high (about 5%), at all speeds.

At "73" on the speedo, the GPS says around 69.35. At lower speeds, the difference is smaller in absolute MPH, but remains close to 5% high, as close as I can tell.

Jeff

 

This is a very common problem with motorcycles. I'm not exactly sure why, but some people think the optomistic speedos help keep speeds down. There is a device called a 'SpeedoHealer', http://www.speedohealer.com/eng/intro.htm, to correct the problem, but they don't appear to have one for cars.

I didn't purchase one because I didn't care to fix the analog speedometer on my bike. It is mounted too low and requires me to loook at it for a while to actually determine my speed. I mounted a digital bicycle computer on my motorcycle to get accurate readings. I have it mounted up high on the dash next to my Valetine One remote display. When the V-1 alarms I can look at it and the speedo simultaniously and determine the direction of the radar and my speed at one glance.

Anyway, I degress. I wonder if there is a SpeedoHealer like device for cars? It's a simple in-line electronic device that adjusts the pulses sent to the speed display.....

 

Yellow box should work, but can't we simply change the little speedometer gear inside the transmission?

 

Over a 6 and a 1/2 year bump!
Sweet

 

People say do not post new threads on a subject that is already covered, and then complain when someone bumps an old thread... So basically everyone should just not post at all to keep everyone happy?

Also my speedo is perfectly accurate with any gps I have tried, not +/- anything.

 

I would agree with that ...

Good point.

 

My '11 Sport is the same.

Just tested it on a to and from trip from Denver to Las Vegas, NM last week.
Spot on, or within 1 mph in each direction according to my GPS.

I wonder what they changed between the Series I and Series II cars to make the speedo spot on?

BC.

 

The reason I searched for this thread was I want to change the dif ratio to 4.10 or 3.90 from 4.44 (2007 MT/SC); can the speedometer be corrected with a gear, do we need a flash or a yellow box?

 

Another thread on here said the speedometer gets its data from the wheel speed sensor so as long as you have the stock wheels and tire size then it should read fine.

https://www.rx8club.com/tech-garage-...dmeter-132716/