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Discussion Starter #1
During my Thanksgiving Day trip, I noticed speedo error of 3-4mph. I knew there was about 1-2 mph difference between actual needle reading and ICC reading but 3-4 difference was more than what I thought.

After setting ICC at 79(speedo needle indicating 81), I pushed Ave. Speed button on trip computer. 0.5 mile passed and it said "74.4" :? . I know this is not an instant reading but shouldn't it say 79+/-?. Tried same test at various speed and they all came with 3-5 mph difference.

Any thought on this?
 

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You were using ICC. Did you take into account that it probably slowed down because of traffic ahead and never did maintain 79mph for the entire 0.5miles?
 

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Discussion Starter #3
05fxguy said:
You were using ICC. Did you take into account that it probably slowed down because of traffic ahead and never did maintain 79mph for the entire 0.5miles?
With your points in mind, I did this on open flat road(I-88 toll). I also counted in other slow-down factors like road grade, wind, etc but was never able to get speedo error margine smaller than 3mph.
 

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What size tires are you running? I think I remember some threads where you were looking at a different size tire than the stock ones?
 

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Although I have not formally tested it; my ICC setting appears to be ~ 1 mph slower than the Speedometer needle. I'm glad (sort of) that I'm not the only one with a discrepancy.
 

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Right, if we all temporarily agree that that the speedo is calibrated for 18" tires and we say those with stock sport 20" tires will notice an over-registering speedo of +1 or +2MPH, if you go with a slightly larger tire then I guess it could easily result in another +1 or +2MPH on the speedo.

However, if you have a cheetah painted on the side of your car there's no telling what effect that might have on your speedo, ICC or NAV system... :wink:
 

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ShadyJC said:
Right, if we all temporarily agree that that the speedo is calibrated for 18" tires and we say those with stock sport 20" tires will notice an over-registering speedo of +1 or +2MPH, if you go with a slightly larger tire then I guess it could easily result in another +1 or +2MPH on the speedo.
Having 18" or 20" wheels shouldn't make any difference at all. The bigger wheel size is compensated for by a shorter sidewall on the tire, so that the total diameter is the same, and therefore so is the circumference (i.e. distance traveled per tire revolution stays the same). I'm pretty sure that even when someone puts on a set of 22's, the tire sidewall goes down and the overall diameter stays the same.

That said, my speedo also registers ~1mph greater than what the ICC says. :)
 

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My mistake, didn't realize that the 18s had a larger sidewall at 265/60/18 when the 20 sports are 265/50/20s.
 

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ShadyJC said:
My mistake, didn't realize that the 18s had a larger sidewall at 265/60/18 when the 20 sports are 265/50/20s.
By deffinition, that's how it works. The sidewall height of "60" is 60% of the width of 265mm, 50% for the 20s. The 20's have a outer circumference 0.3% smaller than that of the 18s, which only works out to a difference of .2 MPH. Keep in mind there is always going to be some slight error as tires wear as well.

Here's a good tire size calculator that you can use to compare diff. size tires: http://www.miata.net/garage/tirecalc.html
 

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Discussion Starter #11
So, has anyone of you get this reset or fixed by the dealer? I can live with 1mph difference but not with 3-4 mph.
 

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I know this is an old thread, but I think my speedo is off by approx. 5 mph. Only thing I have tested against are the road signs that supposedly tell you how fast you are going. Ever since I got it (10/31/04), I have felt like the speedo was recording faster that I was actually going.

Anybody had any experiences with this since November?
 

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My speedometer is correct to within the thickness of the needle according to my GPS. (WAAS enabled tracking 10 satellites)
 

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Hi all,

here's my experiences :) donno if it helps.

according to the MIL EMARKS on the northeast highways and tollways, the odometer (currentl reading 254XX miles) and trip meter is off by only 1% or so.

also, checking with the OEM nav, the odometer is correct.

Also, as I tested on long journeys (> 100miles) the average fuel consumpltion X miles ravelled confirms very nicely with the gallons of gas refilled.

So, I believe our speedometers are very good (assuming the odometer is fueled by the speedo) around highway miles (55-100). below or above that I can't confirm.
 

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Comparing my GPS to my needle, my needle is 3 mph faster.
 

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manta8888 said:
Comparing my GPS to my needle, my needle is 3 mph faster.
I believe that most FX's err to reading faster than what they're actually going.
I believe mine to be an exception.
Tracking ten satellites (WAAS enabled) I hit 130 last fall. My speedometer said exactly 130. Dead nuts.
My speedometer appears to be right on through all range of speeds.

Driving with another 03 FX35 last fall on another cruise, we hit a nice looking straight stretch and took off. My speedometer and my Garmin GPS showed 120 as a top speed. The other FX speedometer read 125.

These cars, both the 35 and 45, are governed to 130. That's it. Top speed.
No question the 45 is going to get there much more quickly than the 35.
It took me probably 7-8 miles to hit 130. Very slow going after 120.
 

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This is an interesting discussion, especially when you take photo radar into consideration. In Scottsdale, AZ the city just installed "the first in the nation" permanant photo radar sites on a public state owned freeway :shock: (six sites in all, over about an 8 mile stretch). I drive past 3 of these sites on my daily commute. The city made it very clear that while the speed limit is 65 mph, they would not trigger and ticket until 76 mph (a 10 mph grace). With such a specific photo radar trigger number I care a lot about what my speedo reads.

My 05 45 has always read 1 mph higher on the speedo than the ICC readout. I'm pretty satisfied that the ICC is accurate. Having driven 30,000 miles so far (most highway) I've noticed that the car tracks perfectly with other traffic going 75 on the highways (typical AZ highway speeds). I always set my cruise with the ICC readout not the speedo when going by the Photo Radar sites so I don't get a ticket. It had better be accurate. 8)
 

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kxfx45 said:
I know this is an old thread, but I think my speedo is off by approx. 5 mph. Only thing I have tested against are the road signs that supposedly tell you how fast you are going. Ever since I got it (10/31/04), I have felt like the speedo was recording faster that I was actually going.

Anybody had any experiences with this since November?
dIGGING UP AN OLD THREAD by searching is a good thing - however it hasn't changed the facts - even in '06 :? http://www.infinitifx.org/phpBB2/viewto ... hlight=mph
 

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Well, I've been meaning to do this for a long time and finally did.
I run 18" wheels and Blizzaks in the winter and took my Garmin along with me today to check for any speedometer error because of the 18" wheels.

We have all generally acknowledged that the tire profile of the 18" wheel is basically the same as the 20". I measured the height of the wheel and tire when I first put the KDW's on the car and they're virtually the same.

However, I just got back and I'm showing about a 1.3 mph discrepancy. My speedometer is reading fast. Tire pressure is at 32 same as my 20"'s.

Come the spring when I change them out again, I think I'll put a level across the top of them and really take a close measurement just for shits and giggles - 8)
 
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