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Old 06-03-2007, 03:50 PM   #1
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New shoes (aka tires)

Hello -

I just got these Continental Contact-Pro tires :

Attachment 530

Attachment 531

New tire size : 195/65 R15
Stock tire size : 185/65 R15

These tires are one size up from stock. Rated up to 51 PSI, currently running 42 PSI (will inch up over time).

I used this site to figure out my difference in speedometer readings :

Miata.net Tire Size Calculator
http://www.miata.net/garage/tirecalc.html

It told me that the speed will read 2.1% too slow.

I don't use my ScanGauge for my mile/distance readings, I use my car odometer.

Question : Since I use my car odometer, can I calculate that the odometer should read 2% more when I enter the data into my Gaslog, or would that be cheating?

Thanks,

CarloSW2
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Old 06-03-2007, 04:21 PM   #2
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Don't automatically assume that the speedo and odometer are off by the same factor. I just went through the same thing with my '95 Honda Civic and discovered that the old 175/70/13 tires, although they only measured .2% smaller than the new 175/70/13 tires (different brands), the new tires make the odometer read 4% too low. The old tires made the odometer read 2.6% too low. However, the speedometer reads exactly the correct speed regardless of the tire. I determined all this by using my GPS to measure my MPH and distance traveled.
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Old 06-03-2007, 05:30 PM   #3
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cems70 -

Thanks, I didn't know about speedo/odo differences. I was thinking I would have to use a GPS for the best possible test I need to see if the feature set on my wife's GPS will show total miles travelled (not in a straight line). I am guessing they all do, I just need to read the manual.

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Old 06-18-2007, 01:05 AM   #4
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Hello -

Overall the tires are good. I still need to do a GPS test to get the odometer/MPG calibration right.

The one problem is that on freeways that are grooved in the direction I am traveling, the car "wiggles", aka the tread tracks into the grooves like a phonograph needle and the car has a noticeable wiggle. I had a feeling this might happen, but no proof until now. Thank goodness my main commute doesn't have this kind of freeway.

Back in the day it was much worse on my motorcycle, but it's still no fun in terms of road feel for new tires .

CarloSW2
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Old 06-18-2007, 01:27 AM   #5
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yeah the best thing to do is verify the odometer with GPS. Instead of reling in the specifications.

On my old Daewoo, the speedometer was right on with stock size tires, odometer off, then with oversize diameter tires, the speedometer off a little and odometer dead on.

The road grove following is a side effect of higher TP. Im my experience, it gets worse with more air. That is when you know you have reached the limit for that tire. Also sometimes you can "feel" paint stripes though the handling.
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Old 06-18-2007, 01:51 AM   #6
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Try using the DML calculator - it will give an odo correction .
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Old 06-19-2007, 12:13 PM   #7
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MnFocus -

Quote:
Originally Posted by MnFocus View Post
Try using the DML calculator - it will give an odo correction .
Ok, I just used the handy DML calculator and it gave me a 2% error at 100 MPH, which is the same as the Miata calculator :

Quote:
Actual Speed
100 mph (160.90 kph)

Indicated Speed with Tire B
97.95 mph (157.60 kph)

Speedometer Error
-2.05 mph (-3.30 kph)
I then ran the car with the GPS and got these samples in miles :

GPS:Car Odometer
19.6:19.0 => 19.6 / 19 = 1.031 => 3.1% error
37.2:36.15 => 37.2 / 36.15 = 1.029 => 2.9% error

Soooooo, I think the GPS is telling me that my Odo is too slow by an additional 1%.

I think I will stick with 2% error for now, because I don't want to accidently inflate my numbers and tire wear will eventually bring this correction down over time (i.e. always losing tread => always reducing tire circumference => always reducing odometer error over time).

CarloSW2
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Old 06-19-2007, 07:04 PM   #8
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The tread looks very similar to my mich harmonys. Three main grooves with similar siping. Are they high silica rubber?
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Old 06-19-2007, 07:42 PM   #9
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lovemysan -

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The tread looks very similar to my mich harmonys. Three main grooves with similar siping. Are they high silica rubber?
I don't know. There is some fine print on the tire near the wheel rim, so I will read that and see if it specifies it. I went here but didn't find anything :

http://www.conti-online.com/generato...master_en.html

CarloSW2
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Old 06-19-2007, 07:52 PM   #10
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It doesn't appear to be. It no biggy. I love my harmonies. Night and day wet traction difference.
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