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Unread 01-19-2014, 10:40 AM   #1
goonbash
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Cj Odometer and onboard mpg computer with larger tires

I know my speedo is off by 1-4mph between 30mph and 100mph with my new 275/65r18 tires. What I am wondering is does this also mean I won't rack up mileage on the odometer quite as quickly since the speedo reads a little low, or is the odometer based on gps or something?

Also what is the effect of larger tires on the onboard gas mileage calculations and is the 'range' number for how far you can drive on the fuel you have still accurate?
I wasn't sure if the mpg and range numbers were based on actual fuel consumption so they would still be accurate or if they were in some way related to tire revolutions or something like the speedo so they would no longer be accurate.

If some of you super knowledgeable tech guys could chime in I
Would sure appreciate it. Thanks

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Unread 01-19-2014, 11:05 AM   #2
XJ2WK2
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Naturally, your mileage is actually slightly better than it says because you went further than the odometer thinks.

Here's the funny thing, I'm running the same size tires as you, I set my evic to show the speed, it would read the same or off by 1 km/hr compared to the 430n and even any speed app on my phone! Have you actually done this?

I know when you do the math, our tires are 3% bigger circumference, so I'm stumped.
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Unread 01-19-2014, 11:36 AM   #3
ColdCase
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How did the speedo compare when you were using the OEM tires? Not that I'm skeptical, but a speedo off by a couple mph is not unheard of, and you may have just corrected it by using larger tires ? 3% at 60mph is less than 2 mph.
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Unread 01-19-2014, 11:45 AM   #4
goonbash
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I figured that might be the case with the odometer but I have no clue how the onboard mpg and range calcs are done, any ideas on those ?

I have not done any speedo comparisons but it's great that yours is so accurate with the larger size.

I recently ordered a blue tooth device for the obd port to use with a torque app on my phone, don't know if that will allow for such comparisons. What is a 430n, a GPS device?
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Unread 01-19-2014, 02:26 PM   #5
XJ2WK2
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Quote:
Originally Posted by ColdCase View Post
How did the speedo compare when you were using the OEM tires? Not that I'm skeptical, but a speedo off by a couple mph is not unheard of, and you may have just corrected it by using larger tires ?
hehe good point, I've had larger tires since 2000kms on the odo, so never checked

Quote:
Originally Posted by goonbash View Post
I figured that might be the case with the odometer but I have no clue how the onboard mpg and range calcs are done, any ideas on those ?

I have not done any speedo comparisons but it's great that yours is so accurate with the larger size.

I recently ordered a blue tooth device for the obd port to use with a torque app on my phone, don't know if that will allow for such comparisons. What is a 430n, a GPS device?
430n is the factory nav unit
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Unread 01-19-2014, 03:07 PM   #6
goonbash
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Thanks. I have nav, just wasn't aware it gave speed to compare against the speedometer.
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Unread 01-19-2014, 04:55 PM   #7
goonbash
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I rarely use my Nav system where do I see the road speed on it to compare against the speedo ?

Thanks
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Unread 01-20-2014, 06:17 AM   #8
zr2s10
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Typically the speedo will read slightly above the actual vehicle speed. That way some nitpicker doesn't try to get Chrysler to pay for their speeding tickets by saying their speedo led them astray. They are much closer on newer vehicles though. I think our WK2 reads just 1 or 2 over what my GPS says (been a while since I compared), but my old truck was off by almost 5 when the tires were worn down.
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Unread 01-20-2014, 09:26 AM   #9
goonbash
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Reading over would be good because then with larger tires like I have now the speedo reading will be closer to accurate.
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Unread 01-20-2014, 08:44 PM   #10
goonbash
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I compared my speedo to the vehicle speed listed in the Nav system trip computer (not sure if it gets it's reading from gps or what) and similar to Xj2wk2's result I couldn't get them to read more than 1 maybe 2mph difference. I think it was more like only 1mph difference but the Nav trip computer vehicle speed readout has a delay when you speed up or slow down before it adjusts its reading. When I was able to hold a constant speed between 20 and 50 mph it never seemed more than 1mph different than the Evic speedo.
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