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Speedometer recalibration

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Old 12-25-2018, 07:35 AM
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Default Speedometer recalibration

I want to put some knobby tires (like Wranglers) on my 2002 Xtreme, but the stock size is 235/60R16 and in those, knobbys are unobtainium at a decent price. So I'm looking at 235/70R16, whose circumference is 7% bigger.
As I appreciate it there is no gear to fix the speedo, you have to take it to the dealer. How much does that cost?
​Also, if I decide to live with the difference, are their other issues I should be concerned about?
 
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Old 12-25-2018, 10:25 AM
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I don't think that small of a change would be worth recalibrating. Probably wouldn't even notice.
 
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Old 12-25-2018, 11:08 AM
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If you make that size change, add 7% to whatever speed your speedometer reads. Example - if the speedometer reads 65mph, you're actually doing 70mph.

https://www.discounttiredirect.com/learn/tire-size-calculator
 
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Old 12-25-2018, 11:31 AM
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I'm not sure how much you want to spend on it, but I used a Jet Accu-Speed Calibrator on a Suburban once and it worked perfectly. Pretty easy to install and adjust, too.
 
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Old 12-25-2018, 12:15 PM
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I was wondering if 7% would make a difference in shift points, rpm, sensors information, mpg, that kind of thing, or is it just remembering to go 5 mph faster?.
Also, would a tire store have a tool like Tom A shows and they would recalibrate for me with purchase?
Finally, I can get the diameter and the tread I want in a 17" or a 15" tire. What is the consensus on fitting one of those onto a 16" rim?
 

Last edited by puttster; 12-25-2018 at 12:25 PM.
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Old 12-25-2018, 12:41 PM
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I don't think you can put a 17" or 15" tire on a 16" rim. Can you not get the tire you want in 16"?

Even if you could somehow stretch it to your rim, I wouldn't want to be anywhere near you on the highway
​​​
 
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Old 12-25-2018, 01:46 PM
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Find someone with a Tech 2 , I've recalibrated tire size on my 02 Astro and 05 Jimmy so I'm pretty sure it would work for you.
 
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Old 12-25-2018, 03:48 PM
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Originally Posted by puttster View Post
I was wondering if 7% would make a difference in shift points, rpm, sensors information, mpg, that kind of thing, or is it just remembering to go 5 mph faster?.
Also, would a tire store have a tool like Tom A shows and they would recalibrate for me with purchase?
Finally, I can get the diameter and the tread I want in a 17" or a 15" tire. What is the consensus on fitting one of those onto a 16" rim?
So you got me curious so I looked up what I could find in my 1999 GM service manual. What I found was that the shift speeds for the transmission are a function engine speed in RPM and to what degree the throttle is open (% TPS), not speed of the vehicle.

What confused me is that there appears to be three different calibrations for the 4L60E depending on rear end ratio but these ranges overlap:

A; 3.08/3:43
B: 3.08/3.42/3.73
C: 3:42/3.73

The relative shift points are at lower engine speeds for the higher gearing. (see attached) Does anyone know about these different calibrations?

There might be other inputs into the ECM etc. but this is what I found.

With a larger tire you have effectively changed the differential gearing ratio; i.e. for a tire with 7% larger circumference, you travel 7% further for the same number of turns of the drive shaft or that a differential gearing of 3.73:1 becomes 3.47:1. To calculate your mpg you will need to know the actual distance traveled, not what is on the odometer.

With regard to acceleration; although the torque at the axle is unchanged, with a larger diameter tire the force applied becomes smaller (torque = force * lever_arm)
 
Attached Files
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Trans, shift speeds 7_49.pdf (649.8 KB, 7 views)
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Old 12-25-2018, 09:30 PM
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Originally Posted by cleburne red View Post
I don't think you can put a 17" or 15" tire on a 16" rim. Can you not get the tire you want in 16"?
Even if you could somehow stretch it to your rim, I wouldn't want to be anywhere near you on the highway
​​​
I brought it up because I saw this ad on Craigslist, someone selling four 16" tires installed on 14" wheels. https://houston.craigslist.org/pts/d...763693128.html
 
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Old 12-25-2018, 09:33 PM
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Originally Posted by puttster View Post
I brought it up because I saw this ad on Craigslist, someone selling four 16" tires installed on 14" wheels. https://houston.craigslist.org/pts/d...763693128.html
I think those are off of a 2014 Jeep. Not 14" wheels.
 

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