When regearing a diesel.....

[TurboChris]

Is it necessary to go in and change the gear ratio in the computer (taser) so that the trip computer remains (somewhat) accurate? Or is it just as long as the MPH is accurate the trip computer is accurate.

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[Rusty PW]

If changed the gear ratio in the diffs. You have to change it because of the shift points in the transmission. Plus change the tire diameter if you went to bigger tires too.

 

[Zachanadandy]

How would the MPH be accurate if you didn't change it? I guess you could leave the gear ratio but adjust tire size to make it accurate, but why not just adjust both to be right? If you got it accurate by putting the wrong tire size and gear ratio then it wouldn't matter as far as driveability as shift points, etc would still be correct. The computer wouldn't know the difference. But why?

 

[TurboChris]

Lol, I was only asking about the relationship between the gear ratio in the computer to the MPG on the display. I just got re-geared and all of the sudden I was getting 5 mpg better on the freeway on the way home. My speedo perfectly matched my gps but I was doubtful that the mpg could be accurate so I posted up real quick asking.
I got home and adjusted it to 4.56 using the taser and it seemed to make the difference. A quick 10 mile drive seems to confirm brought it down to a reasonable number.
I'll fill up tomorrow and run a tank and hand calculate to compare.

 

[Vtur]

Yes as you have noticed. The computer thinks that you had drove a longer distance due to lower pinion ratio.

 

[TurboChris]

Not sure about that as the speedo was 100% accurate.... but the display mpg was off.

 

[Camaroboi13]

Out of curiosity, is your phone plugged into your Jeep when reading the gps? Because if it is, your gps will automatically correspond with the speedometer. It’s weird but it does. When I went to 37s I checked my phone gps when the speedo said 65, and the gps matched it. The next drive I left my phone unplugged and did 65, and the gps showed 73 which made more sense.

 

[TurboChris]

Now that is interesting...... it might have been! I'll double check it today!

 

[TurboChris]

Ok that was the intent of my original question. Thanks for that!

 

[kevman65]

Any time you're trying to verify accuracy of the JT speedo, you HAVE to have an independent source.

I have a Garmin GPS and a radar detector that uses GPS to verify vehicle speed.
I used these two items to get my Speedo right after going to 35's and using a Tazer.

I will have to do this again next weekend after the 37's go on.

IF you use a GPS on your smart phone, make SURE the smart phone is NOT mated to the JT in any way.

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