Impact of bigger tires and no computer update?

[AjMac]

I have been running 37's for a few thousand miles now, and never updated anything. According to my gps, my speedometer is about 3-4 mph low. I understand why... but, what about gas mileage? Is my computer showing lower than it is? And what about mileage? Is it somehow logging accurate rotations, or is my mileage counter low also?

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[glassjawkid32]

Odometer will be off over time. I just put 35 yokos on mine and I can't wait to get the tire size reprogrammed. It'll get you accuracy on your speedometer/Odometer and I can tell the shift points are off so hopefully it will fix that too.

 

[Dougstdig]

Yes and yes.

If you measure your rolling circumference (mark the tire and ground then roll 1 revolution…mark the ground where the chalk is at the same angle to the ground as the 1st mark…measure the distance between the caulk marks) divide it by pie and enter that number in to a tazer it should match the GPS.

 

[ecidiego]

When you sell the vehicle you have to disclose that the mileage is not accurate or you could just be dishonest but that's not what I would do

 

[DailyMoparGuy]

I went from the stock 31.5” tires to 32.8” and didn’t change anything yet. It’s been 6k miles….I should probably get that JSCAN thing going and update it lol.

 

[49Gramps]

You can figure with about a 12% increase in tire circumference, your speedometer and odometer are also off that much. Do nothing, but regear to 4.56 and you're within about 1% of where you started!

 

[AjMac]

Odometer will be off over time. I just put 35 yokos on mine and I can't wait to get the tire size reprogrammed. It'll get you accuracy on your speedometer/Odometer and I can tell the shift points are off so hopefully it will fix that too.

How are you gonna go about getting it set up?

Yes and yes.

If you measure your rolling circumference (mark the tire and ground then roll 1 revolution…mark the ground where the chalk is at the same angle to the ground as the 1st mark…measure the distance between the caulk marks) divide it by pie and enter that number in to a tazer it should match the GPS.

I dont have a tazer...or anything else like that, unfortunately. It comes up a lot in this forum, so it's probably worth having.

I went from the stock 31.5” tires to 32.8” and didn’t change anything yet. It’s been 6k miles….I should probably get that JSCAN thing going and update it lol.

Same, I dont have anything either to do that.

Anyone know how significant the mpg impact is?

 

[AjMac]

You can figure with about a 12% increase in tire circumference, your speedometer and odometer are also off that much. Do nothing, but regear to 4.56 and you're within about 1% of where you started!

Thanks, thats as good an answer as I can get! I am looking at 4.56 in the near future, I've only seen great things with the diesel running that!

 

[glassjawkid32]

How are you gonna go about getting it set up?



I dont have a tazer...or anything else like that, unfortunately. It comes up a lot in this forum, so it's probably worth having.



Same, I dont have anything either to do that.

Anyone know how significant the mpg impact is?

Currently waiting on the blue Santa as we speak to drop off my tazer. Was just going to get one of the cheaper programmers but it has other features id like to have like hood alarm and others. That being said make sure you look at compatibility because I'm not going to be able to update the tazer itself until I can borrow a windows pc from a friend. Mac is a no go.

 

[49Gramps]

Thanks, thats as good an answer as I can get! I am looking at 4.56 in the near future, I've only seen great things with the diesel running that!

Those calculations are based on the circumferences of tires actually measuring 33" and 37", which ain't always what's found in the real world!

 

[AjMac]

Will a JSCAN also update shift points when I update tire size?

 

[learjet]

just my experience…swapped from stock tires on my LE Rubi to KO2 37s which measure closer to 35. Shifted like crap, mpg and speedo obviously off. used an aviation certified gps and programmed the tire size change in my taser. started with the chalk mark and measure method to get close, then made small tire size changes till the speedo matched the gps. ended up programming just under 36” tire size if i remember right. speedos dead on now and she shifts like dream again.

hth

lear

 

[AjMac]

just my experience…swapped from stock tires on my LE Rubi to KO2 37s which measure closer to 35. Shifted like crap, mpg and speedo obviously off. used an aviation certified gps and programmed the tire size change in my taser. started with the chalk mark and measure method to get close, then made small tire size changes till the speedo matched the gps. ended up programming just under 36” tire size if i remember right. speedos dead on now and she shifts like dream again.

hth

lear

Thanks for the response. Im gonna be ordering something tonight, probably the JSCAN for the price.... I have only some basic needs lol

 

[ShadowsPapa]

I have been running 37's for a few thousand miles now, and never updated anything. According to my gps, my speedometer is about 3-4 mph low. I understand why... but, what about gas mileage? Is my computer showing lower than it is? And what about mileage? Is it somehow logging accurate rotations, or is my mileage counter low also?

If the odometer is off - which it is, then mpg calculations by you or the cluster will be wrong. Can't have one thing wrong and the other correct since mpg is - MILES per gallon and your miles will be off.

TIP - do not use the speedometer to tell when you have the correct tire diameter plugged in. Use the ODOMETER.
Put a GPS in the truck or use a GPS app on your phone. Set your tire size in the system, then ero out your trip odometer, and fire up a GPS or GPS app (don't use the truck or plug the phone into the truck, keep it independent)
Now drive about 20 miles and when you stop compare what the trip odo on the truck says compared to a real GPS. Only when it's the same to the 1/10 of a mile is it right no matter what the speedometer says since that rounds numbers off. It may say you are going 60 but are really going 59.7 and maybe NAV or GPS says you are going 61 but you are really going 61.2 - meaning you are actually farther off than 1 mile per hour.
Odometer readings are the only real way to know if you have the tire diameter correct.
And what you are "told" to plug in may be only a starting point - there's such a thing as rolling diameter and I plugged in a very accurate measurement on mine - but still had to adjust the number in the system after a test drive.

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