Do i need to adjust tire size up or down?

[dslfreak]

I dont know if this should be tires, tazer, or coders so asking here....

So 40m left in VA to NC border, I decided to use mile markers to calculate how far off I am on tire size, I reset the trip ODO as I hit the 40 mm on 95, when the mm hit 0 the trip ODO said 38.5 (IIRC) so I know I'm off but do I need to tell the computer larger or smaller tires with tazer, I believe I set it for 34.04 but ill be checking tomorrow.

TIA

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[Pigman67]

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[dslfreak]

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Lol thanks , but that didn't answer the question, I got the tazer already, I hadn't heard of ECRI but I know tazer can enable cargo bed outlet and front camera

 

[Barnaby’sdad]

If the actual elapsed distance is greater than what’s recorded by your dash, you need to recalibrate to a smaller tire size with your tuner. I.e. 34.4” instead of 35.0”.

You shouldn’t have to make a WAG. What Make, model, and size tire are you running? The manufacturer website should say EXACTLY what the diameter is with XX PSI in them to take out the guesswork.

 

[Pigman67]

Lol thanks , but that didn't answer the question, I got the tazer already, I hadn't heard of ECRI but I know tazer can enable cargo bed outlet and front camera

Yeah, i figured out I read it wrong and tried to delete.

 

[dslfreak]

If the actual elapsed distance is greater than what’s recorded by your dash, you need to recalibrate to a smaller tire size with your tuner. I.e. 34.4” instead of 35.0”.

You shouldn’t have to make a WAG. What Make, model, and size tire are you running? The manufacturer website should say EXACTLY what the diameter is with XX PSI in them to take out the guesswork.

Personally I did not change the tires, I purchased the jeep used. But I do know it is not the stock 32, the spare is the stock tire. So im just trying to get it closer to alignment of what it should be. If there is a website with this info please point me to it. As you can see it's a 35x12.5 fuel gripper m/t

 

[Barnaby’sdad]

See the link below. The speed rating 121Q. For some reason, I can’t scroll any farther to the right on my phone (reference the table below). There should be a column with actual diameter if you keep going to the right.

https://www.fueloffroad.com/gripper-tires/truck/

 

[Jefe1018]

Take a measuring tape, measure from the ground to the top of the tire on all 4 wheels, take the average, use that measurement.

 

[dslfreak]

Take a measuring tape, measure from the ground to the top of the tire on all 4 wheels, take the average, use that measurement.

I didnt think of all 4, I only used the rear driver, came to 34 with a level and tape measure

 

[Jefe1018]

I didnt think of all 4, I only used the rear driver, came to 34 with a level and tape measure

You can get it close, but it will never be perfect. Remember there are a number of variables at play here. Even cops will typically give you a 2-5 mile an hour freebie due in part for this very reason.

Other things broscience has taught me:

1) How much did your air pressure change overnight vs after driving for a few hours?

2) how many curves in the road?

3) How good was the technology when the guy put the mile marker signs in?

4) Have you ever had Google Maps tell you 3/4 of a mile before your exit where the highway sign tells you 1? I see it happen every once in a while.

The best thing you can probably do is get a GPS Speed app and match that to your speedometer. That'll get it as close as possible.

 

[ShadowsPapa]

If the actual elapsed distance is greater than what’s recorded by your dash, you need to recalibrate to a smaller tire size with your tuner. I.e. 34.4” instead of 35.0”.

You shouldn’t have to make a WAG. What Make, model, and size tire are you running? The manufacturer website should say EXACTLY what the diameter is with XX PSI in them to take out the guesswork.

You can't go by that and be accurate. It's the unladen diameter and even that is their target, not reality. My stock tires were 32.1 by the tire maker, the next set was 32.1 by the other tire maker - speedometer and odometer should have matched since the tires were the same size - but they weren't the same.
Rolling circumference will be different and that's what the JT's BCM uses.
It's measured in MM although tazer and others use inches and fractions and then converts it for the BCM.

I used a GPS and my truck's trip meter/odometer and what I had to enter to make it match perfectly wasn't even close to what the tire makers said.
The weight of the truck figures into it. A given tire with xx psi will have a different diameter or rolling circumference depending on the weight of the truck.

Use odometer, not speedometer. Speedometers round the numbers and you could be off by almost a mile per hour but the numbers match.
It depends on how OCD you are. I made mine match perfectly - after 17 miles of driving my GPS distance traveled matched to the tenth of a mile what the truck's trip meter said.

 

[Barnaby’sdad]

You can't go by that and be accurate. It's the unladen diameter and even that is their target, not reality. My stock tires were 32.1 by the tire maker, the next set was 32.1 by the other tire maker - speedometer and odometer should have matched since the tires were the same size - but they weren't the same.
Rolling circumference will be different and that's what the JT's BCM uses.
It's measured in MM although tazer and others use inches and fractions and then converts it for the BCM.

I used a GPS and my truck's trip meter/odometer and what I had to enter to make it match perfectly wasn't even close to what the tire makers said.
The weight of the truck figures into it. A given tire with xx psi will have a different diameter or rolling circumference depending on the weight of the truck.

Use odometer, not speedometer. Speedometers round the numbers and you could be off by almost a mile per hour but the numbers match.
It depends on how OCD you are. I made mine match perfectly - after 17 miles of driving my GPS distance traveled matched to the tenth of a mile what the truck's trip meter said.

Yes. It’s not going to account for wear either, but it’ll give you a starting point (as would a tape measure).

 

[Rusty PW]

To drive you OCD types crazy. New 35" MT tire with 21/32" of tread. You take them off at 2/32" of thread remaining. Does it still have the same OD as new? You lost around 1 3/8" in height. Think your speedometer and your odometer still correct from when you reset it when you first got your tires?

 

[ShadowsPapa]

To drive you OCD types crazy. New 35" MT tire with 21/32" of tread. You take them off at 2/32" of thread remaining. Does it still have the same OD as new? You lost around 1 3/8" in height. Think your speedometer and your odometer still correct from when you reset it when you first got your tires?

I'd have taken MTs off well before only 1/16" of thread remains - they'd be closer to racing slicks, and worthless in mud. They'd be at the recycler's.

Your example is full off the truck diameter. So although there's a fair amount off the diameter - gotta give ya that - the rolling circumference loss would be a bit less, and your truck would only sit about 5/8" lower than when new.
But then - That's enough to see - how can you stand your truck dropping that much?

OK, I'd probably shoot for the middle of the road on tires that lose that much - that way over the life of the tire, it would average out. Program in smaller than a new circumference, but larger than a worn circumference. And if your wife gripes about things being off, tell her you need to drive the truck more to make it accurate. Then when they are worn, you have an excuse to replace them sooner.

Yes. It’s not going to account for wear either, but it’ll give you a starting point (as would a tape measure).

The tire maker's diameter is not what the truck sees and shouldn't be used is my point.
It's not even a good starting point.
The guys using a tape measure on an installed tire measuring from the ground up are going to be closer. Check all the posts from guys who used the tire chart numbers and then came asking "what should I do" because it was so off.

The tire charts are also max PSI, not what you'd run on your own truck.

The tire pressure you use on your truck, and your truck's weight affect the rolling circumference. If in doubt, check out how ITPMS works and how much impact tire pressure has on the rolling diameter or circumference.

 

[Escape.idiocracy]

See the link below. The speed rating 121Q. For some reason, I can’t scroll any farther to the right on my phone (reference the table below). There should be a column with actual diameter if you keep going to the right.

https://www.fueloffroad.com/gripper-tires/truck/

These are some incredibly light weight tires!!!! ?

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