Speedo wrong after adjustment

[Longarmtj]

So I'm running 315 70 17 and set the tire size at 34.4 close to the measurement and my speedo is reading 4 mph above GPS reading. What do you guys have your tire height at for this size?

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[johnparjr]

Who makes the tires some manufacturers list the size other you have to measure

 

[j.o.y.ride]

Just keep tweaking until it's close. If 34.4 doesnt work try 34.0. If 34.0 isn't right adjust again.

 

[Renegade]

I found that measuring from the ground to the hub center, and multiplying by 2 got me where I needed to be. In theory, it's more about tire circumference, but my method worked for me.

 

[KurtP]

So I'm running 315 70 17 and set the tire size at 34.4 close to the measurement and my speedo is reading 4 mph above GPS reading. What do you guys have your tire height at for this size?

The only way to properly load your tire size is to measure the circumference of the tire, mounted on the wheel, at your street pressure, and math the diameter. Dont go by what’s listed on the tire or the manufacturer’s website.

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[hjdca]

So I'm running 315 70 17 and set the tire size at 34.4 close to the measurement and my speedo is reading 4 mph above GPS reading. What do you guys have your tire height at for this size?

After I put in the 5:13 gears and the 37s (30 psi), even when I set the true values correctly on the Truck (ie. 36.25), the mph was minus 2 mph, especially at high speed compared to my escort radar detector mph. To get the proper mph synchronized with my escort radar detector, I had to set the tire size over 37 inches. Now my escort radar mph is precisely synced with my truck mph at any speed. I tested using cruise control for multiple days at various speeds. Note: I have the manual transmission which has a lower 6th gear than your 8th gear, so, that maybe the reason.... Also, stock, my 33" tires were set at over 33" on the computer from the factory, something like 33.07 I think, maybe more...

 

[Longarmtj]

I'll keep adjusting around because it feels like the transmission isn't as happy as it could be right now either

 

[AZJT]

As others have said, just keep playing with it until you get it right. I put the Tazer Mini on some months back, started with the OEM spec and found it to still be off. I then measured by hand, put that # in, and it was still off. It ended up being somewhere in the middle - go figure.

 

[XraytecH]

Tried the stated size. Way off, almost 5 miles off.
Tried measuring. Closer but still off.
Tried the toothpaste method. Better but just a little off.

I ended up tweaking the tire size using my TAZER until the speedometer is within 1mph.

 

[AZJT]

Tried the toothpaste method. Better but just a little off.

Pardon my ignorance, but - what IS this method?

 

[hjdca]

Tried the stated size. Way off, almost 5 miles off.
Tried measuring. Closer but still off.
Tried the toothpaste method. Better but just a little off.

I ended up tweaking the tire size using my TAZER until the speedometer is within 1mph.

If the GPS is 1 mph higher than your speedometer, then, go up in tire size 1/4 inch or 1/3 inch on your computer. If it is 1 mph lower, then, the opposite applies. There is no reason that you cannot get it perfect. I was able to get mine right on the GPS.

 

[XraytecH]

If the GPS is 1 mph higher than your speedometer, then, go up in tire size 1/4 inch or 1/3 inch on your computer. If it is 1 mph lower, then, the opposite applies. There is no reason that you cannot get it perfect. I was able to get mine right on the GPS.

Only problem with that is when using the TAZER, it displays very specific tire sizes. I can't enter exactly what I measured. I adjust one click up, I'm over. I adjust one click down, I'm under. There's no in between.

 

[XraytecH]

Pardon my ignorance, but - what IS this method?

Draw a line of regular white toothpaste across the tread of a tire. Drive forward one rotation. The toothpaste marks two lines on the ground. Measure the distance between those two lines, aka tire circumference, then divide that number by 3.14.

 

[RichardCranium]

Pardon my ignorance, but - what IS this method?

I'm assuming he means the chalk test method for getting optimal PSI for even tire wear.

 

[XraytecH]

I'm assuming he means the chalk test method for getting optimal PSI for even tire wear.

Almost, I use toothpaste to measure the tire circumference not proper tire inflation. I suppose chalk will leave enough of a mark on the ground.

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