What tire height are you guys using with HPTuners and 285/70r17. I recently swapped to these taller tires but speedo seems incorrect...

I used to use an old GPS to get the speedo calibrated, but it's now gone...

Appreciate any help...

 

Download the speedview app if you have a smart phone.

 

Just use the gear/tire wizard to enter the tire size.

They are like a 32.7in tire if you look that size up.

Never had an issue using that feature.

 

I used the drop down box for the tire size but it seems to be way off.

 

Then use a app on your phone to double check it.

Adjust it as needed.

 

The important number is the pulses per mile. There are 40 teeth on the speed reluctor so... Pulses = ((5280*12)/(diameter*pi))*40*rearend

So a 31" tire with 3.73s would be 97,116


Or something like that

 

32.83" is the tire's advertised height.

I'll try the app and play with the tire height to sync them. That's how I did it with the old GPS I had. Thanks for that app suggestion.

I'll have to pass on trying to figure out Atomic's formula...that's way beyond my pay grade, lol.

 

What rear end gears do you have?

I would suggest actually measuring the rolling diameter of your tires instead of going by the advertised. Make a mark on the bottom dead center of the tire to the ground, then roll the tire exactly 1 revolution until that mark is BDC again and make another mark on the ground. Now measure the distance between those 2 marks on the ground and divide by 3.1416 and that is your actual diameter.

 

3.73's.

Now that you've written the formula in English, lol, I can understand it. I'm just a dumb old mechanic...

What I did in the past, was went a steady 40mph, on level road, turned on cruise control, and compared to gps...then just skewed the tire height measurement, trial and error, and compared it to GPS, until they matched.

 

Yea thats one way to do it haha

Here is the actual way (see attachment):

The VSS pulses per mile is my equation above, so in your case =((5280*12)/(32.83*3.15159))*40*3.73=91365.7

The number to the left (gear/trans pulses) is basically the equation above divided by 40, so 2284 in your case.

This is what the gear/tire wizard does, but if you need to do it manually this is how. It is important to actually measure your tire size like I described a few posts ago if you want it to be accurate.