Tire size calibration

[Brusky]

I got new larger tires. Which of course threw the speedo off, so no big deal, came home looked tire height up on line. (33.3in) so I reprogrammed with my superchips 1715 to 33in. And it threw the speedo off even farther but in the other direction saying I was doing 3mph faster than I actually was. So I then put a tape measure to them while still on the vehicle and it measured 32in. So I set the programmer to that and it's still off saying I'm doing 1 mph faster than actual. Any help would be greatly appreciated.

All checked with 2 different GPS devices that both read the same

 

[toms89]

Can you find the tire diameter specs direct from the tire manufacturer? It should be the most accurate. Tough to measure because of the squish factor...lol.

Do you have the correct gear ratio programmed in?

1 or 2 mph off at 60mph is not bad. You are also dealing with some margin of error at both ends. GPS and speedo.

Unless you carry it out to the 1,000ths it will not be precise but it should be within a few percent. Tires do wear with age so it changes. Ideal number to use would be the 1/2 life diameter.

If you had to measure the tire yourself I would measure the circumference and calculate diameter from that. Should be much more accurate. Formula or calculators for this is available online..

With my custom tuning software it uses tire revolutions per mile which is much more precise. The tire manufacturer publishes this info.

 

[Lightnig]

...

With my custom tuning software it uses tire revolutions per mile which is much more precise. The tire manufacturer publishes this info.

Actually, that one would be really easy to do at home (so to speak) without any fancy equipment.


Take a friend, a piece of chalk, and a long tape measure to an empty parking lot. Mark a spot on a tire and the ground, and then drive slowly ahead while your friend watches the mark on the tire. Once it has gone, say 10 (more would be more accurate of course, but being able to measure the distance becomes more difficult) revolutions stop and then measure how far the truck actually travelled along the parking lot.

Then just sit down and do the math...

 

[Lightnig]

...

Then just sit down and do the math...

Which would be: distance measured in inches / ( 3.1415 * Number of tire revolutions) = tire diametre in inches


for Example, if a guy travelled 962" with 10 tire rotations:

962 / ( 3.1415 * 10 )

962 / 31.415

30.622" tire diametre...

 

[toms89]

Actually, that one would be really easy to do at home (so to speak) without any fancy equipment.


Take a friend, a piece of chalk, and a long tape measure to an empty parking lot. Mark a spot on a tire and the ground, and then drive slowly ahead while your friend watches the mark on the tire. Once it has gone, say 10 (more would be more accurate of course, but being able to measure the distance becomes more difficult) revolutions stop and then measure how far the truck actually travelled along the parking lot.

Then just sit down and do the math...

I'm saying the tire manufacturer actually publishes this figure. I simply load it into the software. Revolutions per mile takes almost all the error out of it that the math provides when you are off fractions of an inch multiplied by ??

 

[toms89]

Which would be: distance measured in inches / ( 3.1415 * Number of tire revolutions) = tire diametre in inches


for Example, if a guy travelled 962" with 10 tire rotations:

962 / ( 3.1415 * 10 )

962 / 31.415

30.622" tire diametre...

That is a great way to find the circumference and calculate the diameter.

 

[Brusky]

Thanks for the responses. I have had this posted in trans and drivetrain for a couple days with almost 100 views without 1 response. Lol

Nittos site says tires are 33.3in. With tread depth of .67in. I set the size at 33in. Left the gears at stock setting. And it was way off saying I was going much faster than actuall.

It is now set on 32in and stock gears and still says 1 mph faster than actual. If anything it should be the other way.

Should I set it back to 33 and change gear from stock to the actuall size of the gears and see how close it is. ?

 

[toms89]

If you have 3.73's and the software believes you have 3.55's the speedo would read about 5% high. The opposite would be true if it was reversed. Numerically lower gear ratios raise indicated speed as it should. Maybe the gear ratio is incorrectly identified in the tuner.

Have no idea what gear ratio you do have and not familiar with the superchips.

 

[toms89]

Thanks for the responses. I have had this posted in trans and drivetrain for a couple days with almost 100 views without 1 response. Lol

Nittos site says tires are 33.3in. With tread depth of .67in. I set the size at 33in. Left the gears at stock setting. And it was way off saying I was going much faster than actuall.

It is now set on 32in and stock gears and still says 1 mph faster than actual. If anything it should be the other way.

Should I set it back to 33 and change gear from stock to the actuall size of the gears and see how close it is. ?

Don't add for tread depth!! Stick with the advertised diameter.

Edit: Ops! Never mind.. misread your post..

 

[Brusky]

The tuner has a stock setting for everything which the gears are set at stock. Your saying mabe they are possibly wrong? I have 3.55 according to the sticker. Should I set it back to 33in and manually set the gears in the tuner to 3.55? It's worth a shot.

 

[toms89]

The tuner has a stock setting for everything which the gears are set at stock. Your saying mabe they are possibly wrong? I have 3.55 according to the sticker. Should I set it back to 33in and manually set the gears in the tuner to 3.55? It's worth a shot.

The mph would read low if you have 3.55's and the tuner was incorrectly set for 3.73's so its likely just error in calculated tire diameter.

Or... you actually have 3.73's and its programmed for 3.55's as you said but I trust you know your gear ratio.