Measuring tire diameter for speedo recalibration

[Air Biscuit]

I am planning on getting a tuned PCM and might as well have the speedo recalibrated for my 265s (245s stock)

 

Can someone tell me how to measure the diameter of my tires? I know it sounds stupid but I was wondering if I should measure it vertically (including the flat spot where it sits on the ground) or if I measure it horizontally across the tire. The measurements are understandably different so any help would be appreciated.

 

I checked the diameter on the Tire Rack site. It shows 31.8" for my tires but mine are definitely not that big...Possibly because they measured them unloaded on a 7.5" rim while mine are mounted on the truck on a stock 7" rim.

 

Thanks.

[Bish]

I am planning on getting a tuned PCM and might as well have the speedo recalibrated for my 265s (245s stock)

 

Can someone tell me how to measure the diameter of my tires? I know it sounds stupid but I was wondering if I should measure it vertically (including the flat spot where it sits on the ground) or if I measure it horizontally across the tire. The measurements are understandably different so any help would be appreciated.

 

I checked the diameter on the Tire Rack site. It shows 31.8" for my tires but mine are definitely not that big...Possibly because they measured them unloaded on a 7.5" rim while mine are mounted on the truck on a stock 7" rim.

 

Thanks.

 

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Put a vertical chalk mark on your tire and on the ground. Push, drive, roll forward 1 revolution until the verticle line on the tire is straight up and down. Measure the distance between the 2 chalk marks and divide by 3.14157. This will give you the tire height or diameter.

[mudbug660]

I've never thought about that I need to try this also. I need to get a retune also just got some 285/60r17, stock was 235/75r16

[Butch 02 Denali]

Another option is to measure the loaded tire height. The distance from the axel centerline to the ground and multiply X2 to get the diameter, or multiply it by Pi (3.1416) to get the true circumference.

 

Since it's difficult to see or measure from the center of the axel, take the Measurement from the top of the rim to the ground, then measure the rim, then subtract 1/2 the rim measurement to get the true loaded radius.

[SS Crew]

Here try this:

 

Tire Calculator

[redvett]

Why dont you just tell the tuner you have 265s :-)

[Air Biscuit]

Why dont you just tell the tuner you have 265s :-)

 

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The tuner wants actual measurements as all 265s are not equal in terms of diameter (significant variations according to brand, tire type, tread depth, load range, inflation pressures, etc).

 

I like the rollout technique. Seems like it will get me about as close as close can be.

 

Thanks for the help, guys.

 

-jeff

[pcrow]

My BFG TA/KO's 265/75-16 on 8" wheels are rated at 31.6" As measured by 1 roll on the driveway they are only 30.55"

[99-2door]

Why dont you just tell the tuner you have 265s :-)

 

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The tuner wants actual measurements as all 265s are not equal in terms of diameter (significant variations according to brand, tire type, tread depth, load range, inflation pressures, etc).

 

I like the rollout technique. Seems like it will get me about as close as close can be.

 

Thanks for the help, guys.

 

-jeff

 

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Are you gonna get a new tune everytime you buy tires?? If not, just go from the tire size.

[Bish]

My BFG TA/KO's 265/75-16 on 8" wheels are rated at 31.6"    As measured by 1 roll on the driveway they are only 30.55"

 

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The Manfucaturer's spec is based upon a brand new tire. As it wears, it will change slightly. Also could change based upon inflation. It's not an exact science. I always figure if I'm within a half inch (would equate to 1.6% difference in speedo) I'm close enough. 1.6% would mean 100mph would actually be 98.4 or 60mph would be 59mph.

[Air Biscuit]

Why dont you just tell the tuner you have 265s :-)

 

<{POST_SNAPBACK}>

 

 

The tuner wants actual measurements as all 265s are not equal in terms of diameter (significant variations according to brand, tire type, tread depth, load range, inflation pressures, etc).

 

I like the rollout technique. Seems like it will get me about as close as close can be.

 

Thanks for the help, guys.

 

-jeff

 

<{POST_SNAPBACK}>

 

 

 

 

Are you gonna get a new tune everytime you buy tires?? If not, just go from the tire size.

 

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Well. I don't buy new tires too often so I might. The re-tune is only the cost of postage

 

j/k. Point taken. I'll still go with the rollout since my tires are relatively new.

[Aquamoose]

My BFG TA/KO's 265/75-16 on 8" wheels are rated at 31.6"     As measured by 1 roll on the driveway they are only 30.55"

 

<{POST_SNAPBACK}>

 

 

 

The Manfucaturer's spec is based upon a brand new tire. As it wears, it will change slightly. Also could change based upon inflation. It's not an exact science. I always figure if I'm within a half inch (would equate to 1.6% difference in speedo) I'm close enough. 1.6% would mean 100mph would actually be 98.4 or 60mph would be 59mph.

 

<{POST_SNAPBACK}>

 

 

 

 

 

 

Heck, roll the truck forward 3 revelutions, measure and divide by 3 to come closer to the perfect measurement! Of course, you could do 5 or 10 for an even better measurement!