Tire size & diameter

PaulW · Post by **PaulW** » Thu Aug 13, 2015 5:58 am

Some tire size BS. Disregard if you are tire smart:
Example
Tire diameter for BFG TA KO
295/75R16 32.2" Tire RPM=626
315/70R17 33.6" Tire RPM=601
More
(The ones I recently looked up)
35x12.5R15 Baja 34.8" RPM=580
35x13.5R15 Krawler 34" RPM=594

Every tire is different in spite of them being models labeled size. Change brands an sure enough your speedo will be wrong for the same tire size.
Your sensitivity to a wrong speedo varies from person to person Ha.

Go to the manufactures tech site and look up the tire rev/mile and convert to diameter.
Formula:
(5280*12)/(Pi*RPM)= diameter in inches. The constant Pi is 3.1415
Use the tire RPM for calibrating your speedo (or use your tuner). For Broncos use the built in speedo electronics.

Wrightracing.net · Post by **Wrightracing.net** » Thu Aug 13, 2015 8:24 am

[/quote]For Broncos use the built in speedo electronics.[/quote]

Not all Bronco's are Electronic speedo's I am not sure what year it goes up to but my 86 is gear driven. You have to change the nylon gear size to recalibrate the speedo. You can get Ford OEM gears that are color coded. For 33" tires with 4.10 gears I ran the 21 tooth red gear to get my speedo to the correct calibration.

ChaseTruck754 · Post by **ChaseTruck754** » Thu Aug 13, 2015 9:00 am

GPS speedo is another way around this "problem".

Wrightracing.net · Post by **Wrightracing.net** » Thu Aug 13, 2015 9:30 am

My phone has an app
And it is really accurate.

PaulW · Post by **PaulW** » Thu Aug 13, 2015 11:56 am

Electronic speedo starts around 1996
The bottom line is tire diameters are not as marked on the sidewall. Adjust accordingly if you care.

Hauls Assington · Post by **Hauls Assington** » Wed Aug 19, 2015 3:08 pm

I phone 4 speedo, is accurate until you lose, satellite, or it gets too hot to function.

Wrightracing.net · Post by **Wrightracing.net** » Wed Aug 19, 2015 5:06 pm

Hauls Assington wrote:I phone 4 speedo, is accurate until you lose, satellite, or it gets too hot to function.

Get a Samsung. My Note 3 works as a GPS even in the middle of Plaster City and Glamis.

robertcrav · Post by **robertcrav** » Thu Aug 20, 2015 5:32 am

Hey David,

How do you like the Note? Wife and I are considering upgrading from out Galaxy S4s to the Note4. . . What GPS app do you use?