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Old 06-23-2013, 05:40 PM
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Tire size v. MPH

Ok, since you guys are experts, tell me if I'm right.

Wifes truck, 2000 F150 Lariat.

Currently has P235 70 17. 63MPH on speedo means 60MPH real.
Door sticker says P275 60 17. That would be the same sidewall, which would be 63MPH speedo and 60MPH real correct?

Tire Size for 2000 Ford F-150 Lariat
Using that website, it gives me a list of possible OEM sizes.
Using the P265 70 17, I think I used the calculators right and it would actually correct my speedo, right?




General question, MY truck states 215 85 16 or something like that is OEM size. But using 235 85 16, my speedo is actually right. Is that common?
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Old 06-23-2013, 06:37 PM
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No your speedo is incorrect. The 60,70,85 etc is the percentage of the tire width. So for instance if you have 275/60r20's the sidewall is 60% as tall as the tire is wide. Make sense?
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Old 06-23-2013, 06:43 PM
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No I understand that. I'm saying right now, on 235 70 17, it says 63MPH and is GPS rated right about 60MPH.
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Old 06-23-2013, 06:50 PM
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Then your speedo needs recalibrated.
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Old 06-23-2013, 06:54 PM
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Quote:
Originally Posted by Tuscany View Post
No your speedo is incorrect. The 60,70,85 etc is the percentage of the tire width. So for instance if you have 275/60r20's the sidewall is 60% as tall as the tire is wide. Make sense?
X2

First number is in mm. Second is % of first. Conversion is divide by 25.4 to get inches, then double that plus rim.

So. Your door says correct tire size, specifically height, must be 30".

275.6=165 165/25.4=6.49 round up 6.5x2 13+rim17=30"

Whatever tire/wheel setup you want without recalibrating pcm to keep mph correct, just plug in the numbers to get as close to 30"

Sent from AutoGuide.com Free App
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Old 06-23-2013, 06:55 PM
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Maybe this will help. My stock tire size is 265/75r16. I have 315/75r16 and my speedo difference is 9.3% over my reading. There are charts on google that will tell you the percentage. You put in stick tire size and the other size and it will tell you over or under percentage. Can't find the link or I'd post it.


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Old 06-24-2013, 03:57 PM
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Speedometer Calibration / Tire Size Change Calculator
I used this and another.

From Tire Size for 2000 Ford F-150 Lariat <- that website, it lists three possible OEM sizes. 235 70 17 and 275 60 17 have the same sidewall size.
The other, 265 70 17 has a larger sidewall.
Basically, I'm making sure that if I run a larger sidewall, my speedo will drop in displayed speed.
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Old 06-24-2013, 06:02 PM
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Quote:
Originally Posted by jesilvas View Post
No I understand that. I'm saying right now, on 235 70 17, it says 63MPH and is GPS rated right about 60MPH.
Quote:
Originally Posted by jesilvas View Post
Speedometer Calibration / Tire Size Change Calculator
I used this and another.

From Tire Size for 2000 Ford F-150 Lariat <- that website, it lists three possible OEM sizes. 235 70 17 and 275 60 17 have the same sidewall size.
The other, 265 70 17 has a larger sidewall.
Basically, I'm making sure that if I run a larger sidewall, my speedo will drop in displayed speed.
235 70 17 is 29.95" tall
275 60 17 is 29.99" tall
265 70 17 is 31.61" tall

In total, there is only a 5% difference between these tires. 31.61/29.99=1.05 Guess what? Your speedo is...wait for it.... 5% off 63/60=1.05. Everything is working like it is supposed to.

In short, your speedo was calibrated for the tallest of the three tires. Ford most likely did it because it's better to go slower than indicated than to go faster than indicated.
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Old 06-24-2013, 06:14 PM
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Ok cool, just wanted to make sure I thought it all through right.

That's so dumb, because my truck is also the same way.
215 85 16 = 182.75mm sidewall is sticker oem.
235 85 16 = 199.75mm sidewall is what makes speedo right.
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Old 06-24-2013, 06:24 PM
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Keep in mind that different tire manufactures can have a different OD for the same size tire, hence why I'll never buy tires at Pep Boys ever again.
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