Dumb question :( Oversized tires vs speedometer

[fishingsetx]

Had a 1979 f150 Ranger back in high school that showed 55 at 70 mph. Tires were 38.5' - 12.50X15. Only way to fix it was to regear the speedometer. it was like that when I sold it!

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[Saltyag15]

My uncle has an 07 Dodge 2500, and he has some bigger tires on it. Nothing overly huge, probably a 35in. equivalent. His speedometer is 10% slow at all speeds. He failed to mention that to me when we were driving to the ranch in Junction. I had my cruise set on 80 in the 80mph zone, and was actually going about 88. He didn't relay the information to me until the blue and whites came on behind us.

Luckily, the police officer that pulled me over just happened to have a similar truck with the exact same tires. He told me what my uncle already knew, and gave me a warning. Good dude!

[MONSTERKEN]

Slower. My speedo is 5mph with my 35's.

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[150class]

According to the Mcmullen county sheriff my old trucks tires threw the speedometer off by 7 mph at 75 mph

[CabezaBlanca]

Here is a good calculator that shows speed differences between tire sizes.

http://tireconvert.com/