I have a 71 FJ40 and the speedometer reads high. There are many posts here about tires and errors but not this.
At 60 on my gps, the speedo says 70. Assuming that the speedometer came reading up to 4mph high I’m 6 mph or 10% off. In my way of thinking it should be because the tires on it now have a 10% shorter rolling circumference than stock due to tires shorter than stock...
Except, my tires are taller than stock. My truck came to me with 31 x 10.50 r15s. A radius of 15.5 puts my rolling circumference at 97.34 inches.
Stock tires per multiple posts were 235 75 r15’s. Or 29.2 inches tall putting the stock rolling circumference at 91.69 inches.
5.65 inches extra travel per rotation should have the truck going faster than the speedometer not slower.
Why is my error the opposite of what I’d expect?
I’ve thought of two things. Could the rear end be different? Higher or lower? How would I know?
Could my instrument panel be from a different year? Again, how would I know?
All of this because, besides wanting a correct speedometer, my 31s seem shorter than tires on trucks I see online. My truck has two inches of lift. Is that enough for the 31s to seem short? They are way wider than stock and squarer. Maybe that’s it.
I‘d be thankful for someone giving me answers.
At 60 on my gps, the speedo says 70. Assuming that the speedometer came reading up to 4mph high I’m 6 mph or 10% off. In my way of thinking it should be because the tires on it now have a 10% shorter rolling circumference than stock due to tires shorter than stock...
Except, my tires are taller than stock. My truck came to me with 31 x 10.50 r15s. A radius of 15.5 puts my rolling circumference at 97.34 inches.
Stock tires per multiple posts were 235 75 r15’s. Or 29.2 inches tall putting the stock rolling circumference at 91.69 inches.
5.65 inches extra travel per rotation should have the truck going faster than the speedometer not slower.
Why is my error the opposite of what I’d expect?
I’ve thought of two things. Could the rear end be different? Higher or lower? How would I know?
Could my instrument panel be from a different year? Again, how would I know?
All of this because, besides wanting a correct speedometer, my 31s seem shorter than tires on trucks I see online. My truck has two inches of lift. Is that enough for the 31s to seem short? They are way wider than stock and squarer. Maybe that’s it.
I‘d be thankful for someone giving me answers.
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