Speedometer reads 8-10 MPH too low, why?

[philo_beddoe]

Could it be the cable or guage as to why my speedometer is reading 8-10 mph too low??  Where should I start?

It always felt low, then when I went through one of those stationed police radar guns that tells you your speed, sure enough it was about 8-10 mph too low.

One mechanic thought is was an improper tire size, which makes sense, but my cable hooks to the tranny. Actually my tires are way too fat, I think....could it be the wrong tire size??

[timthescarrd]

I'm assuming 2-wheel drive, what year is your truck? electronic speedometer or not?  The speedometer is driven by a pair of gears in the transmission (or transfer case in 4wd).  If the rear gear ratio or tires are changed from stock and these gears are not changed, it will throw off the speedometer reading.  Taller tires = speedo reads lower than you are going Shorter tires = speedo reads high.  width of tires doesn't affect it.

If that is what is causing it, your speedometer will read off by some percentage (So if speedo says 65 when your at 75 then it will say 39 when your doing 45 [reading about 8.5% low])

My truck came with 235/75R15 (about 28" tall) and Now I run 31x10.5x15 (about 31" tall). When my speedometer says 45 I'm going about 52, when it says 65, I'm going about 75

[philo_beddoe]

Excellent response!!  Makes perfect sence, i have the wrong tires. Regardless, i'm going back to stock.   Thanks!


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[philo_beddoe]

Oh yeah, i have a 77 c-10, 2wd.


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