Ahhh, the speedometer story...
Well, when the truck was new, I took the liberty of measuring my odometer against the mileage markers along the freeway. Over 10 miles, my odometer moved 10.0 miles, so the odometer was right on the money, new.
The speedometer was a different story; Our local police has a habit of placing this goofy trailer along roads where people speed a lot. It doesn't produce a ticket, but it has radar and it shows your speed as you drive by (it also has a big speed limit sign on it as a friendly reminder). Well, I drove by this thing at 35 mph and it said I was going something on the order of 32 mph, so the speedometer portion of my truck was off.
Now, after I changed the tires to the new 265's, I did the odometer thing again and found that after covering 10 miles, the odometer only moved 9.5, a 5% error. This figures properly with the tire diameter change which also calculated to about 5%.
The speedometer also changed 5% as one would figure, but it is now very close. I was following Ryan Penner through Colorado on the way to Moab and asked him how fast his GPS said we were going. I can't remember the exact speed, but it was within 1 mph of my speedometer reading.
So, after this long story and if you are still reading, I did not change anything to correct the speedometer. I'd rather have the speedo reading correctly, and 5% on the odometer isn't all that far off anyway. Besides, I think that a reprogramming would be in order to change it on these newer ones. I haven't looked but I remember reading that the speedo/odometer is controlled electronically rather than by a cable. I could be wrong.
Now, as to why they aren't both accurate is beyond me, but anyone who has a theory is welcome to present it. Maybe they want you to think you're going faster than you really are.
