tire size speedometer offset

[stumblefoot]

We have probably all seen the gear ratio guide for larger tires, how about a chart that tells me how far off my speedometer will be when I put taller tires on? If there is a mathematical solution, there should be some sort of chart. For those who havent seen the gear ratio guide its @ Lift Kits, Tires & Wheels by 4 Wheel Parts

[krabysniper]

Dosn't work that way. Speeddometer's are calibrated to work with a given drive train when the vehicle is built at the manufacturer. When you change tire sizes you alter one of the factors in that calibration and your speedo will now be off by a % based upon your actaul speed (because of how a speedometer works (this quote was the best description I have found and comes from this link How a speedo works "How does a speedometer work? Talk to your local speedometer repair center or mechanic, but in most cars the same principle is used today as in the Model A Ford. The standard revolutions of the cable are: 100 rpm equals 60 mph. The cable turns a magnet inside a drum which holds the speedometer needle. The faster the magnet turns, the more the drum moves. Through precision manufacturing, modern speedometers are highly accurate."

So if you alter wheel size you alter this input to the magnetic drum, by a percentage. For instance lets say you change your tire size and it alters your speedo by 5%, at 50 mph it's off by 2.5 mph, but at 100 mph it's off by 5 mph. Now lets say your tire size alteration changes the calibration by 10 %, now at 50 mph it will read off by 5 mph, and 10 mph at 100 actual mph.

[HoosierFarmer]

If you're not 100% sure, here's a simple solution, check with the cops. I work for the Sheriff's Dept in Indiana. I have had people ask me in the past 'I put new tires on my truck, is there a way you can tell me how far I'm off?' As long as I'm not busy, I dont mind sittin on the side of the road and clocking them when they pass, then tell them how fast they were ACTUALLY going. Thus, they know if it's off 3mph or 13mph. Worst they can do is tell you they're busy.

[13" shawn]

Either way this is a decently easy fix, you can change out the gear on the tail shaft of your transmission that will change the revolutions of the speedo cable there for resetting your speedometer calibration for your new tires.

[hunter1991]

how would that be changed in the chevys 88-98? cuz i thought they have a sensor and not a gear like on the older fords? and the chevy sensor is in the t-case i thought