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I want to correct my speedometer

95 dodge half ton ram slt larmie sport, 2 wheel drive 318 auto… it has 245/75/16’s and has since new[including the spare tire] but the speedometer reads faster than your actual speed[ checked it with a gps and against polce radar and against our other car. its 2-3 mph at 30 and more like 3-5 mph off at highway speeds like 60/65. the truck has a sticker in the door jam that says its a 225/75/16 equiped truck, if it had this smaller tire, it would worsen the inaccuracy issue. the truck is an electronic speed sensor not a speedo cable equipped truck. how do i fix the problem?, new sensor?? and is that a dealer only item?? P.S. i want to put the 225/75/16’s on next time i’m buying tires. so what can i do?? also is the sensor easy to replace or is it a special tool/skill type of repair?

In larger cities there is usually at least one shop that will take care of that, but remember it is intentional. Most all cars come with their speedometers reading about 3-5% fast. This is to avoid problems if the calibration is off and reading too slow and you get a ticket.

However… You say you have a 1995 and electronic. I am not at all sure what kind of system you have, but I would have to guess it is still correctable.

Your proposed new tyres will result in an additional 4% slower display.

If you find a shop that can recalibrate your speedo they will only do so with the tires currently on the car. Perhaps you should wait to correct the speedo until after you have the new 225’s on the truck.

Changing the speed sensor will not alter your speed reading, it doesn’t work that way. Many (don’t know about yours) can be corrected by reprogramming the computer in the cluster. A friend of mine used to (maybe still does) sell a small electronic device that he designed that you would connect between the speed sensor and the rest of the car. It would scale the output of the speed sensor so that it read any speed you wanted it to.

“so what can i do??”

Just keep this discrepancy in mind when you’re driving.
I wouldn’t spend a dime over this issue.

I agree & I have the same “problem” on my 87 Ranger.

Cruising at 75 MPH means setting the CC at 80.

4% slower makes it very close to exact.