I have a 1994 f150 4X2 and am planning to put a Bridgestone Dueler A/T or firestone destination A/T 31X10.5 tire on it. Will going to a bigger tire affect the speedometer? if so by about how much?

thanks

 

Here's the deal. Your speedo is probably linked to your tranny via cable so it dosen't care what size tire you have. It will always read what the tranny is turning. Upping the tire size will not effect the reading but it will affect your actual speed. Unless you are drasticaly changing your tire size I would not worry about it. I went from a stock 33 to a 35 which added about 6.5 inches of tread per rotation but my speedo is still reading close to my actual speed. The difference is so negligable that when I pass one of those radar speed limit signs I am usually dead on with what it says I am doing.

 

He's got a '94, so his speed is sensed off the tone ring in the rear differential. There's an adjustment in the odometer to compensate for different tire sizes. I don't recall the process.

Jason

 

Not true. First off he has a 94' so the reading is taken from the differential. Second, tire size WILL ALWAYS affect speedometer reading. Unless your speedometer uses some sort of ground speed mearuring sensor it must use the roational speed of the wheels to determine the speed. Changing tire size will always affect the speedometer reading regardless if the measurment is taken at the transmission or the differential. With that said the change will be more dramtic depending on what size tires your speedometer is calibrated for and what size you are changing to.

 

I believe that Rotor meant the same thing as he referred to actual speed.

The circumference of the tire changes the distance traveled for each rotation of the tire and thereby impacts the speedometers ability to display true vehicle speed.

Why not just calculate: inches per mile/tire circumference = number of rotations of the tire to travel one mile. Compare this to the stock tire circumference and you will know what the error is when using the larger tire.

 

check the artical/spec section for PSOM adjustment at the top of this page. tells you how to fix this problem--easy.

 

Go to www.1010tires.com somethin like that and they have a
tire size calculator that will give you speed off set, 60mph will
equal ???? with a bigger size tire.

 

Expect about a 10% difference. That's what I saw with my (bald) 31" tires. If I was doing 60, I was really doing 65. I verified it with a GPS. That's not to say that the speedo was all that accurate before.

 

On my f250 original tire size was 235- 85 -16 I went to a bigger
tire with my lift to 315-75-16 and the % change was 9% meaning that with stock tires i was reading 60mph on the speedo
and with the new tires i was 5mph slower on the reading of my
speedo at 55mph. So 75 would be 70. 65 would be 60 and so on.

 

In the old days, we changed speedo cable drive gears. Today, we program it.

Get 10% bigger tires, and your formerly accurate speedo will show 60 while you get a ticket for 66 mph.

Sure, you can calculate the changes. But it's easy to run those measured mile sections of the freeway with a stopwatch, and check your own speedometer error after the tire swap is made. Or maybe you know a friendly radar cop who will help you out.

 

here you go to save you some time here is the link to the fix for adding larger tires to your truck

https://www.ford-trucks.com/article/...ND_BRONCO.html