How much will my spedo be off with 31s?

[WannaXP8]

I have the stock 235/75/15s and stepping up to 31s? so how much will my spedo be off?

 

[Byrd91]

31 / 29 = 1.07 or 7% off or 3.5 MPH at 50 MPH

 

[expo5.0]

your sig says you are going to get the same tires as me, and their 29s? whats the deal?

 

[WannaXP8]

well I was going to be lowered..but I decided to lift..

 

[Blue Steel]

my speedometer was a bit off. when i was going 70mph i was going really 80mph. when i was going 45 i was going around 50mph. you can get a speedo gear with another tooth to it to correct your speedometer.

 

[Robert]

Originally posted by Blue Steel
. . .you can get a speedo gear with another tooth to it to correct your speedometer.

Not for '98's and up.

 

[Okrazie1]

A '98 doesn't have a speedo gear. It would require reprogramming the computer to correct the speedo.

IMO, I wouldn't worry about it. With my 31's, my odometer reads about 9.4 miles per 10 miles actually travelled, but the speedometer seems to be pretty close as checked by the portable radar stations (trailer with a digital speed indicator) set up by the local police to remind you of your speed. Ford states that for an actual distance of 10.0 miles travelled, an odometer reading of 9.6 to 10.4 miles is acceptable and considered accurate.

 

[Fischer]

My GPS tells me I am going 72 when my speedo in truck says 65 or so.

 

[Brian8]

Damn i just put 31's on mine and figured it would throw off the speedo some but didn't think it would be that much. I haven't checked mine yet. Wish i would have remembered last night b/c I drove ny one of those speed trailer things.

 

[Fischer]

I think it is a algorythm type of increase of speed. The faster you are traveling the more off the speedo is.

I am almost on at about 35 or so. Though the truck speedo is not rearly as detailed as the gps. It is just in the area of 35 or so.

 

[FourXFred]

My truck use to read fast anyways, and the speedo is actually perfect now. Guess i was lucky. Too bad my gas mileage went down like 3mpg. Oh whel, the tires look cool.

 

[Robert]

Your speed will be off by the same percentage as the change in tire size.

(new diameter/old diameter) * indicated speed = actual speed

Those with '97 and earlier can fix with a $7.50 gear and 5 minutes of your time. Those with a '98 require a speedometer calibrator or Ford to reprogram it.

 

[Sport Trac SVT]

You can go to Tire Size Calculator and calculate the diffrences.

 

[Sport Trac SVT]

You can also go here.