tire size change=mph change?

[KSears]

I recently upsized my tires from the stock bfg rugged trail 265-70-17 to some Toyo open country AT 285-75-17's, Does anyone know how much of a change it will make to the speedometer reading the current speed. I can't imagine it being too much different but if so what is the easiest way to have the speedometer corrected to read the acurate speed. I do not have a Smarty, nor know anyone that does. Thanks in advance for any help on the matter.

 

[BDaugherty]

Use this calculator to find out anything you would want to know about tires and gearing options:



Gear Ratio Calculator

 

[R.Taylor]

-2mph was what my old rig's speedo was off when I went from 265's to 285's. My new rig has 35/12. 5R18's and that throw's the speedo off -5mph.

 

[KSears]

thanks BD, great tool!, shows about 4 mph difference at 2000 rpms,

 

[Bob4x4]

If you take the revs per mile(from the toyo website)to the dealer they can change the pinion factor in the abs module on your year

 

[Jeepr]

Bob4x4, that is only true on 2005 and older. In 06+ it has to be a factory available tire size. Unless you have one of the 3rd party programmers, many of them can do it also.

 

[MAllen1]

If you know someone that owns their own shop, or, more specifically owns their own scanner (high dollar, like a snap on or factory), you should be able to recalibrate the computer so you don't have to go down the freeway adding 5 m. p. h when you pass a state trooper or diesel cop (d. o. t). I don't know about 3rd gen. , but, that is what I did with my 2nd gen when I went from stock 245 to 285/75R16. Good thing is that you can also calculate your fuel mileage better. Before I recalibrated I was getting 15 mpg and tried to recalibrate and alway came up with 17-17. 5. When I recalibrated the computer to read the speed with the 285's instead of the 245's, I was actually averaging 19. 5 mpg's.

 

[Bob4x4]

Bob4x4, that is only true on 2005 and older. In 06+ it has to be a factory available tire size. Unless you have one of the 3rd party programmers, many of them can do it also.

:-laf I am well aware of that. The OP has an 05

 

[Bob V]

The correction factor is a %, the faster you go the farther off it gets. 10% = 5mph off at 50, 10 mph at 100.

 

[JRone]

I went from 265/70 17 to 295/70 17 and my speed changed 2. 5mph according to GPS.

 

[KSears]

I went to the local Dodge dealer today in hopes they could recalibrate the truck computer to correct the odometer and speedo, to accommodate larger tires, I have an 05, they sware it can't be done to anyhing other than original optional tire sizes, I wonder if they are right , lazy, or just misinformed. Has anyone else experienced this? if they are wrong I would love to prove it to them, any suggestions?

 

[firemansdiesel]

I had my old '05 CTD speedo recalibrated w/ 35" tires through the dealership. He just inputed the RPM's of the tire, in which it was 600. Obviously you have a dealer tech that is not educated on his job, it can be done. Anything over 2006+ can only go to a 285/70 series (32. 9"). .



I had one of my LEO buddies radar me after the recal and it was dead on.

 

[KSears]

it is a true shame these professional mechanics don't understand their jobs. I called two other dealerships, one said they would get back to me in a couple days, being the main guy was in some schooling, the second refused to do anything at all and suggested some outside garage, Said they could not even consider something like that. Sounds like one will get a different story for every phone call made.

 

[Motorhead]

it is a true shame these professional mechanics don't understand their jobs. I called two other dealerships, one said they would get back to me in a couple days, being the main guy was in some schooling, the second refused to do anything at all and suggested some outside garage, Said they could not even consider something like that. Sounds like one will get a different story for every phone call made.

Find someone that runs a Smarty jr or Smarty sr. Find the tire OD size and your done.

 

[KSears]

I don't know anyone personally that has a Smarty, If anyone living near Manassas, Va. does and reads this, and would feel inclined to help, I would definately appreciate it,thanks, still haven't heard back from the dealershiup that was going to check with their tech guy, thanks everyone.

 

[JMarketos]

I can't address how to correct the in-dash speedo reading, but I have another suggestion that might be useful for you... If you have access to a dash top GPS (like a Tom-Tom or a Garmin etc. ), most of those have a trip computer feature which includes a speedometer. So long as you have good clear day so the GPS has good reception, you can use it to check your actual speed. Check when you're going at a pretty constant speed- the GPS speed measurement is a differential measurement, and tends to be extremely accurate when you're going steady speed.