2nd Gen Non-Engine/Transmission how far off is speedo with 315s

[DBentley]

Just put on a set of 315s that measure about 33. 5 inches tall. Is there a % difference that i could use to figure fuel milage.

 

[Stamey]

What size were your original tires? Are the 235/85s in your sig the size?

If so, 235/85 to 315/75 = 9% larger.



Chris

 

[DBentley]

door tag says 245 75r 16 were original size. So mabey 10 or 11% off now ?

 

[RDelissen]

just use a GPS

 

[Stamey]

door tag says 245 75r 16 were original size. So mabey 10 or 11% off now ?

14% then.



Chris

 

[EricBu12]

Use the Superlift "Tru-speed" Speedo corrector box to dial it in. My 245 70R -19. 5 is a 33" an I have it dialed in to within 100 Ft in 13 miles.

 

[NVR FNSH]

I use a correction factor of 6% - calculated using GPS.



Brian

 

[nps]

To determine tire diameter, divide the width by 25. 4 to get inches. Then multiply by aspect ratio to get sidewall height. Then multiply by 2, and add in the rim size.



For 315/75 16's, it works out to ((315/25. 4) * . 75 *2) + 16, or 34. 6 inches.



For 245/75 16's, it is 30. 46 inches.



So 34. 6/30. 46 = 1. 1359, or your 315 tires are 13. 59% larger than the 245s.



You can use this formula to determine the tire diameter on any size tire. The dealer or any tool with access can dial in the new tire size if you want the spedo corrected.



My truck was originally 265/75s. Then I put on an EZ and the overhead MPG computer was off. Now with 315's and the EZ, the overhead comp reads correct avg MPG as compared to hand calculated. So I've resisted getting the spedo corrected - just remember that the spedo is about 5mph off.

 

[jraker]

Go to this website. They have little calculators that do this for you. www.4lo.com

 

[NVR FNSH]

Using the calculated values for tire diameter does not account for actual tire size (read loaded radius) and the speedometer error from the factory.



Brian

 

[LOtero]

i found mine to be about 6 mph off.

 

[RDelissen]

a good way to adjust it is with the Truspeed

It might be a bit pricey at 180 bucks compared to 15 bucks for a new tooth gear, but that way it'll be more accurate and you can adjust it for 2 different tire sizes if you run different ones for instance in winter and summer. That way all you have to do is flip the switch.



Just installed a unit on my Jeep Grand Cherokee and calibrated it with the GPS. Works like a charm (I was running out of 'this time I'll only write you a warning, just make sure you get it fixed' free passes )

 

[DBentley]

Thanks for all the advice. Im thinking this is a good excuse for a new gps.