From what i have seen when i go up 2 inchs on my past trucks the mph was off by about 3-4 at 70 so my gage would say 70 and i would be going 74ish.
So i would say if you are going down in size that it would be about the same but the speedo would say 70 and you would be going 66. But i maybe wrong with gearing and all. If you can just get a gps and check it that would be the best way.
07 GMC ccsb 1500 4x4, front replacment bumper, 285 70 r17 on 17" black helo max 6, Dual flowmasters dumped.
If its only off by 5 or so I can deal with that. Im just iffy about if I had it close to off by 10 or more
06 Intimidator SS- Tuned, CAI -DD
03 Silverado- Last incarnation 13.6... RIP
Ls1, Pnp heads, Polluter camshaft, spray, 4k stall on the way
"Seize the day like you seize your noodle" - Shin Chan
dont worry about it, just drive it. Get a GPS unit that shows your MPH and live with it, thats what I do since mine is off by about 4 MPH at 70. I correct the speed and I get a antilock brake code so I just put it back to stock and compensate for it.
See my truck data in the "My Garage" section here... http://www.ls1truck.com/forums/my-ga...tml#post191709
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