285's and Speedop question
Butt-Head
Joined: Apr 2004
Posts: 9,448
Likes: 2,934
From: Reed Point, MT
A GPS showed my speedometer to be off 2 MPH slow at 75 MPH (indicated) with 285/75's. However, the speedometer was 1 MPH fast with the stock size of 265/75's. I think that you'll find a lot of people on this forum that have actually "fixed" their speedometers by going to the larger tire. Unfortunately, there are VERY RELAXED guidelines on actual speedometer accuracy set by the feds. Besides not every 285/75-16 is 285 mm across and 32.?? inches tall. Every brand is just a little bit different.
Post Fiend
Joined: Feb 2006
Posts: 11,892
Likes: 2
From: Middle Tennessee
My current 285s check out spot on, (well, maybe 1 mph low) via radar. That is considered well within tolerance.
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Hotshot
Joined: Nov 2006
Posts: 11,286
Likes: 263
From: North of Greenville
There's another technique for checking speedometer accuracy for those of us who do not have the convenience of a GPS... it's called Cruise control plus mile markers plus a stopwatch. Works great.
My '02 F250, with stock 265/65/R16 Michelin LTX M/S's, is 1.5-2.0 mph slower tahn the speedometer says. I'm going to increase to 285/70/R16's, which should be almost the exact correction and will look better to boot (depending on tire manufacturer, of course).
My '02 F250, with stock 265/65/R16 Michelin LTX M/S's, is 1.5-2.0 mph slower tahn the speedometer says. I'm going to increase to 285/70/R16's, which should be almost the exact correction and will look better to boot (depending on tire manufacturer, of course).
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Posting Guru
Joined: Sep 2006
Posts: 1,309
Likes: 2
From: Plainfield, IL
Originally Posted by Iceman31
Mine is 3kmph slower. When I show 97km I am actually going 100km. Sorry for the Km but I am Canadian.
. I run 285's and with the gps it is dead on a 60 as well, but at 70 the spedo is 1 or 2 mph slow.
Last edited by yamaguy; Apr 29, 2007 at 06:33 PM.
