How far out is my speedo????

[Barc]

Quick question for you tire junkies out there - I went from 245/75-16 to 255/85-16 - how far out is my speedo going to be - dealer tells me likely around 10 kph (6 mph) Does this sound right??


Where can a guy post pics instead of Picturetrail???

[torque]

255R85 means 255mm wide and 85% times 255mm high.
245R75 means 255mm wide and 75% times 245mm high.

If you double 85% times 255mm times .04mm/in and add 16in (rim size) you get 33.07in which is the diameter of your new tire. Do the same for 245R75 and you get 30.47in for the diameter of your old tire. Divide 33.07 by 30.47 and you get 1.085 or an 8.5% error (slow). Figure that you're going 65MPH when your speedo says 60MPH.
It's real, I made the same swap and checked it with a GPS.

[torque]

245R75 means 245mm wide by 245mm times 75% high.

[ccds]

If you want to post pictures try:

http://photos.yahoo.com

they have a service that is free or you can expand for a small cost.

[img]smile.gif[/img]

[DMAX Daddy]

I bookmarked this site to play with all the differing tire size questions

http://www.c5-corvette.com/tirecalc.htm

On the left side put int your old tire as tire A, your new tire as tire B. Use the center selection for comparison, and voila:
Speedometer Error

Actual Speed Indicated Speed with Tire B Speedometer Error
5 mph (8.04 kph) 4.61 mph (7.41 kph) -0.39 mph (-0.63 kph)
10 mph (16.09 kph) 9.21 mph (14.83 kph) -0.79 mph (-1.26 kph)
15 mph (24.13 kph) 13.82 mph (22.24 kph) -1.18 mph (-1.90 kph)
20 mph (32.18 kph) 18.43 mph (29.65 kph) -1.57 mph (-2.53 kph)
25 mph (40.23 kph) 23.04 mph (37.06 kph) -1.96 mph (-3.16 kph)
30 mph (48.27 kph) 27.64 mph (44.48 kph) -2.36 mph (-3.79 kph)
35 mph (56.31 kph) 32.25 mph (51.89 kph) -2.75 mph (-4.43 kph)
40 mph (64.36 kph) 36.86 mph (59.30 kph) -3.14 mph (-5.06 kph)
45 mph (72.41 kph) 41.46 mph (66.72 kph) -3.54 mph (-5.69 kph)
50 mph (80.45 kph) 46.07 mph (74.13 kph) -3.93 mph (-6.32 kph)
55 mph (88.50 kph) 50.68 mph (81.54 kph) -4.32 mph (-6.95 kph)
60 mph (96.54 kph) 55.29 mph (88.95 kph) -4.71 mph (-7.59 kph)
65 mph (104.58 kph) 59.89 mph (96.37 kph) -5.11 mph (-8.22 kph)
70 mph (112.63 kph) 64.50 mph (103.78 kph) -5.50 mph (-8.85 kph)
75 mph (120.67 kph) 69.11 mph (111.19 kph) -5.89 mph (-9.48 kph)
80 mph (128.72 kph) 73.71 mph (118.61 kph) -6.29 mph (-10.11 kph)
85 mph (136.76 kph) 78.32 mph (126.02 kph) -6.68 mph (-10.75 kph)
90 mph (144.81 kph) 82.93 mph (133.43 kph) -7.07 mph (-11.38 kph)
95 mph (152.85 kph) 87.53 mph (140.84 kph) -7.47 mph (-12.01 kph)

*EDITED to 95MPH for obvious reasons*

[ 02-11-2003: Message edited by: DMAX Daddy ]</p>