Tire size and speedometer reading....
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Lead Driver
Joined: Feb 2003
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From: Oak Harbor, OH
Tire size and speedometer reading....
Hey guys, I need some help.. My dad and I got into a debate over tire size VS speedo reading.. It seems that I remember reading somewhere that smaller tires will read faster, and taller slower. Is this right?
Posting Guru
Joined: Nov 2005
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From: the side of a mountain
if you have bigger tires than stock you are goin fater than the speedo says, smaller tires than stock you are going slower than the speedo says.
edit: here is a calculator to show that.
http://www.4lo.com/calc/gearspeedo.htm
edit: here is a calculator to show that.
http://www.4lo.com/calc/gearspeedo.htm
Post Fiend
Joined: Nov 2001
Posts: 14,947
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From: Topsfield, MA
Doesn't it depend on the device used to pick up the speed? Whether it's a cable operated speedo in the tranny/TC or in the rear diff?
My truck has 35's on it and if I'm going 70 my speedo says I'm doin' 65 or so cause I havn't recalibrated it. This is just like he said but I wonder if it's different on the cable operated stuff...
My truck has 35's on it and if I'm going 70 my speedo says I'm doin' 65 or so cause I havn't recalibrated it. This is just like he said but I wonder if it's different on the cable operated stuff...
Posting Legend
Joined: Sep 2001
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Cable or speed sensor makes no difference. The electronic speed sensors can sometimes be easily calibrated with a tuner etc. Check the chip forum for more info. A cable operated system requires a gear change.
Post Fiend
Joined: Nov 2001
Posts: 14,947
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From: Topsfield, MA
Our 92-96 trucks can be changed w/o a tuner 
What does matter is a gear ratio change - no affect on the axle sensor but does have an affect on the cable version. I was thinking along those lines...
What does matter is a gear ratio change - no affect on the axle sensor but does have an affect on the cable version. I was thinking along those lines...
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Junior User
Joined: Jan 2005
Posts: 54
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From: London Canada
I do want to hijack this thread but how does the different tire size play in for gas mileage. I had 235's on my truck and put on 255's. Would i see better or poorer gas mileage on the highway and/or city driving.
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More Turbo
Joined: Apr 2004
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From: nortonville illinois
the taller the tire the lower you want your gears to be , so that you dont couse stress to the tranny/transfer case and possibly the motor
if you put taller tires on you will be giving it more gas when you are taking off to get them to turn
and smaller tires you will be able to get a faster take off but wont have as much top end
if you put taller tires on you will be giving it more gas when you are taking off to get them to turn
and smaller tires you will be able to get a faster take off but wont have as much top end
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Lead Driver
Joined: Feb 2003
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From: Oak Harbor, OH
I have a '79 LTD, and the tire size on the door sticker is labeled "FR78-14". This roughly translates to 195 75R15.... It has 215 75R15 (makes the tires taller as the height is a ratio of the width) so it's reading 60 when I'm actually doing around 65 to 67. My GF was following me the other day and said she was having to run almost 70 to keep up with me.
FTE Legend
Joined: Aug 2004
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From: North Central Rural Sask.
Stock tires on my 150 when I bought it was 245/75-16, and I changed them to 285/75-16. The speedometer is off, and I am always going faster then the speedomter reads. For mileage, in town it is worse, but on the highway it is a little better.