Tire size

I'm trying to determine how far off my speedo should be with the 275/40/17's on my car. The tires are way shorter than the tires I took off, which would imply that my speedo would read faster than I am going. I'm sure it reads a good 10% slower than I am going but would like to use the tire sizes to help verify. I got pulled over and the speed the cop told me seemed much faster than i thought I was going, luckily i just got a verbal warning. My current method was drive 100 mile markers and see how far the odometer went (90 miles was the result) which seems like a great idea but doesn't seem to work so good in practice for some reason from previous experience. I have the stock tire size on the build sheet but it doesn't make sense to me, I'm too young for the crazy old tire sizing, I just know the section width / aspect ratio / rim diameter sizing method. How do you determine how tall a tire is using the old way?

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1968 Ford Torino GT fastback - 289, auto, 2.79 rear
2004 Ford Lightning
Recently Sold - 1994 Thunderbird - DOHC 4.6L, 4.10's, big sway bars and lots of accompanying suspension mods.


<font size=-1>[ This Message was edited by: 4camtbird on 8/9/06 2:35am ]</font>

awesome, thanks. Damn that car's speedo must have been a long ways off from the factory. F70-14 is 26.24 and the 275/40/17 is 25.7 and it is currently off as if the tire were too tall? WTF?
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My first car-a 1976 Elite, had been my Parents car, they bought new in 1976 off a showroom floor. When I got it, my stepdad got me a set of 14" magnums for it, and I stuck some stupid wide low profile tires, a hell of alot shorter than the 235/75/r15 tires it was stock with.
At 25mph on the speedometer, I was only going around 17 MPH because of the shorter tires.
In highschool, I fuzzily recall a time I got nailed, I was going according to the speedo 40mph, but I was doing something like 27 in a 25 zone, and got nailed.
I had the dashpad off, and told the cop I was trying to get a new speedocable for it, and got away with a warning and the riot act.
I ended up sticking 15x8 rims and some 245-ish 60 series tires out back and tha seemed to fix my problems.
Worse case, you could always upgrade to a new speedo, and use one of the electronic programable units?
My dad had a 36 ford sedan a while back like that, press a button going on the highway, do a mile, and click it again at the mile marker, and it was set.
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I found a "your speed is" sign tonight and it agreed that my speedo reads 10% slow as the odometer suggested. I may have been going faster a few miles before the cop pulled me over and just not known he was back there. I wasn't really sure when he got behind me, when I look in the mirror at headlights I don't expect the SUV to be a cop. Why can't they just stick to one car for us? [img]/forums/images/smiles/icon_smile.gif[/img]
[addsig]

Try this link

http://www.1010tires.com/TireSizeCal...?action=submit

Do you plan on changing the plastic speedo gear to get the speedometer to read correctly? I know how to swap out and calculate what to use for gears as far as the 60's Chevy ones goes - have to figure out the Ford ones as I am going to need to change the ones in my car too.
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