Just wondering if it told somewhere what speedo gear came stock in my truck without pulling it first? I recently switched from the stock 255/70R-16 with 3.55 rear to 255/55R-18 and from what I can figure with the help from previous posts it should have come with 19 tooth and I should now have 20 tooth. I am only about 2 mph over now, will 1 more tooth be too much?

 

Your old tires were 30.1" and the new ones are 29" so the difference is about 3.8%.

If you change the gear you need 19*(30.1/29) = 19.72 teeth (or 20 teeth). You will read slightly low after the change (about 1.5%).

One note of caution: all the tire dimensions quoted by the manufacturers are nominal and are probably accurate to less than 2%, so measuring your actual error over a measured mile is the best way to see where you are at.

Personally, for an error of 4%, I wouldn't bother to make the change. My speedo from the factory was out 2%.

Ian

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99 Expedition EB
5.4l, 4x4, 4.10 LS
WMS Tube, Bassani Cat Back Exhaust, Superchip Flipchip

 

Thanks Ian N, actually I measured my tires and they were 29.750 and 28.375. I have been going past a police speed indicator for the past few days and when going 34 on speedo it said 32 so I just figured I was adding unnecessary miles to the odometer and I didn't know how many mph each tooth would change the speedo.

 

For me personally, 2 MPH isn't worth the effort.

When I switched from 235/70Rx16 to 265/75xR16 the difference on my speedo/odo was about 10%. So when I would see 50MPH on my speedo I was actually going 55MPH! Scary considering I like to cruise at 75MPH on the Thruway ;-)

I lived with this for 2 years before I realized that you could change the VSS gear (Ford dealer said I couldn't). Thanks to F150 online.

Now my speedo/odo are right on. I used the mile markers on the expressway and my odometer to measure accuracy. It took a couple weeks to get used to driving the true speed. At first I was driving 50MPH ;-)

Mark

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So what gear did you end up going to? I have the same set up original was 235/70/16 and I'm wondering what gear I need. Actually my summer tires are 265/70/17 an my winter are 265/75/16. These are very close to the same height. Thanks for any help.

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1997 F150XL 4x4 SC 4.6L Oxford White, A.R.E Cab High FB Topper, SI/DO Performance Exaust, Head light, tail light and SC window covers,Ventshade bug shield, Cooper 265/75/16 tires and Pilot driving lights mounted inside the grill.

[This message has been edited by sierra99 (edited 04-22-2001).]

 

Haven't tried yet, have to take truck in for the third back window so far and thought I would ask how much for a 20 tooth and see how much difference it makes.

 

sierra99, just noticed where you were from after last posting, I love it up there, I usually spend 3 weeks a summer at the State Park and a couple of other weekends at the Days Inn just down the road, I'll let you know when I find out anything.