Speedometer change with 30" tires

rpc625 · #1 10-24-2001, 10:40 AM

I changed my stock tires from P235/75R15 to LT30x10.50x15 Bridgestone Dueler AT (Great Tire) and the odometer is off by 2/10 of a mile and the speedometer reads a lower speed than the accual. What can I do?

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fordman3 · #2 10-24-2001, 02:18 PM

rpc625,
What year model is your truck? If it is a '97 or '98, I believe you can simply change out the "speedo gear" (not the real name, but you get the idea) at the tranny with one with one less tooth. With 30" tires, you need the 20-tooth gear, I believe, and you should have the 21-tooth with 235/70R16's. Just ask for the gear that would have come with 255/70R16's, also 30". If it's '99 or above, I think you'll have to have it reprogrammed at Ford or with a Superchip. Again, it should be straightforward, since you are changing to another stock tire size. If you were going for an odd gear ratio or a really tall or really short tire height it might get more difficult. I hope this helps. Good luck!

rpc625 · #3 10-24-2001, 02:22 PM

It is a 1985 ! Can I change the gears on this year, and where can I get the gears?

fordman3 · #4 10-24-2001, 03:12 PM

rpc625,
Sorry about that! I didn't notice the "15" in the tire size, I just assumed you were talking about a late model. On a 1985, it should be as simple as a speedo gear change also. Your truck still has a "mechanical" odometer, doesn't it (not digital)? I believe you will still just need to remove the gear you have and replace it with a one-tooth smaller gear. Your local Ford dealer should be able to help you, and if not, you could probably go to any speed shop or look in any parts catalog (Jegs, Summit, etc.) and find one. Again, I hope I've helped some. Later!