Hi i have a 97 F-150 4x4ORP with a 4" lift and 35/12.50/15 tires. the deal is i just changed my gears but with the 3.55 gears i had at 55mph the speedometer was off by about 8 or 9 mph so i should have been going about 63mph. but now that i have 4.56 gears it seems like it is the complete opposite now i can do lets say 75mph and everyone seems to be passing me like i was actually doing under the 65mph speed limit. My question is how can i determine if the speedo is off and then how can i correct it?
Or will i have to take it to a mechanic?
Sorry i guess i wasnt paying attention when i posted this in the wrong room .


[This message has been edited by Bigern6697 (edited 05-17-2001).]

 

The easiest way without having to do alot of calculations is the old mile post marker count. Just start crusing down the road, note the mile post number and then note what your odometer reads. As you pass the mile markers, in your case, your odometer will read "more miles" then you actually travel. So for example, you pass by two mile posts and your odometer says you went 2.1 miles. This means for every 20 miles you travel, your odometer will say you went 21 miles. This would be a 5% differece. Subtract 5% from what your speedo says and that's how fast you are going (still approxomate, but close) You are probably close to being 9-10% off by what you said about showing 75 and everyone passing you. I'd get it figured out soon. Your mileage is going to soar and not reflect the actual mileage you have on the truck.

To fix it, I think you can get a new speedo gear to compensate (since you have a 97). I'm just the opposite, and need to change mine too, because I show "less" miles then actually traveled. From what I understand, it's a pretty simple fix. Just need to get the correct speedo gear.

[This message has been edited by MitchF150 (edited 05-18-2001).]

 

Just have a friend drive next to you at 55 and then have him set it up on cruise control...then set yours and drive about a mile and make sure u cant gain or slow down...then adjust the speedo.

------------------
97 4.6L F150 Super Cab Lariat

Current Mods
- Billet Grille
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Soon To Come:
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- carbon fiber dash kit
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- Superchip
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Far Future:
- 20 inch Titans or T-rex's
- Bagging it
- Hella loud system

 

First, you do not have 15" wheels. Do you have the 16.5"s?

So the 35/12.5/16.5 is 34.7" tall.

second, what was the stock tire size? I am going to guess the crappy 17s which are 31.6" tall.

So use a calculator to figure the difference such as my sheet here:

http://members.home.net/mjbobbitt/f150/tirecalc.html

So to get back to stock performance, it would take 3.90 gears - basically 4.10s in the ford. So without changing gears, at 65 on the speedo you would actually be doing 71.38mph. Now with the gears you are probably reading about 6-8 mph slow.

Go to the ford dealer or the place that did the gears for you, they should be able to calculate or let you purchase a few different speedo gears to try. You should be able to get close to stock.

The one nice thing about the 97s is that you can just change the speedo gear to correct things.

 

Here is another great link for you!

http://www.finer.com/Internet/pages/speedo.htm