I have a 2005 f-150 that we bought used. It has had a 6 inch lift kit put in it, it has 37" tires plus an evolution chip installed. How do i find the stock gear ratio's and then how do i figure what the correct gear ratio should be for how the truck is now?

 

Should be a sticker with the stock ratio on the driver's side door sill.

 

there should be an axle code on the sticker in the door jam. there also is a tag on the differential that will tell you what the gear ratio is and if it is a traction lock rear. If you want the engine rpm to be the same with 37" tires as it would have been with stock tires divide 37 by the stock tire diameter and multiply this by the stock gear ratio to determine what the new ratio would need to be.

 

Just post the axle code, please.

 

The number I come up with by using your method is 1.72?

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Your answer is a little confusing.

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The current gear ratio is the stock 3 73
I need the gear ratio for the set up I mentioned above . If someone could tell me where or how to calculate it that would help.
Thanks

 

For that, I would recommend 4.56.

I've got 4.10 on normal tires and I love the gearing... so with 37's, you'd want a slightly larger ratio.

 

http://4lo.com/calc/gearratio.php

A bit confused - are you wanting to regear the truck, or are you just looking for the data to enter in the programmer to correct the speedometer and odometer?

If you want to regear, get 4.56. If you just want to do the correction, use trial and error till the speedometer corresponds with a GPS.

 

I corrected my math in my previous post.
If you want the engine rpm to be the same with 37" tires as it would have been with stock tires divide 37 by the stock tire diameter and multiply this by the stock gear ratio to determine what the new ratio would need to be.

If you don't want to do this calculation tell us what the original tire size was and we will do it for you.