Hey fellas,

Need help figuring out which speedo gear I need in my car. I just had the rear end geared to 3.80 and a rebuilt C-4 put in and I'm taking it this weekend to have 16" tires/wheels fitted. Obviously the speedo is showing slow right now. I know there's some formulas out there to figure it if you just change the rear end, but what about the combination between gear ration and tires?

Any help would be appreciated.

Thanks

 

there are multiple calculators on the web to determine speedo gear, but here ya go. You indicate a C-4, with 3:80 gears, i'd pull your current gear and go upseveral teeth sizes. Bigger teeth count = slower MPH, smaller teeth count = faster MPH. I found this link, check it out - http://www.fordforums.com/f497/falco...-chart-127835/ by what I'm seeing, you're lookin somewhere in the 24-26 teeth range, depending on tire height.

 

Copy all that. Now how do I figure out the #Drive Teeth part of the equation?

 

Typically the drive gear is 7 if rearend 2.7-3.5 and 6 if rearend 3.5-4.3 for toploaders. I think my drive gear teeth is a 7 on my tranny with a 3.50 rearend but I have toploader. It may be different for an auto. Take yours out and go two teeth higher then check it against a friend driving another car. You don't know if it was right to begin with. I have found that one tooth makes about 7-10 mph difference in my car, so I clocked it and worked backwards. Ended up going one higher, but I bought both and tried them.