Speedometer gear for TH400 transmission
 

My truck's speedometer shows about 75mph when I think I am going about 45 to 50 mph. Can anyone tell me how many mph one tooth on the driven gear will change the speedometer? The driven gear that is in there now is a 40 so do I go up in teeth or down. I am thinking going up will bring the speedometer down. Any help will be great! Thanks Jim

Re: Speedometer gear for TH400 transmission
 

correct !! you need to confirm actual speed and one tooth = 5% of speed.... do da math....

Re: Speedometer gear for TH400 transmission
 

Stan, Does that 5% mean about 5mph?

Re: Speedometer gear for TH400 transmission
 

depends on actual subtracted from gauge

Re: Speedometer gear for TH400 transmission
 

Only at 100 mph. 2 1/2 mph at 50.

Re: Speedometer gear for TH400 transmission
 

Thanks Bob for the input, I will try a higher gear to see if that will lower the speedometer.

Re: Speedometer gear for TH400 transmission
 

One more question, Does it make a difference if the drive gear is 15 or 16 tooth?

Re: Speedometer gear for TH400 transmission
 

Yes it does make a difference on the driven gear. You want to go up in size on the driven gear or down in size on the drive gear to get a lower indicated speed. Here are some links that may help you.
Good luck

http://www.wallaceracing.com/speedo-calculator.php

http://www.tciauto.com/tc/speedometer-gear-calculator/

http://www.gtsparkplugs.com/SpeedoGearCalc.html

Re: Speedometer gear for TH400 transmission
 

What is your rear gear ratio and tire height?

Re: Speedometer gear for TH400 transmission
 

My rear axle is 3.73 and tire height is 27 inches