Hey guys my mechanic is having trouble getting the gear you put in the transmission after you change the rear end. I increased my rear end from 3.25 to 3.73 and I have an FMX transmission. Where can I find that gear (so I can correct my speedometere and odometer) and what is it particularly called that I am looking for?

Thanks,

Dave

 

All you are looking for is a basic plastic speedometer gear. On can be gotten on eBay or from suppliers like NPD. If you tell me how many teeth are on the one you have I will tell you how many teeth need to be on the new one to get the correct MPH.

 

honestly I'm not sure. I was just told by my mechanic to look for the tranny gear that will correct my speedometer and odometer because of the rear end change and the only info I would need to know to find it is the 3.73 gears with my FMX trans. Should I call my mech and ask him how many teeth I'm looking for?

 

Yes different things like tire size, number of teeth on the drive gear in the trans, rear end ratio, and speedometer adjustment can affect the accuracy of your speedometer. If it was correct before the swap than to keep it accurate, and nothing changed other than the rear end, it is best to figure it based on what you currently have. Have the mechanic count the teeth on the gear that is currently on the end of the speedometer and I can use that number to figure out which gear you need.

 

everything was working correct before the rear end change and everything else is still the same. As far as I know the current setupwith the speedometer gear isstock. My tire size is 225 70R 14 which is what I had before on the car. I'll ask my mechanic if he remembers how many teeth are on the speedometer gear, but he currently doesn't have a car and he's not really local to me to stop by from him to check it out. Thanks for the help!

 

sounds good. let me knwo when you have the number

 

Count the # of teeth on the old speedo gear, multiply that by 3.73 and then divide that number by 3.25 ... Voila!

 

Just got the car back from the shop where they were supposed to put in the new gear. Drove it on the highway next to my wife and it's still off 10mph!! They had it for 3 days and they couldn't get it right. Not to mention I had the rear end done during the fall last year. I have my old gear though. It's orange and has 20 teeth on it. Would that mean that I need a gear that has 23 teeth (20 x 3.73 / 3.25= 22.95)? Any suggestions where to go to get the new gear?

Thanks!

 

what size are your rear tires daze will tell you the right one to get

 

I am assuming 10 miles an hour to fast at around 60??? based on that a 23 gear will fix it (I calculated it another way based on 10 MPH to fast at 60 MPH and the result was 23.3333). The math you did is a correct way to figure this out (there again and this is VERRY important) if and only if the speedometer was correct before the rear end swap. If you speedometer were slightly off before with the original rear end that would account for the .35 differences between the two ways of figuring it.