Speedometer Recalibration Box Settings after gear change

[Motor City Mad Man]

The housing on my Specmo RB-2120 speedometer recalibration box melted, and I am afraid the internal circuitry may have been damaged as a result. My Viper had a non-stock rear gear installed by a previous owner. I was never quite sure what the new gear ratio was. The dip switch settings in the Specmo box translate to a .8929 ratio according to the table on their website. Can I assume that I have a 3.45 rear gear installed then since the stock rear gear is 3.07 and 3.07/3.45=.8898 which the box setting of .8929 is closest to? Am I calculating this correctly? Also, does this mean that without a speedo recalibration box when my speedometer says I am doing 89MPH am I actually doing 100MPH? Without a speedo recalibration box, will the odometer not roll up accurately now too? Specifically with a rear gear of 3.45 for example will the odometer roll up less miles that it actually is?

I have emailed Sean Roe at Roe Racing and he sells a speedo recalibration box in which the black box installs inside the car instead of in the engine compartment. This seems preferable to me since I won't run the risk of it melting again. But, this supposedly only corrects the speed to the speedometer. The Specmo unit can be connected from the speed sensor to the PCM to correct the speed for the entire system. What speedometer recalibration units do any of you out there use? Which type is preferable?

 

[Ulysses]

I can't comment on your specific questions since I am not familiar with the recalibration boxes, but I can tell you a really good way of telling exactly what gear you have. Get the car up in the air mark the driveshaft with a light colored marker (crayon or the type they mark wood with or a taylor's garment marker, they are all the same) and the top of the tire. Be careful not to move the tires or driveshaft while you do this. Have a helper rotate the tire one full revolution. While under the car, count the number of revolutions that the driveshaft makes in relation to one full rotation of the tire.

 

[David Jenkins]

Remove the four cover screws and open the box. Is the "link" cut? What year is the car? What dip switches are on? I have the formula to figure the right rpm/speed. Call or email and leave me a fax no. and I'll send it to you. dj

 

[Kid97GTS]

While I can't help you out with figuring which gear set you currently have, if it is an aftermarket mod, it is likely a 3.45, 3.54 or 3.73 - all of which are a "lower" gear than the stock 3.07. Thus, if your speedo is not recalibrated for the new lower gear and instead "thinks" you have a 3.07, you will actually be going SLOWER than your indicated speed. Another thing to consider is that since your speedo and odometer think you are going faster/farther than you actually are with the lower gear, your odometer will read a higher mileage than you have actually gone. This may have an adverse impact on your mileage both for resale and warranty purposes. Granted, the .5 or so difference between the stock and modded ratio is not great, but it still is ticking off miles quicker than you are actually driving them. Good luck!