Speedo is 10 Miles per hour off

[JKVIPER]

I have a 2001 GTS which I recently bought. The seller stated the car has 3.55 rear end gears in it. I have had the car for about a month and when I drive it on the hiway I normally go 10MPH over the limit and noticed I was going appreciably faster than the rest of the traffic. Plus my 0-60 times did not seem really impressive vs. my 1999 vette.

I verified today that my car is about 10MPH slower than it should be. That is, the speedometer says I am doing 50 and I'm really doing approximately 60. Thus, I have the following questions.

1)Does this make sense if the rear was changed from stock and updated with a 3.55 rear?

2)What can I do to fix this?

Any help from the Viper nation would be appreciated.

Thanks,

 

[motor602]

i think i may have the same problem, although i have the stock gears, it seems as if my car only can get up to 50 at the top of first instead of the published 58-60. i think there may be a calibration kit out there for when you change gears and wheel size, anyone know what to do?

 

[KenH]

Putting in 3.55 gears should cause the speedo to register a higher speed than your are actually going, not lower. At 50, you would register a speed up in the upper 50's.

You can get an electronic box to correct the speedo. One came with my 3.55 gears. Not sure what would cause the speedo to read too low. Maybe they snuck in some 2.73 gears on you!

 

[Ulysses]

Calibration boxes can be obtained.

 

[KenH]

If you do a gear change from 3.08 to 3.55 and keep the same tires, you will be reading slower mph. A little less than 10mph.

Calibration boxes can be obtained.

Are you sure about that? I thought the Gen II took the reading off the transmission, so the lower gears would take more transmission turns to achieve the same road speed. I could also be off in left field somewhere on this one.

 

[Ulysses]

I was wrong, I misread my gear calc spread sheet, sorry.

You are correct, the drive shaft rotates at 3.55 times during one rotation of the tire versus 3.08 times versus one rotation of the tire. If the speedo is based on RPM at the tranny, you would be reading a higher MPH versus what the tire is truly travelling.

 

[JKVIPER]

If a calibration box was already installed. Where would it be located?

KenH,

Thanks, I agree with your premise that if no calibration unit was installed with the 3.55 gears then the speedo should read faster than actual.

 

[FikseGTS]

my car has the stock gears and wheels..... the speedo has always read about 6 MPH lower than I'm really going.....

 

[99 R/T 10]

If a calibration box was already installed. Where would it be located?

KenH,

Thanks, I agree with your premise that if no calibration unit was installed with the 3.55 gears then the speedo should read faster than actual.

Check by the PCM, this is where some tend to tap the signal coming from the trans, also check for the little black box under the dash. If and when you find it, there will probably be "switches" you will mess with. You will have to open the box to get to them. You'll have to play with it to find the right combo, or call the manufacturer to help with sync'ing it to your gears. Good luck,

 

[JKVIPER]

Thanks 96 R/T 10,

I'm new to this car. Where is the PCM located?

 

[DodgeViper01]

i think i may have the same problem, although i have the stock gears, it seems as if my car only can get up to 50 at the top of first instead of the published 58-60. i think there may be a calibration kit out there for when you change gears and wheel size, anyone know what to do?

Mine does the same thing. That is kinda weird.

 

[txlen]

call sean at roe racing...he carries unitrax speedo boxes and can set you up with what you need....Len

 

[C O D Y]

I think the stock speed for 1st is 52 mph. This site might help for people who have changed wheel and tire sizes.....

http://www.miata.net/garage/tirecalc.html

 

[joe117]

For you to be going 60 when your spedo is showing 50 there is only one explanation.

The rear was changed from stock to 3.55 and the calibration box was put in.

Then the rear was changed back to stock 3.08 without removing the cal box.

 

[KenH]

If a calibration box was already installed. Where would it be located?

KenH,

Thanks, I agree with your premise that if no calibration unit was installed with the 3.55 gears then the speedo should read faster than actual.

It's located on the drivers side under where the big red electrical connection is next to the fuse box. Between it and the front edge of the side sill cover. It is mounted under a cover (so you can't really see it).

The cover comes off with a #10 torx driver. My Unitrax calibration box is mounted under the cover.

-- Ken

 

[3snakes]

gps would tell you your speed.

 

[JWVIPER]

Joel, erJK

I checked mine by having buddy run next to me at 40 50 and 60 and we compared speedos, Joe117 seems to make sense with the swap in and out, when do you have to shift to 2nd, what speed are you at ?

jw