How do I correct my speedometer for changing the gears from 3.07 to 3.75's. Does anybody have an easy solution to this. Have their been any articles in any of the magazines have articles on this. If so magazine year and month. :steering:

 

Charles,

I too had the same problem. For years, after installing 3:75 gears, I drove around with my speedometer reading 55...when it should have read 40.

I tried buying the plastic gears from my local Chevy dealer, but I just could'nt get the right combination for my 4+3, '86 Vette.

In 1999, I got a catalog from Eckler's, and they have listed a 'Speedometer Calibrator'...for '84 to '93 Vettes. (P/N: 35073)

I purchased it for $169, and a mechanic installed it for me.
There were no problems with the install, although it did take me several 'runs' at 60 MPH, with my buddy driving his car beside me...before I was able to get the correct combination tuned in.

It's a great unit, and I highly recommend it. Hope this helps.


[Modified by UDLUV12, 12:48 PM 12/22/2002]

 

It depends on the year of your car. I think the break comes between the OBDI and OBDII cars. The earlier C4s can go with a speedo gear, and sometime a sensor change. Later models have to be reprogrammed, like with a Hyper Tec reprogrammer. I have a Corvette parts book. With the tire size, trans type and before and after ratios, I can tell you what you would have to buy to correct an earlier car. Gears are a fraction of the cost of the $170.00 calibrator.

RACE ON!!!

 

Did this correct your odometer too?

 

War Machine,

Yes it did correct my odometer reading too.
Both my speedometer and odometer were off due to the change in gears, from 3:07 to 3:75.

To my knowledge, Chevrolet does not make gears that corrects the speedometer, and odometer reading for 3:75 gears.

I tried several combinations of their 'plastic gears', but my readings were always off between 5 to 7 MPH.

It wasn't until I added the RB-2120 Ratio Calibrator from Eckler's, that I began to get the correct readings.

 

UDLUV12,

So even if I don't fiddle around with the plastic gears to get mine close, this unit will correct it anyway? I was about to dump some serious coin with a trans shop to fix this. He said the best he could get me was close and that will be with alot of trial and error. That sounds just like your experience with the plastic gears. If all I need to do is order the RB-2120 Ratio Calibrator then that would be SWEET!!! :D

 

War Machine,

That 's exactly right. You will not have to go through the expense that I did in trying to get it 'close'.

'Close' to me isn't good enough. Why spend that kind of money on an install just to get our readings 'close'?

That's why I drove around for 3 or 4 years with my speedometer off by 15 MPH.

When I noticed the add in Eckler's catalog...I jumped at it.

I know of a mechanic who is very knowledgeable when it comes to the install of 'electronics' in Vettes, so when I approached him about it...he said "buy it",
and he would install it...but I would have to calibrate it...as it can be 'time consuming'. Of course I said "OK" to this.

It took me about 6 runs (where my buddy drove beside me at 60 MPH, so I could get a 'read' as to where my speed reading was in relation to his 60 MPH)
before I finally got it to near 'dead nuts on'. I just kept 'pulling over' to make the adjustments, as suggested in the 'directions' that come with the unit. It took about an hour of time. Not bad at all.

The way I checked everything out, was: by finding 'mile markers' out on the Interstate Highway, bring my car up to 60 MPH, and then 'time' my run for 60 seconds...which at 60 MPH...should bring me to the next 'mile marker'...which I was able to do, after another two tweaks on the calibrator. (I didn't need my buddy for this exercise).

I do not have any 'false' lights coming on from my 'dash cluster' such as the 'shift now' light, or the 'ABS' light, as reported by some other forum members...as being a concern.

What I can tell you about the install...is this particular calibrator...offers 6 different 'diagrams' on how to install the unit. My mechanic choose diagram #3, which 'corrects the signal BEFORE the ECM.'

He also 'soldered' any connections where wires needed to be connected...then added a little bit of electrical tape, and finally some black 1 inch diameter tubing, so as to protect the wires from exposure.

I hope this helps both you and Charles, and anyone else who may be having similiar problems.

Feel free to contact me if you need more information.

John

 

UDLUV12,

:cool: Thanks for the info! I'm getting the calibrator! :cheers:

 

Here's more info on the Ratio Adaptor.

http://www.abbott-tach.com/era.htm

 

Where did you have the calibrator mounted?

Thanks,
Ken

 

Has anybody successfully used this Abbott ratio adaptor on a automatic vette with the OBDI? I have a 92 which just got 3.55 gears, and it needs something done, cause I'm adding to the mileage way too fast.

 

I'll find out soon with my auto and OBD1. I just recieved a quote from my mechanic for $125 + shipping on the Ratio Adaptor. That stomps a mudhole in the Ecklers price. ;) I hear ya on the milage adding up. I'm going to tell him to order it a.s.a.p.!

 

I just ordered the Abbott ERA corrector. It looks like I can mount it inside the engine compartment. 4 wires: +12V, ground, to tranny, and to ECM. I plan on identifying the wire coming into the ECM from the tranny (with help from my shop manual), cutting the wire, and splicing in the Abbott box. Finding the correct DIP switch settings should be easy, since I have been driving around with a GPS device since the gear change, and so I can identify the correction ratio with precision. I'll let you know how the device works out. I'm very happy with my 3.54 gears, but not happy with adding all those phantom miles to the odometer!