[Motor Mouth]

Having taken my new GT out for a drive yesterday I spotted one major problem, my speedo is way out.

The car has had a 5speed Getrag 240 box fitted and at 3000rpm the speedo is indicating about 160km/h (100mph) where as I think it should really read about 115km/h or (70mph).

If it helps the car is running on 195/50/15.

Anyone got any idea what I can do to rectify the problem as I don't want to get stopped by the boys in blue!

 

[ConreroGT]

First place I'd start would be to make sure you have the correct speedo gear for the cable. The getrag uses a different one then stock. Opel GT Source sells them.

not sure about the wheels....of course, they would certainly effect it if they didn't quite match the original 13's...

 

[RallyBob]

With that tire size, and assuming the standard GT 3.44 rear differential ratio, plus the normal .805 overdrive ratio for a Getrag 240, you should be at approximately 76-77 mph @ 3000 rpms. Your Getrag probably has a blue speedometer gear, there are also red, yellow, and brown ones that I've commonly seen in other models.

Bob

 

[baz]

only way i can think of is to bight the bullit and have the speedo calabrated to new setup
the people to do it are normaly to be found in max power /fast ford and the like in classifides

 

[greensmurf20]

my GT did the same thing after a tranny swap, and i couldnt just change the gears because the trannys had different style tailshafts. so i had to use a "gear reductor" it just goes in place of the speedo on the tranny, and the speedo cable then goes into the reductor.

 

[Motor Mouth]

All,thanks for your help so far.

Greensmurf20, where did you manage to get one of these reductors that you talk about?

 

[Gary]

Mark,
Do you have the "Blue" speedometer drive gear in the Getrag? It is normally used in an automatic trans.

 

[greensmurf20]

i had a gear reductor on one of my spare trannys that i got with my 70GT. it even says VDO right on it.

 

[kwilford]

I think that Jared is referring to the older-style (1968 to 1970) Opel 4-speed transmission (with the speedo drive on the passenger side) that used a right angle drive. I didn't think that they were a "gear reducer", but I can't say I ever checked on the gear ratios inside.

Mark, the speedo drive you need is determined by the speedometer you have. The standard North American "mph" speedo is a "w=1062", which is related to the speedo drive ratio (I believe), and uses a "black" plastic speedo drive gear. The automatic transmission in N.A. uses a "blue" plastic drive gear, which is what you need to install in a Getrag with the "1062" speedo. But I don't know what the Euro speedometer ratio is. Or is the same with just "km/h labels? Look on the face of your speedo. I kind of doubt it since the odometer is also in km. Perhaps one of our Euro members would know which drive gear to use with a Euro speedometer. Or Otto (aka "tekenaar", from Texas)? You might try PM'ing him to ask, as he is really the resident Getrag expert.

 

[greensmurf20]

keith: you are more knowlegable about the opels than i am, and maybe this can be enough for you to be sure what it is. the GT was a seventy so i'm not sure if that makes a difference. but i changed the tranny to another tranny froma 72 opel, and without the "thing" it said we were doing about 95 when we think we were only doing around 65 or 70. so i stole the "thing" from another tranny because its one of those better do it just to get dad off your back, and he was right, it did fix my speedo. maybe i am talking about the right angle drive that you are refering to, as its like a short extension into a small block, and then another shorter extension that the speedo screws into..........probably what you were talking about.

 

[Motor Mouth]

Thanks for your help so far guys, I guess I'm going to have to get under the car this weekend and check out the colour of the speedo drive.

I suppose my next question has to be, does anyone know if you can buy these speedo drives new?

I like the sound of the reductor drive on the speedo, but this sounds pretty difficult, again any help would be appreciated.