Switching From 120 MPH to 150 MPH Speedo

[Bruzilla]

I replaced the 120 MPH speedo in my Roadrunner with a 150 and it's now reading too high. Do I need a new drive gear for the tranny, and where do I get one?

 

[696pack]

No, you need to take the speedo to a speedo shop to have it recalibrated to the 150 script.

 

[YY1]

Out of curiosity- does the needle read right for the 120 scale?

 

[bigredbird]

I swapped a 120 out for a recalibrated 150 and "zero" sits at about 4mph now. At 70mph the difference is +/- 6mph; i.e. 76 on the speedo is right around 70 on the GPS readout.

 

[Bruzilla]

Out of curiosity- does the needle read right for the 120 scale?

It was spot on with the 120 MPH speedo.

 

[Bruzilla]

No, you need to take the speedo to a speedo shop to have it recalibrated to the 150 script.

??? I didn't change the face. I took a gauge cluster with a 120 MPH speedo out and installed one with the 150 MPH speedo, so the new speedo should already be calibrated correctly.

I was thinking there's a drive gear where the cable plugs into the tranny that needs to be changed.

 

[69Bee]

Should work fine, as the unit is calibrated. You can only change the unit out with another, as long as your tranny gear is correct. I think the 150 unit itself needs a little TLC. After years of use and sitting, they are not going to work like they were new. There are plenty of places where you can get it rebuilt/refurbished, or do it yourself.

 

[68gtx]

who calibrates them, my 150 speedo is out too

 

[AirborneSilva]

This has got my interest as I'll be putting in a 150 speedo too. I plan to send it in to a gauge restoration shop prior and they should calibrate the gauge itself. I would think that you would only need to change the gear in the tranny if you changed tire size.

 

[69Bee]

The grease in the speedo gets hard, and the magnet spins erratically, making the needle side worse. A guage resto shop hould be able to go thru it, clean and regrease it. A google search brought up Bob's website, and says they have been doing this for 80 years.

http://www.bobsspeedometer.com/1/120/index.asp

 

[RR78654]

Was 150mpg speedometer available for 69 roadrunner? I thought the 150mpg was just for GTX.

 

[chtampa]

Was 150mpg speedometer available for 69 roadrunner? I thought the 150mpg was just for GTX.

GTX and Police Certified

 

[RR78654]

GTX and Police Certified

Thanks

 

[Nacho-RT74]

If you are not too picky you can "calibrate it" adding tension to the return spring, using a drill with reverse speed and a piece of speedo cable with the square end to fit into the speedo.

60 mph equals to 1000 rpms so if you get a 1000 rpms drill can be taken to getbthe needle to read 60 mph. If not 1000 rpms drill, any other will work just making the maths. A 750 rpms speedo will mean 45 mph... and so...

I had to make it after fix the needle shaft bushing with epoxy because got broken, making the return spring be free on assembly

 

[Ron H]

Yeah - the tranny speedo gear would need replacing if you go to larger or smaller diameter tires or change the rear end gearing. When I increased the wheel size I ordered a lower profile tire and overall diameter was pert near the same. Had to put in another gear when changing the end gearing.

 

[Nacho-RT74]

Tranny speedo gear requires a change just if making a wheels or any axle related change but not a speedo change.

 

[Nacho-RT74]

BTW, some ppl here is thinking on standard 68/70 cluster where the scale script is on lense and you can change the speedo and lense with scale separatelly. We are on 71/74 board, where speedo and scale is one assembly, not on lense

 

[ODZKing]

OK, I have read the various posts and seems like it goes from one thing to another. So let's be clear, can you simply take a 120 MPH speedometer out and put it a 150 MPH speedometer? Will it work properly with out modification?

 

[68gtxman]

OK, I have read the various posts and seems like it goes from one thing to another. So let's be clear, can you simply take a 120 MPH speedometer out and put it a 150 MPH speedometer? Will it work properly with out modification?

I did it and other than reading the wrong speed, it operates. BTW, it was reading the wrong speed before the change too (different gears in the rear). I bought the more correct speedo gear but I haven’t installed it (for 15 years now). GPS speedos are very accurate and if you want accuracy, buy a cheap Chinese heads-up-display unit that sits on your dashboard and reflects on your windshield. You can put it in your glove box during car shows. Buy it here: https://www.aliexpress.com/item/330...chweb0_0,searchweb201602_1,searchweb201603_53

 

[Nacho-RT74]

OK, I have read the various posts and seems like it goes from one thing to another. So let's be clear, can you simply take a 120 MPH speedometer out and put it a 150 MPH speedometer? Will it work properly with out modification?

YES!!!

The only problem and is what some ppl is getting confused around here is about those where the scalle is divorced from speedo itself i.e.: 68/70 standard cluster B bodies. Where you need to change not just the speedo but also the lense.

but on ANY case, you won't need ever any other change related with speedo gear on tranny.