Ok, I used to spend a lot of time on here a few years ago doing things like speedometer gear calculations and what-not to match gear changes. Anyway, while looking into guages, I had a weird idea that I want to know if it was feasible before I started actually trying to run numbers.

Everyone wants the 145mph speedo that came on IROCs and Z28s for their cars, whether they be for the 82-89 or 90-92 cars. Also, there are white face overlays for the 145mph speedo clusters.

Now, what I want to do here is put the 145mph overlays into a 115mph cluster and adjust the speedo gears to get it to read accurately. Before I start trying to run numbers, does anyone think this idea is even remotely possible, or should I start looking at different angles?

 

On 90-92 cars the overlays will look like crap if you do that, especialy at night. It might work if you use the "glow" type overlays that don't use the stock lighting.

Now I dunno how you would go about finding what speedo gears will correctly convert the speedo from 115 to 145. But I'll tell you, the odometer will be all messed up.

You really can't beat how easy it is to just swap in a 145 MPH speedo into your car, rather than changing the speedo and the speedo gears.

 

I already have a 145 cluster, and yes, it is a very easy thing to do. But they are going for $200, while you can get a 110 cluster for around $20-40.

If I can find a way for people to use the stock 110 cluster and use 145 overlays, it would make speedo changes a lot easier. But yes, the odometer would be completely useless at that point.

Doing the calculations would be surprisingly easy. All I would have to do is calculate the speedo gears to spin at a little more(.3) than 79% of the drive train speed. Problem is, and the calculations I would have to do, would be to find out if the speedometer gears exist to allow it to run at that speed for certain gearing.

Although I have no way to correct the odometer when doing this (maybe somoene else might - cause it would only be tracking 80% of your miles), I think another possible solution would be to have a new set of custom overlays made that will work with the stock lighting.

 

I just did some quick calculations after looking at the 110 cluster. In order to use the stock lighting, I can grade by 15 instead of 10 and have a 165mph speedometer. Then I would have to calculate the speedometer gears to spin at 69.7% of the drive train. Which would also make the odometer even more accurate.

Anyone know how to change gearing on an odometer?

 

I doubt that the odometer gearing can be changed. And how is 67.9% more accurate

I bought my 145 Cluster for $100, and I've seen them advertised in the classifieds for $100-$125. I just think your barking up the wrong tree.

 

I had meant to say more "inaccurate".

Yeah, I very well might be. But if I can find a way to make it work without going too crazy, I think it would be a good idea.

Perhaps if I go at it from another direction. Instead of changing the speedometer gears, which I found I would most likely have to have custom gears made, I keep the gearing as is, and just adjust the signal going to the speedometer... which I think is already done. Of course, this would limit it to 90-92 speedometers. So if you are adjusting the VSS signal, all you would need is the overlays.

But then again it comes to the odometer, which will still probably be off. Now if I could have two different signals sent...

 

what was a stock speedo for a 1991 firebird formula with a 305,and auto tran.i here ya talk about 110 115 145 and 165 speedos.need help