my 86 IROC only has an 85mph speedo face but i want to make it into a 140mph. since i have installed Macewen overlays i am thinking of ordering a 140 from him then install it and have a speedo shop put the correct gears in the tranny....will this work properly? i was under the impression something had to be recalibrated (like the vss) but ED WRIGHT at FASTCHIPS says there is nothing on the chip to change so i thought i would ask to see if anyone did it without any problems

thanks
terry

 

No it won't work, we've been over this 100 TIMES people. Not to be rude, but do a damn search, I see this question every other day. No you can not just put a 145mph overlay on your speedo and install gears. It has nothing to do with the gears in yoiur tranny. Its the speedo itself that must be replace with a 145mph speedo. Its as simple as taking out the one and plugging in the other. Otheriwse it will not work!

 

un4given, however rude he is, is right. (Just messing with you man, besides you said it)

Basically if you re-calibrate your speedo via gears to match a 140 then your Odometer would be off, we don't want that do we. Actually because you will be essentially reporting to the speedo that you are going slower on the same scale, your miles will increase alot slower, not a bad thing until you get caught by the feds, I believe its a federal offense (ONLY IF YOU SELL THE CAR)

What might be possible, and Unforgiven might have forgotten this, is to adjust JUST the speedo without making an adjustment to the Odometer.

Its been a while since I have looked at a mechanical speedo so bear with me.

What you will notice when you remove your speedo are two ears that spin around a little drum/cylendar thingie, as the ears go round and round the magnets draw the little disc/drum thingie to adjust for speed. The faster it spins, the more the needle moves.

NOW FOR SOME BASIC PHYSICS

Since we know the faster the little ears move, the further the speedo is pulled, and we can deduce that the further the magnets that are spinning are from the cylendar, the less of an effect they will have on the needle.

SO what you MIGHT be able to do is to take the little ears, and spread them to make them further from the little drum/cylendar/ thingie to cause less of a pull when they fly by. You will have to figure out a way to adjust them accurately for it to be DEAD on.

To adjust it accurately, what you might consider is to to take something that you KNOW spins at a constant rate at a certain RPM. spin the speedo and take note of the reading. IE 64 MPH, write it down. take your new speedo and figure where 64 mph will be, and spread the little ears until it hits that mark.

Does that at all make sence?...

I have not done this to the extreem that you want, I have done some minor adjustments ONLY on an 83 Riv, I will look at my 83 speedo from a firebird in the back to confirm the parts and pieces are essentially the same. but its worth a shot IF and only If you pick one up from a junkyard and do it to that one...

Disclaimer: I do not RECOMMEND doing this, I am only stating that it MIGHT be possible, I am not responsible nor condoneing in part or in whole for messing with your speedometer or any other part when working on your car. I am just throwing Ideas out there... If you mess with it and break it, its your bag...

JOhn