110 -> 150 Speedo Overlay

[FunkZ]

I bought a 150mph dash overlay for my car. I currently have the 110mph speedo. I'm having Ed reprogram my PCM. I need to know what gear ratio to have programmed so my speedo reads correctly.

On the new overlay 80mph is at 12 o'clock, current overlay shows 60mph.
On the new overlay 145mph is at 3 o'clock, current overlay shows 110mph.

Calculating based on 25.7" tire diameter and .74 final drive:
3.42 @ 2650rpm = 80mph
4.50 @ 2650rpm = 60mph
3.42 @ 4800rpm = 145mph
4.50 @ 4800rpm = 110mph

So if the computer thinks I have a 4.50 ratio, and I'm driving along at 2650rpm, the computer is going to tell the speedo to put the needle at 12 o'clock. Similarly, at 4800rpm (not that I could go this fast) it's going to put the needle at 3 o'clock.

Am I calculating this correctly?

Since the computer is going to be telling the speedo to indicate a slower speed than I'm actually going, will the odometer also be registering less miles traveled?

[Question]

I dont think its going to work that way for odometer, but one could only hope! lol.

[FunkZ]

I've got 150k+ on the clock, so I'm not really concerned whether the odometer is correct or not. ;) But I do want to get the speedo fairly close. So nobody has put a 150 overlay on their 110 or 120 speedo?

[TopHatJimmy]

Why not just get a 150mph V8 guage cluster, then only your tach will be off...much easier.

[FunkZ]

One, more expensive... I only paid $2.50 for the overlay. Two, then my tach will be off, and you can't correct that with a PCM program.

[1DangerousV6]

i bought the "yellow box" recalibrator from blackrobotics.com for my 155mph indiglo flame overlay ...just splices into your speedo wire and you set the new calibration...cost about $100 and works great [img]smile.gif[/img]

[FunkZ]

Thanks for the info, I considered one of those speedo interface units, but that's all they do for $100. And I've heard most cause the cruise to stop working too. I figured better to put that money into a PCM reprogram.

What calibration did you set with the unit? What I mean is how did you figure out what to set the box for after switching to 3.42 gears and the 150 speedo?

[1DangerousV6]

yeah mine caused the cruise control to stop working...thats why i might pull it out and just handle it with HP Tuners...but its worth cuz my gauge face looks INSANE at night...and i hardly ever use the cruise control anyway...basically you just figure out what the change ratio is...going from 125 to 155 was like...20% or something like that...so you just splice it in...set it to 20%...and thats that...if you're somewhat interested in going this way you should email blackrobotics and tell them exactly what car you have...and what you plan on doing...they're really good about getting back to you with awesome info...give em a good day because they're in australia [img]smile.gif[/img]

[arthansen]

You guys might find that PCM tuning will not help the cruise control problem. The problem is the PCM compares the speedometer reading to the calculated value. If it is off, the cruise will not work. If you adjust the final drive ratio in the PCM, your indicated speed will not match the calculated speed. What you need is something that changes the signal between the PCM and the insterment cluster. However, our cars use a single data wire to feed all info to the cluster so I do not think this is possible.

My final drive ratio was off (due to a rear end swap) and cruise did not work. I corrected it using HP Tuners and now it works. If I were to change the raio again for an overlay, reason states that the cruise control won't work. Maybe I am wrong, it might be worth a try. Good luck.

[Antman]

Originally posted by TopHatJimmy:
Why not just get a 150mph V8 guage cluster, then only your tach will be off...much easier.

I've installed the 150mph guage cluster in mine and the Tach is fine.

Basically, the feed to the tach is different on our cars than on the V8's.

Trust me, it works fine.
Would certainly be a lot easier to install overlays on a 150mph cluster.

[speedy-v6-camaro]

Originally posted by Antman:
</font><blockquote>quote:</font><hr />Originally posted by TopHatJimmy:
Why not just get a 150mph V8 guage cluster, then only your tach will be off...much easier.

I've installed the 150mph guage cluster in mine and the Tach is fine.

Basically, the feed to the tach is different on our cars than on the V8's.

Trust me, it works fine.
Would certainly be a lot easier to install overlays on a 150mph cluster. </font>[/QUOTE]Me too....Dakota digital tach box took care of that problem. This way your odometer won't be off as well.

[1DangerousV6]

Originally posted by Antman:
</font><blockquote>quote:</font><hr />Originally posted by TopHatJimmy:
Why not just get a 150mph V8 guage cluster, then only your tach will be off...much easier.

I've installed the 150mph guage cluster in mine and the Tach is fine.

Basically, the feed to the tach is different on our cars than on the V8's.

Trust me, it works fine.
Would certainly be a lot easier to install overlays on a 150mph cluster. </font>[/QUOTE]nice...i just might have to look into that myself [img]smile.gif[/img]

[Antman]

It's a straight swap.
No modification necessary.

Tach reads fine.

Easy way to tell, we know what speed the engine idles at (and on the 3.8, revs are limited to 4000rpm when in Park)

Swap the cluster, start the car and you'll see that the tach reads correct for idle. Rev the engine and watch the needle bounce at 4000.

[FunkZ]

I have heard from others that swapped in a V8 cluster that the tach works, but that it reads low because it is looking for 8 signal inputs and only receiving 6?

3.4 doesn't have a rev limiter so that test won't work for me.

[Antman]

Originally posted by FunkZ:
I have heard from others that swapped in a V8 cluster that the tach works, but that it reads low because it is looking for 8 signal inputs and only receiving 6?

3.4 doesn't have a rev limiter so that test won't work for me.

I had wondered that myself prior to looking into it, however, the feed for the Tach comes from different sources on the V8 than the V6.

In other words, and this is how I understand it, the signal supplied to the Tach is modified by whichever module supplies it, so that it reads correctly.
This kinda makes sense, (although, not sure if it applies on 3.4's), as this allows GM to fit the 150MPH speedo to 3.8's fitted with Y87 without having to produce a completely seperate cluster. Remember, cost is a major factor.

I have no idea where the signal comes from on the 3.4, but i'll have a word with my mate who has one, as he also has an Alldata account, so I can compare the system diagram from his with mine.

Probably won't be until the weekend though, but I'll report my findings.

However, I can say, with confidence, that the 3.8 cars can take the 150mph cluster without any need for modifications.