shermanwizard

iTrader: (2)

Has anyone else ever questioned the accuracy of the factory tachs in their cars? My 92 RS has 3.08 gearing but the tach indicates that it is a 3.73 geared car. If anyone else has run into this problem how did you fix it without using an aftermarket tach?

JamesC

iTrader: (5)

The problem is common.

JamesC

gregsz-28

iTrader: (1)

Very common problem, but you can just get a tach from any '90-'92 Camaro with the same engine as yours, to replace your tach. If your RS has a v6, I have a gauge cluster I'd sell you, but with 3.08 gears you probably have the L03, right?

shermanwizard

iTrader: (2)

I tried 2 other clusters and had the same false reading on the factory tach.

gregsz-28

iTrader: (1)

Are you sure you don't have a tach from a v6 car?

shermanwizard

iTrader: (2)

Both doner cars were TBI V8. I had the same thought but the error margin of the factory tach is closer to 4 cylinder than V6. When the sunpro tach shows 1800 the cluster tach shows 3200. Whatever is indicated by the cluster you have to reduce the reading by nearly 60% to find the actual reading.

okfoz

I owned a Car where the tach was off because the needle had been turned on the pin. (not sure how) When the car was off it would be at 0 (straight down) but while running it was WAY low. When I disassembled it I found that the tach was actually in the wrong position, I had to push the needle so that it was in the correct position. Pretty simple really...

Another thing that may cause an inaccurate tach reading is if you have a different size tire. Smaller would cause it to read higher and larger would mean lower.

V6 tach will give you bogus readings (as mentioned before) with a V8 Apparently not your problem...

IIRC 70 Mph
2.73 = 1750 RPM
3.27 = 2100 RPM
3.73 = 2500 RPM

Also if your car does not go into Overdrive it would make it seem like it is reading higher than it should because the car did not go into OD...

Just some random thoughts... Good luck.

xsupermann69x

how much do u want for that gauge cluster? cuz i need a new cluster sense my gas guage doesnt function right and my tach doesnt work. it is for a 91-92 with the yellow righting?

gregsz-28

iTrader: (1)

Okfoz, tire size or gear ratio won't affect tach readings.

xsupermann69x, pm has been sent, and here are your pics.

Edit: sorry the pics are so small...

JamesC

iTrader: (5)

Speedo, yes.

JamesC

shermanwizard

iTrader: (2)

Tach is now fixed. I don't know how long it will work but it's fixed.

chris.niemann84

[quote=gregsz-28;4138648]Okfoz, tire size or gear ratio won't affect tach readings.

thank f'ing god that someone replied with this already!! i'm just now home from work and had the same problem with my car so i thought i'd read it over to see if i could help you out any... but then you started talking about gears in your rear end and then some other guy talked about tire size and for a minute i thought i was reading a thread by someone that just lost his pedal bike and got a car..... never in the history of man kind has your engine rpm needle varied what so freaking ever because of your gear ratio or tire size. the only time that it will differ is if it has spun on the needle or the sensors are just bad. in my case my 85 is slowly losing her factory senses and i've had to install a water temp gauge and an aftermarket tach.

it's a scary jump but i did it, bought the smallest freakin tach available (needed it for the track and my speedo don't work either). mounted it to the screw under the dash directly above the cigarette lighter and then ran the wires through the lighter hole (don't smoke in car so don't need it). looks clean, out of sight, and it's not a rice grinder size to be ugly, it's black to my interior with black face and soft white glow at night..... i love it!

just get an aftermarket and save yourself the hassel $30 at oreilly's for the smallest cheap one!

okfoz

SINCE I am soooo Wrong... Tell me why at 70 MPH your tach would read the same with a 3.73 or a 3.08 as the initial question stated?

Let us think about this...

At 70 MPH actual speed if you have a taller gear IE 3.73, 4.10 etc your RPM WILL BE HIGHER than if you have a Lower (road) gear such as a 2.73, 3.08 etc. ALSO if you change to a SMALLER tire diameter you will also increase your RPM at a GIVEN speed.

NOW it should be understood that if you do NOT change your speedometer gears to compensate for a different gear ratio your speedometer and tachometer will be in sync and at 70 MPH... Going back to my original statement where a 2.73 gear is about 1750 RPM, you are correct no matter what you change your tire size to the Ratio between your tachometer and your speedometer will be a constant...

Yes there will be a test in the morning.

John

BlueIroc-Z

iTrader: (6)

I'm now interested in where this whole thing about the tachs accuracy being affected by tire size is going to go... Here's my input: Yes, different tire size would affect RPM at any given speed, but its accuracy (which is what I thought this post was about) would not be affected in the least. Assuming a tach reads properly at idle, how would it suddenly become inaccurate when the car is in motion? Also, yes a different tire size would affect the relative position of the tach to the speedo at a given speed, but again how does that affect the accuracy of the tach?

okfoz

I guess No one bothered to ask... How do you know it is wrong?

What makes you think that it is reading like a 3.73 and not a 3.08? If you think it is off, what are the readings and what do you expect them to be?

If you have nothing else to compare it to, why does it read wrong? Are you using a computer to read the computer output?

John

kcallais

I thought the tachometer is driven by the RMS Voltage waves from the low tension side of the ignition coil? One pulse each time the coil fires, running on the white wire from the coil to the tach.

Yes, a 3.73 gear will read higher rpms at any given vehicle speed than a 3.08 geared car at the same speed. Does changing rear gears cause the tach to be inaccurate? No. Does tire size effect tach readings? Inherently yes, its the same principle as why different rear gears change the reading....but on a much smaller scale. Because of the larger gears, the motor has to spin faster to maintain the same rate of movement as a smaller gear. So when John said:
IIRC 70 Mph
2.73 = 1750 RPM
3.27 = 2100 RPM
3.73 = 2500 RPM
....his numbers may or may not be correct, but his theory is. Its extremely akin to Bernoulli's principle.

Bottom line is.......no, rear gearing/tire size will not affect the ACCURACY of a tachometer. It WILL affect the FINAL reading at 70mph.

Back to the OP question.....its sounds like your tachometer is old. It looks like it may well be time to replace it.

chris.niemann84

OMG......... HOW LONG HAVE YOU BEEN AROUND CARS? DUH!! your tach will be higher with 3.73 than with 2.73 cause it takes more motor to turn those gears!! FREAKIN DUH! this thread is going absolutely nowhere!! we are arguing over a factory tach being off.... if it's off the only thing you can do is try another tach, you have done that, you have tried new clusters too, so now all that is left is to get an aftermarket tach, hook it up to the white wire on your coil, and compare. if they are the same then your factory tach is right regardless of what freakin gear you have. now if you are trying to use your tach as a speedo like i do then yeah, there is a difference, to fix that take your friends little geo and have him do whatever speedlimit you are most concerned with, for me it was 65. ride right along side him and mental mark where your tach is! that's all you can do. if you need your tach for the track like i also do, then get yourself and aftermarket tach, mount it, wire it, and drop this huge retarded argument about tach's with tire size and rear end gears. you use tach for motor and speedo for speed.... a freakin tach does not matter what rear end you are using, you could use 6.11 gears for all i care, when your motor is at 7000 it is at 7000 and mine with 3.73 will be at 7000 also, however i will be moving faster than you, that's all there is to it!!

HOLY FREAKIN WOW!

P.S. ENJOY YOUR TESTS..... whatever that means?

chris.niemann84

just to clarify and keep myself part of this wondeful website, that previous comment was in no way shape or form directed at "okfoz" i just used your statement to make the same point that I was getting at too....

just wanted to clarify that for the records!

Thanks
Chris

crap... and i hit post before finishing that sentence at the bottom too.......LOL phone was ringing

"enjoy your test's whatever that means" - cause i'm not coming to class tomorrow!!

BlueIroc-Z

iTrader: (6)

Ok, yeah, ^that's^ all I was trying to establish. I think the source of all this confusion is the OP said "Has anyone questioned the ACCURACY of their factory tach?" Which I what I took to be the main subject of this thread. The ACCURACY of the factory tach. That's why I jumped on okfoz about his statements. Although, in retrospect it now seems that some people were talking about tach accuracy (myself, JamesC, and gregsz-28) while others (okfoz, and the OP apparently) were talking about how tach readings are affected by rear end ratio... (In fact, I'm going to go out on a limb here and guess that the OP was just using rear end ratios to illustrate how far off he thinks his tach is.)

So, my apologies to okfoz for misinterpreting the basis for his reply and to the OP for not really helping with his problem.

Anyway, I hope we can all be friends now.

gregsz-28

iTrader: (1)

That is how I understood it...

Jason89RS

iTrader: (2)

Since everyone was busy arguing about gearing/tire size and hardly anything useful was posted on how to resolve the problem here's a link on how to fix the factory tach. https://www.thirdgen.org/forums/elec...ml#post2998002