87lt1rx7

with gratitude I have a 95 z28 m6. I have the dilemma of the fact that my rear gears are 3.42s(I just checked 12 pinion and 41 ring). My speedo is accurate(checked with a gps) and as far as I know my tachometer reads accurately. However, I have run all of the rpm calculators and they say I have 4.10s. At 2000 rpm in 4th gear (1:1) with a 26 inch tire I am at 39 mph. I do not know where the missing link is other than it is driving me nuts and I cant think straight. does anyone have any ideas?

drkbluls1

iTrader: (1)

its a Camaro, that explains all....

jessedale98

iTrader: (16)

You cant just change ur tires and expect your tach to read correct. The stock size is 24 inches, u got 26. If u program ur car for 26inches ur tach will be correct.

chicane

You must be using Clinton math....

I figure you are at 45.4 mph with a true 26" tire and 3.42 at 1:1.

39mph at 2000rpm with a 26" tire at 1:1 is approx. a 3.96'ish ratio.

Nor would I count on an SPS type GPS for anything that I could not assume probable error for. SPS systems have an intentionally degraded resolution by the DOD by the use of "Selective Availability." This would equate to about 14fps of error.

That and I can almost bet that your tire is not a true 26.0" either.... what tire are you running.... manufacturer/size/running PSI etc etc ??

87lt1rx7

first of all I would like to clarify that whoeever said that stock tire size is a 24 inch is nuts. stock size is a 25.77 if you want to be dead on. I am currently running a nitto 555r drag radials 245/50/16 with 25 psi at 73 degrees and 23 feet above sea level with an atmospheric pressure of who gives a damn. with a little less than full tread. still, I would like to say that it would be clrealy identifiable if I had 3.42 gears by more than a marginal mile or two an hour difference. there would be more like five miles per hour.

And just because i have a low post count on here does not mean I am an idiot. thanks!!

chicane

First. Chill the hell out. No one is making any point because you have a low post count. Your reply however, does make your assumed point more than obvious though.

Secondly. Tire manufacture type, size and pressure do make a difference. I didnt ask you for the AtmoPsi but thanks for including the sarcasm. Tires with lower set psi can (and do) grow with speed..... which is calculable error.

Now, back to the regularly scheduled program.

The "factory stock" Goodyear GS-C in the 245/50/16 size is 25.7". Unknown if this was an inflated spec.
The Nitto 555R Drag Radial in the 245/50/16 size is 25.6". Known that this is an inflated spec.

So.... neither of them are 26.0" and is accountable for some of your possible error. A 25.6 or 25.7" tire at 2000 rpm with 3.42's at 1:1 equates to roughly 44.7mph. With a 26.0" tire and the same variables, it equates to roughly 45.4mph.

With a 4.10 ratio and the included variables, the 25.6 or 25.7" tire would be roughly 37.8mph. And with a 26.0" tire, it would be roughly 38.9mph.

So no..... there isnt much there in the way of math. The factory TACH in the production F-body is a friggin joke. I have known them to be off as much as +/-750rpm at any given point in range. So this might also induce some error in the error as well.

I agree. Something is amiss here. I say the error is in your factoy TACH and speedo.

Gearhead1

Just to add more fuel to the fire, the manufacturers listed tire heights are for an unloaded tire. After you put the weight of the vehicle on it, it becomes the loaded height and on average drops the height roughly 3/4 of an inch. When trying to calibrate speedometers, I believe some of the tuning software out there defaults at 24.97 inches (loaded height) for a 245/50-16, which actually measures 25.65 unloaded.

DVCrazyCamTAWs6

iTrader: (6)

whats with the hating!
enough with the *** talk , tire inflation is not going change anything dramaitcly in the rpm. It's probably a tuning issue check into that

Carter Hays

CLINTON won a Rhodes Scholarship to Oxford University in 1968

The confused poster is clearly using GWB math.

chicane

So..... centrifugal force and rotational inertial have nothing to do with tire growth ?? Geez... that statement goes against the laws of physics by itself.

Tuning, has about nothing, to do with mechanical values........

Hater.

Ok. You got me there..... he DID actually show signs of intelligence some 38 years ago. But what happened ??

8a8mfh

iTrader: (3)

I come up with 1744 rpm at 39 mph with 3.42's and 25.7" tires. Thats only a 256 rpm error in the tach. And 2090 rpm with 4.10's.

my94blackz

Ball park

1600rpm@70=3.42
1800rpm@70=3.73
2000rpm@70=4.10

8a8mfh

iTrader: (3)

Those numbers are way off, no where near close.

87lt1rx7

Chicane, just to clarify, I do agree that tire growth can play a factor, however the minute details of tire growth are not going to greatly effect my tach to be 300 rpm off nor would it pick up the slack of having 4.10 gears yet everything else gauge wise telling me otherwise. yes, it would be going against laws of physics to say that it wouldnt make a difference about tires, but come on Mr. Wizard, I am cruising at 40mph. its not like I am a top fuel dragster putting an extra 4 inches on my tire.

chicane

Yeah... I know. But with so many variables in this situation, it can lead to tolerance stacking.... or AKA, "The measurement of uncertainty". Ya know, 1% here.... 1% there.... 2% for this.... 1.5% for that. That quickly adds up to 5.5% which could be the answer to your 5+ MPH anomaly.

There is more in this equation than the simpleton approach that is being taken. You wanted (and need) emperical data or "clearly identifiable" evidence...... that, and you did ask for ideas. Which I have provided a logical and discernible approach.