subdude

Well I was sitting around this weekend doing some research on my car and I started looking at gear ratios for my transmission. I did the math using the numbers I knew for sure 3.55:1 rear axle, and gears 1 through 4 on my transmission as well as 235/50 r-18 tires. I guessed my final (5th) gear ratio to be 0.62 initially since there seemed to be a few choices. I measured car speed with both the installed speedometer as well as my GPS. Engine speed was held at 2000 RPM for measurement in all gears and measured with the car's installed tachometer. I got almost exact data for all gears with the GPS and was close with the speedometer for all gears except for 5th gear. I calculated 70.28 MPH/2000 RPM and got 67 MPH with the GPS and 69 MPH with the speedometer. The GPS proved to be more accurate with all other gears so I figure that should be the case with 5th gear. Does anybody know for sure what the final gear ratio is? Could just be that small inaccuracies in reading engine RPM at that ratio produce large errors in vehicle speed.

Hey! it's Sunday and I've got nothing else better to do.....

John

BruceH

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subdude

That calculates out to 64 MPH. So I guess my engine RPM was higher than 2000 when I measured speed at 67 MPH. That is 93 RPM for the 3 MPH that I was off.

Jazzer The Cat

Bruce is correct

You will get some inaccuracy as exact reading on guage is difficult to determine exactly. You will also get some variance as the tires may not be inflated to specific pressure and be off a bit as well.

Jazzer

ezking

Hmm u know I'm not sure how to be exact on that, now I've got a problem to solve this week.

I love these forums.

BruceH

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subdude

You are correct. I first calculated using the tire specs, then I physically measured and they were different. Basically about a 1/4" of the calculated wall height is used for the bead and does not contribute to the overall height of the tire and rim.

John

Norm Peterson

There's at least one other effect going on that shows up at any steady speed and another that introduces error mostly way up near top speed in this sort of calibrating tach vs speedometer.

Tires deform, and one complete revolution of the wheel does not result in exactly pi times the tire's mounted OD. For most street tires this is a 3% to 3.5% effect (more tire revs per mile than the simple calculation "predicts").

The other is that tires require slip in order to generate force. The greater the amount of force that you demand from them, the greater this slip will be. Perhaps the most well-known example is 'slip angle' while cornering. But braking and acceleration traction or even the traction required to maintain a given speed also result in slippage.


Edit - chances are really good that the 0.68 ratio listed is rounded off a little.


Norm

subdude

Yeah, now that you mention it that makes perfect sense. I have seen the effects, most dramatically on top fuel or funny cars but did not take that into account. That also explains why the most error was introduced in the last two gears. Excellent! Thanks.

P.S. Where did you get the 3-3.5% figure from? Is that empirical or manufacturing data and some physics?

Norm Peterson

The original source to me was a hardcover book "Mechanics of Pneumatic Tires" that appears to have been a collection of a wide variety of tire tech articles by a number of authorities. SAE paper level stuff.

All of those effects are things that are best measured, since tire structure is not even homogeneous within individual regions such as tread, sidewall, or even the bead region. When specific construction details differ, these effects vary (there's some difference between radials and bias-ply tires, for example). However, most radial tires seem to fall within a fairly narrow range of values. Some Nittos are maybe closer to 2%.


Norm