checked my rear axle code CCA 3.08 does that sound possible with a 350th tranny and 235/60/15's my rpm calculator says around 331 ratio i may pull the cover if need be because that doesn't sound right runs 2000 from 45-50mph

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members.aol.com/jnkb2cool/chevelle5.jpg (http://members.aol.com/jnkb2cool/chevelle5.jpg) John Krenn
1970 malibu SS look-a-like well kinda
Ft.Worth, Tx

Hey John,
My 56 Vette has a 3.08 with P225/75R15 on 7in wheels. It turns 3000 at about 75-78mph. So, yeh, you're about right. You say it turns 2000 about 50mph. Well half of 50mph is 25mph, so if it turns 1000rpm per 25mph then you are looking at 75mph at 3000rpm. I think you're on the money.

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Tom Parsons

My truck with 3.50's runs 3000RPM @70mph.

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JT
71 SS Vortec 355
www.angelfire.com/co/KellyMotorsports (http://www.angelfire.com/co/KellyMotorsports)

Whats the formula for figuring out what speed your going at a certain RPM with a certain gear ratio?

My speedo is way off, so I'd like to know what I'm going at 3K RPM w/ 3.31 gears, TH400, and 27" tires.

Wes.

john, I drove myself nuts trying to figure out my rear ratio using the math. Like Wes, my speedo was off. I had my wife hld 55 and 65 mph steady in her blazer to give me an rpm read, measure my tire height carefully and calculated 3.55. It was 2400 at 55 and 2850 at 65. 70 is about 3, 80 is about 3300.

My point is, make sure your speedo is calibrated or it;ll be difficult to guesstimate what gears you have.

[This message has been edited by Gene Chas (edited 09-02-99).]

Another thing is tire diameter. 235/60/15 tires are usually listed as 26 inches. Depending on how much pressure you run in them the actual diameter of the tire can change. Try measuring them from the ground to the center of the axle. My calcs show a tire radius of 12". This is probably a little low. Your tire radius is more likely 12.25"-12.50". The rest of the error may be speedo or tach (I have a near new Autometer monster tach that is off by almost 500 rpm at 6000 rpm).

Pull the cover to be sure. My 66 with these tires ran 3000 rpm at 60mph with 3.36 gears. No speedo, this was over a measured mile. Didn't jive with what theory predicted but this was emperical researched showed.

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Pat Kelley
66 & 67 El Caminos

My car with 3.08's and 235/60/15's runs about 3000 at 70 MPH that is somewhat of a gesstimation since I forgot to grab my old speedo gear when I changed trannys and it reads about 82 MPH at 3000 but reads about 1.15 miles over a measured mile

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Frank W
Melrose MA
Gen X Member#22 (MA)
73 Malibu Coupe
hometown.aol.com/folingo/folingo.html (http://hometown.aol.com/folingo/folingo.html)
Mighty 305 Power

Why dont you jack up both tires off the ground, by the rear end, mark a line on the driveshaft to the pinion on the rear end and rotate the rear axle ONE full turn. While doing this count how many revolutions the driveshaft makes. I did this on mine and I got very close to 3 and a half turns. Later when I pulled the cover to change fluid it was right on the money 3:42. Very easy and dont have to take the rear end apart to check.

As far as speed goes, get on a road with mile markers. Holding a steady RPM time yourself to the second between markers. One minute exactly per mile is 60MPH. Two minutes is 30MPH, 30sec is 120MPH, etc. You can ratio it from there.
John Walker

MPH x Gear Ratio x 336 / Tire Dia.= RPM

Yeilds rpm at 1:1

Take RPM and multiply by gear in transmission.

I will email you the chart I use with my five speed.