loopy

iTrader: (1)

Hi There

Im a first time poster, first time Camaro owner from New Zealand.

I recently traded my boat for a 1983 Camaro Berlinetta and spare parts car. ( I really like this car )

As far as I know it still has the stock 305/700r4 combo.

The old owner says it has a 3.73 diff ratio.

I am working my way through the cars problems of which there are a few.

The car was originally injected but the PO took all that off and installed a 600cfm carby.

What has me confused is the relationship between revs, speed indicated and actual speed. Tires are 225/60/15 all round.

At 60mph actual the speedo is reading 75mph and tacho at 3500RPM.

Does this seem right?

I would have expected at 60mph indicated the revs would be around 2000.

Another thing that has me doubting the rev counter is that when I did a full throttle standing start the car took off and before I could select 2nd the rev counter was pegged at 7000RPM (yeah right!!!) I didn't think a stock 305 could pull that hard.

The car seems to be a bit of a bitsa with a body kit from another year/ years
3 different years to be exact.

Somethings going on.

What revs should I get for 60mph indicated assuming the stock gearing for this year.
What revs should I see for 60mph actual given the 3.73 gearing.

I'm just trying to nail down where this could be faulty.

Your thoughts would be appreciated.

Great site by the way. I have learnt heaps since I started reading threads here.

Cheers

Geoff

loopy

iTrader: (1)

Here a couple of pics

sofakingdom

iTrader: (1)

Wouldn't have come with 3.73 gears; although who knows what's happened to it since. More likely would have come with 2.73 or 3.08 gears. Coulda been changed of course, but I'd be skeptical of them being 3.73 until verified.

Since you just bought it, first thing you should be doing, is catching up on all the "deferred maintenance" (aka neglect). One of the first things there would be, as always, the differential fluid. Make a point of looking at the gears while you're taking care of that.

The stock tach in these cars is VIRTUALLY ALWAYS messed up. Usual way it goes bad is, it reads high. I have (or had? might have got rid of it) one in my pile of junk that read 3 times what it should. Best one I have only reads (or, read when I put it in,,, ... who knows what it's done since) about 5% high. Verify it against a piece of actual test equipment.

loopy

iTrader: (1)

Thanks for the reply.

I have read about the rev counter errors.

I am just wondering what revs it should be doing at a given MPH with a given rear ratio.

sofakingdom

iTrader: (1)

There are "calculators" all over the Internet.

It's eeeeeezy enough to figure out yourself though... 2 x pi x the ACTUAL MEASURED RADIUS of the rear wheels (pull a measuring tape from center to ground) gives you how far the tire rolls in one rev. Divide one mile (or km) by that; that's how many times the wheels rotate in one mile. Multiply that by the ACTUAL OBSERVED ring gear tooth count and divide by ACTUAL OBSERVED pinion gear tooth count. That's the drive shaft revs per your preferred distance. Assuming no torque converter slippage, that's how many times the engine turns in one preferred distance, as well, when direct coupled; and thus at 60 mph (or 60 kph), it's also the engine RPM.

As an example (in Imperial units):

While changing your rear end fluid, you OBSERVE that there are 13 teeth on the pinion and 40 on the ring. After buttoning all that back up with nice clean fresh fluid and you get the car set back down on the ground, you get out your trusty tape and MEASURE that the ACTUAL EXACT center of the wheel is exactly 13¼" off the ground. That means the tire's circumference - how far it rolls - is 83.25", or 6.94'. There's 5280' per mile; that means the tire rolls 761.1 times per mile. Multiply by the ring teeth (40) and divide by the pinion (13), that's 2342 drive shaft revolutions per mile. At 60 mph (exactly one mile per minute) it therefore turns 2342 revolutions per minute. If you had a stick shift, then in direct gear (4th in a 4-, 5-, or 6-speed, 3rd in a 3-speed) your engine RPM would also be 2342. With an automatic you might end up with about 5% slippage or so, therefore in 3rd gear (direct) the engine RPM should be about 2342 ÷ .95, or around 2465 RPM. With an overdrive transmission, the RPM in the highest gear will of course be lower, by the factor of the overdrive ratio. (.76 in a 700 if memory serves, or about 1875 RPM) With the lockup clutch, it might be closer to the same as the stick-shift RPMs.

Did I use words like ACTUAL, OBSERVED, MEASURED, EXACT, and such as that, enough? No "assuming", no "calculating" based on the trade size, no "the previous owner said", no "I thought", no "I heard", NONE OF THAT. Stick with ACTUAL, MEASURED, and OBSERVED.

I should also add, yet another potential source of error in your car's "instrumentation", is the speedometer gears; particularly if the rear gears were changed but the speedo gears were not.

Regardless, once you get the ACTUAL OBSERVED gear tooth count and the ACTUAL EXACT MEASURED tire radius, it's elementary school arithmetic to derive every other number you need.

loopy

iTrader: (1)

Cheers.

Rear gears were definitely changed, speedo gears definitely were not.

That's why it reads 75MPH when only doing 60.

The car I think originally had 3.08 now has 3.73.

60/75=0.8 3.08/3.73=0.82

Given variances in tire size that's near enough for me.

I appreciate your reply but I don't think I willing be pulling the diff head just yet. I have other things I would like/need to do 1st.

I was simply trying to get a handle on what to do about the rev counter.

What do you guys do with these rev counters? Throw them away or is there some fix or replacement circuit board that fixes them?

Cheers