Speedometer calibration

[WVZR-1]

Originally Posted by Pumpman

Gentlemen,
Thanks for the detailed replies. I will investigate and get the right gears for the speedometer pulse driver.
Will the VIN# or other numbers on door jam/console cover give me the code for the original rear axle ratio?
When I bought this 1991 car I assumed it was 390HP. I had read someplace that the upgrade in heads to 405HP did not occur until 92 but the data plate on the cup holder cover says LT5 405HP, 385 Ft-lbs torque, 11:1 compression.
Is this correct?

The "cup-holder" data plate is an "owner added item" - your console label should display a GM3 RPO code. The SPID label is located inside the console lid on the '91. That's the original 3.45 -If you wanted to lift both rear wheels and rotate a wheel one complete revolution if my thoughts are correct then the drive-shaft would rotate 4 revolutions +. If it rotates only three and one/half times then I'm lost. The advertised HP changes occurred in '93. Earlier advertised is 375.

Originally Posted by Mystic ZR-1

Your speed shop guy has his head up his a##!!!

I'd have to agree with this comment!!!!

[Pumpman]

Thanks, I suspected the plaque was an owner "add-on". I will mark the DS and roll the tires to check the gear ratio.
And YES the Speed Shop guy was out of line. I will not patronize him.
My Dad worked as a mechanic 7 years in the Ford garage 1947 to 1954 changing parts. Then 7 years in the Chevrolet garage where he learned how to bench strip components and repair the broken parts. In 61 he went into business for himself and I worked with him.
One thing he admonished is NEVER - ever disparage a person's car even if it is a clunker (probably all they could afford).
I guess the Speed Shop owner didn't have a Dad like mine.
By the way my Dad later gave up the family business and worked in a Marina where he went several times to......Mercury Marine for training on outboards an IO's.

[WVZR-1]

Originally Posted by Pumpman

One thing he admonished is NEVER - ever disparage a person's car even if it is a clunker (probably all they could afford).

There's many that could use that information "reminded" to them nearly daily. They just don't "get it"! I had to remind a couple probably more frequently than just "once a day"! Aggravating for sure!

[alnukem]

Where's the most reasonable place to buy the correction gears? Thanks.

[WVZR-1]

Originally Posted by alnukem

Where's the most reasonable place to buy the correction gears? Thanks.

The internal drive gear in 13 tooth that's required for any rear axle ratio over 3.73 is Marc - there's no need to shop it or even consider others. Just buy from Marc. I'd say buy the driven from Marc also. There's no reason NOT to. One stop shopping!

Marc's product page:

http://www.zr1specialist.com/HAT%20W...on%20Gears.htm

You do need to understand that these tooth counts for the driven are based on the 315/35 tire that's stock. The move to any of the 18" or 19" tire/wheel combinations require likely different driven gears so there's math and "loaded rolling tire radius or circumference" required. The advertised diameters don't work.

[WVZR-1]

Originally Posted by Pumpman

I will mark the DS and roll the tires to check the gear ratio.

I don't know that this would make it any easier to calculate but if we changed the tire rotation to minutes you should see your "mark" every 14.6+ minutes or just less than every 90* rotation of the tire for the 4.10 ratio, if the original 3.45 were present every 17.3 minutes or 104* rotation of the tire. There's possibilities maybe of 3.73 every 16 minutes or 90*+ and a 3.91 which would be 15.3 minutes or 90* + very little. There's of course 4.33's or 4.56 possible but very unlikely.

Really large numerical ratios or medium duty trucks this has helped in the past. It's been a long time.

For this you could just place a piece of tape or a paint mark on the pinion flange deflector that would be easily visible. Most every build should have the sheet-metal deflector BUT not all.

[WARP TEN]

WVZR-1 is right--just count the axle. If you turn one rear wheel through one complete rotation the drive shaft will turn about 3 and 1/2 times for a 3.45, about 4 times for a 4.10 and roughly 3 and 3/4 times if it is a 3.73 axle. Marc Haibeck has the gears if find you have a non-stock rear and he can offer some very important hints to ease the job. --Bob

[csavaglio]

Another tip is to turn the wheels 10 times.....it makes the difference between 3.45, 3.73, and 4.10 a lot more apparent when you're looking for 34.5 turns, 37.3 turns, and 41 turns respectively. Sometimes the difference between 3.45 and 3.73 turns when you're looking at the shaft turn is tough.

I would suspect 4.10 would give a bigger difference than what the poster's seeing....3.73 would be more likely, but I haven't done a gear swap in a few years.

Chris

[WVZR-1]

Originally Posted by csavaglio

Another tip is to turn the wheels 10 times.....it makes the difference between 3.45, 3.73, and 4.10 a lot more apparent when you're looking for 34.5 turns, 37.3 turns, and 41 turns respectively. Sometimes the difference between 3.45 and 3.73 turns when you're looking at the shaft turn is tough.

I would suspect 4.10 would give a bigger difference than what the poster's seeing....3.73 would be more likely, but I haven't done a gear swap in a few years.

Chris

If you do the procedure that I broke down in minutes it's a "single" rotation process. Done maybe "twice" to confirm but I've never had an issue where 10 rotations would have helped a thing. Of course you need to understand that I know some souls that can't count that high and are short some fingers and toes.

I did the numbers that I posted using advertised diameters because they should certainly be effective enough for the OP to determine his ratio IF in fact that is his issue. It may very well be another issue for the OP but it seemed the more rational explanation of what likely is going on.

The dash display should not have displayed the error in 10 MPH increments that he mentioned. He should have had a number that would have responded better with the math involved.

The numbers that I posted for comparisons should easily be interpreted by a drive of maybe 5 miles or so.

[Pumpman]

Gents,
Problem resolved. Car definately has a 4:10 rear-end gear ratio. Got the right gear set on it's way from Haibeck Automotive. $130 + shipping.
355+- degree turn of the wheel = exactly 4 turns of the driveshaft.
Haibeck is a great resource.
Thanks again for you help.