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OK, to get back to my original dilemma. To find out why my speedo is so far out to the rpm's, I need to verify that the speedo gear in the tranny is correct. Verify that the 28" diameter wheels are within a reasonable dimension of what the original tires would be. Weather is going to be nice here tomorrow. So I am going to hook up the old school tach and the new one to make sure there are no errors in the data. If the tach checks out, and for now assume that the other two are correct, here is the quandary. At 60 mph(via GPS) the engine is turning almost 2500 rpm. Why? That's a 70 mph rpm. I will post my findings tomorrow.
For now, it's Brandy,some ice, and time to ponder.

Assuming the tachs check out, what's the difference between OD and Non OD rpms or did anyone calculate the non OD.

For NON OD gimme da axle ratio and the overall tire diamter u want to use and i'll calculate it.

Or try it your self:

Multiply tire/wheel OVERALL diametner by 3.1416. Call the answer to that calculation "C".

Divide 63,360 by C. Call that "M".

Multiply M by the axle ratio. Call that "S".

S will equal the engine rpm at 60 MPH.


THen divide S by 6 and that will give the incremental engine rpm for EACH 10 mph of car speed.

From post 1 of this thread the axle is 3.96:1 ratio and the wheel tire combo diameter is 28".

Multiply 28 by 3.14 = 88" ("C")
divide 63360 by 88 = 720 ("M")
Multiply 720 by 3.96 = 2851 ("S") THe engine rpm at 60 mph.

Divide 2851 by 6 = 475 rpm FOR EACH TEN mph of car speed.

SO at 30 mph 3 times 475 = 1425 engine rpm.
at 60 mph 6 times 475 = 2850 engine rpm.
at 70 mph 7 times 475 = 3325 engine rpm.

With the rpm falling right in the middle above OD and below non OD expecteds, will be interesting to find where the difference comes from. Bad tach reading, smaller tire diameter than thought or higher axle ratio than spec'd are the three variables I can think of.

Quote:

Mark, I can understand your concern. Attempting to confirm your actual RPM's @ what you feel is a comfortable cruising speed is important. I think your main dilemma is that you don't want to "push" the engine RPM's up to obtain a cruising speed higher than the car was built for. We had this same discussion awhile back about my Clipper and I basically approached it the same way you are. Just as OD described, I ran my Tach wires through the drivers side fresh air duct and had the tach on the seat where I could observe the RPM's at different speeds. As it turned out, with my 3:23 rearend ratio, P235/75R15 tires and Ultramatic, I'm running 2750 @ 65MPH which is about as high as I want to push it except for maybe a short burst here and there. Once you get the carb straightened out, your car with it's overdrive transmission will probably be fine cruising at 70 all day long. Both Ross and Packard V8 were spot on with their calculations for my car.

PackardV8 - Thank you for questioning my data on the tach. I had only checked at idle. When the rpm's were up in the operating range the, tach was reading about 300 rpm high. When switched to 8 it read about 100 and a bit lower than my old school dwell/tach meter.

As far as the numbers that everyone has posted, my car is right on the money. At 30 mph OD disengaged old school showed just under 1500 as did new. OD engaged - old school showed 1100, new about the same. At 60 mph OD engaged old shool showed about 2200 and new showed 2000. So mechanically the car is good. The mechanic had a when it came to the tach. Speed was done by GPS, so it comes down to how to get the speedometer to read closer to actual. As Howard mentioned in a previous post the spring inside that would work against the magnets may be weak. This has been a good learning experience.

Thank you everyone for all your assistance.

Good to hear a cause was found. One other thing might be to verify the actual tire dimensions vs bias and see if there is enough difference a different trans pinion would get you close to actual reading. You probably have a 17t for the 4:1 but wonder if an 18t for the 4:3 would work and could be found at one of the vendors

Tire dimension is one of my considerations. I have to work the next two nights so next days off I hope to look at that.

Going back a year or 2 i think someone posted a website or mayb chart showing various tire sizes and their rolling radius relative to various other factors. I think it was Owen Dyneto that posted the chart but i'm not sure. Maybe do a search of this website to find it. A google search mite turn one up too.

However note that rolling radii can vary from mfg'er to mfg'er of tires by a 1/2 inch, maybe as much as 3/4 inch.

I've not bothered to make any calculations as to how much even just a 1/4 inch mite vary the readings. Probably not much except for very critical accuracy which i don't think we are dealing with here.

Tire size - I checked the original spare that is in the trunk(that's gotta go) 710-15 which is a 28" diameter tire. The tires I have on the rims are 225-75-15 which are 28" in diameter. So my speedometer readout is the issue. All other issues are resolved.