Speedo Accuracy vs Tom Tom GPS

[55 Fargo]

Hi all, just got back from a ride and to verify my speedo here it is as follows;

30 mph speedo=30 mph GPS

45 mph speedo=45 mph GPS

50 mph speedo=50 mph GPS

55 mph speedo=54 mph GPS

60 mph speedo=58/59mph GPS

Metric

31 mph speedo=50 kmh GPS

46mph speedo=70kmh GPS

52 mph speedo=80kmh GPS

57mph speedo=90kmh GPS

So I have a regular 3 spd trans,19 tooth speedo drive gear, 3.73 diff, P205 75 15 tires (27.1 inch aspect), and the speedo is not out that much at registering over at about 2 mph at 60 mph , which is really showing 62 mph on speedo, when actual speed is 60 mph.

I am happy with this, I changed speedo heads last spring, and figured it was more accurate, now I have verification.........Fred

[TodFitch]

Looks like your speedometer is just about as accurate as an old analog device can be. I'd be very happy with that behavior on my speedometer.

[BobT-47P15]

I'd say you have a darn good speedo there, Fred.

Mine reads around 10 to 15 mph faster than actual speed, so I

just try to account for it and not worry. What messes with your mind is

when you're going 50 your needle says 65......or says 70 when doing

around 55.

And these old cars seem to be going faster than they really are.

[55 Fargo]

I'd say you have a darn good speedo there, Fred.

Mine reads around 10 to 15 mph faster than actual speed, so I

just try to account for it and not worry. What messes with your mind is

when you're going 50 your needle says 65......or says 70 when doing

around 55.

And these old cars seem to be going faster than they really are.

You know Bob, except for some wind noise, around the doors (need new weatherstrip), and the front window too, this car rides really nice at 55 to 60 mph, even in a cross wind, it is very nice with no wind........Fred

[BobT-47P15]

See, Fred.........your old "clunker" has turned out to be a nice car

with the work you've done on it.

[55 Fargo]

See, Fred.........your old "clunker" has turned out to be a nice car

with the work you've done on it.

Wait till I have the roof insulation, headliner, and trunk divider behind the seat insulated, it will be nice and quiet. I have drivers side window rattle, and will need to swap ina new window frame, which luckily I now have.

Then want to rebuild front end, new rear springs or modern rear end, and the list goes on and on..........LOL

[P-12 Tommy]

Just like stone soup, eh?

Tom

[55 Fargo]

Just like stone soup, eh?

Tom

Or the story of "Nail Soup":eek:.................LOL:p

[martybose]

My speedo is dead accurate at 60 MPH, but it is a mile off at 30 MPH (an indicated 30 MPH is actually almost 37 MPH; definite ticket country!)

I'd have to find an oldtimer to overhaul the speedo head to fix it across the board.

Marty

[Jim Yergin]

Here is what I have posted in my car to determine actual speed

Jim Yergin

Speedometer Corrections.pdf

[Young Ed]

Jim I spot checked a few and it looks like you'd be pretty close with a 15% reduction. The speedo thats currently in my car reads 20 at 30 45 at 60 and 90 at 90. So its not off a straight %. I've got a different one that seems great and will be swapping eventually. After about 6 speedo installs this summer I'm done.

[Gary Manes]

These were nice cars (when everything worked)--comfortable, reliable and when driven correctly rock-solid on the highway.

I am amazed how stable my '48 New Yorker is on the 15 mile drive up to my mom's. Both my '99 Honda coupe and my '76 Grand Marquis sway and react to the wind far too readily for my taste--but not the New Yorker!

Our problem is just getting them back into that condition.

[48ply1stcar]

Fred

Thank you for providing the information in your oridginal post. Last week I posted that now that I have elimated the bounce in my speedometer needle I found that by using a GPS my speedometer is about 21% off, reading 38 at 30mph and 70 at 55mph. Sounds like BobT has the same problem.

At least now I can start by checking and comparing the number of teeth on the speedometer gear and the rear end ratio and see if it's the same as yours.

How do you know that your read end ratio is 3.73, is that standard for P-15s?

Harold

[Young Ed]

3.73 is not standard for p15s. 3.90 or 4.10. I put a 3.73 in mine and changed the speedo gear to match.

[BobT-47P15]

Ed, where did you get your speedo gear??

[Young Ed]

Out of a random transmission that was in Dads spare parts. Probably was the same car that donated my diff.

[BobT-47P15]

Thanks for the reply.........I was hoping you had bought it from some

vendor.

I'm trying your trick - hung two different speedometers under the dash

today as a temporary trial situation to see if they are any better than the

one currently in the dash. Used the GPS to compare. One

was right up til about 35 mph, then it was maybe 5 to 8 mph faster than

I was travellng. The battery in the GPS ran down, so I just drove a bit

with the other speedo.....but don't know if its accurte. Will try again

tomorrow.

I did pull the cable out of the housing, wiped as much of the lithium

grease as I could off it. Sprayed some liquid graphite lock lubricant

down the housing, and also put it on the cable before reinstalling.

When that lithium grease gets cold enough, it makes the speedo go

crazy. I actually disconnected it last winter for a while.

[Frank Elder]

Fred

Thank you for providing the information in your oridginal post. Last week I posted that now that I have elimated the bounce in my speedometer needle I found that by using a GPS my speedometer is about 21% off, reading 38 at 30mph and 70 at 55mph. Sounds like BobT has the same problem.

At least now I can start by checking and comparing the number of teeth on the speedometer gear and the rear end ratio and see if it's the same as yours.

How do you know that your read end ratio is 3.73, is that standard for P-15s?

Harold

Fred doesn't have a p15, he has a big chrysler.

[55 Fargo]

Fred

Thank you for providing the information in your oridginal post. Last week I posted that now that I have elimated the bounce in my speedometer needle I found that by using a GPS my speedometer is about 21% off, reading 38 at 30mph and 70 at 55mph. Sounds like BobT has the same problem.

At least now I can start by checking and comparing the number of teeth on the speedometer gear and the rear end ratio and see if it's the same as yours.

How do you know that your read end ratio is 3.73, is that standard for P-15s?

Harold

As stated by Frankie, I have a 47 C38 Chrysler Royal Coupe, 218 Canadian 6 cyl Flat head, no fluid drive, 3 spd trans, dry clutch, and 3.73 rear end.

How do I know I have a 3.73 diff, because I installed the 3 rd member into the axle housing, and it was stamped right on the gear "3.73", as well as on the housing itself.

You can check on top of your hog head, and see if you can see what number is stamped up htere, or mark the driveshaft, and roll the back wheels, then count revolutions of back tires to 1 complete revolution of your drive shaft, if it is just under 4 revolutions of the wheels, you may have a 3.9 to 1 diff as an example.....Fred