Speedo Reading Corrections?

[55 Fargo]

Hi all, just took out the 47 Chrysler, had a friend follow in his Van, his speedo is accurate. My car was reading 60 mph, my actual speed about 51 mph.

I thought after I had switched my rearend from 4.30 to 3.73, it would have corrected this, but it didn't, it has improved as before 50 mph would read 65 mph in the car.

Now the engine and 3 spd trans are from a 1951 Dodge, I would think a 51 Dodge and 3 spd standard would have come equipped with a 3.73 rearend, may not though.

Does anyone know, what the typical tire size was for a 1951 Dodge, I would think 6.70 15s, I have P205 75 15 tires, they are rated at 27.2 inches tall.

Would there be any other item, I have not mentioned that would cause this speedo to read high.

Any idea what the sppedo gear tooth count should be for a car with a 3 spd trans and a 3.73 diff.

Would the speedo it self have anything to do with this problem, I do know the odometer is not turning, maybe everything is stiff and needs a good cleaning, but whay would it read a higher speed than the car is going.

Having said this, at 52 the rpm is noticeably lower with 3.73 gears then with 4.30 gears.......Thanx Fred

[Phil Martin]

I went from 410 to 321 in my dodge truck and my speedo reads about 15 mph low.

[55 Fargo]

Hi Phil, that makes sense, but my speedo is 10 mph faster than actual speed @ 60mph,which is really 50 mph, this is with a 3.73 rearend and 3 spd trans.

Now depending on what speedo gear I have, it could be as it is now, or if I get a speedo gear with less teeth it should bring it to a more accurate reading.

I am not saying it has to be dead on, 2 mph in either direction is acceptable, but for me 10 or so mph is not.

Just came back from another test drive,this time on the highway, had it up to 65 to 70 mph on the speedo, which is about 53 to 58 mph, with the 3.73 gears, the engine isn't winding up so much like it was with 4.30 gears. I do find at about 45 to 55 mph the exhaust (smithy muffler) is loud, I do not have a tail pipe yet, but will get one, and hope it tones it down at that speed.

[TodFitch]

I went from 410 to 321 in my dodge truck and my speedo reads about 15 mph low.

At what speed? You changed your final drive ratio by 22% so if you did not change your speedometer gears then your odometer and speedometer should be off by 22%.

[55 Fargo]

Tod, that is correct, but the speedo was out and reading higher with the 4.30 rearend, which was not stock and put in this car at some point in the past.

I have now installed a 3.73 diff, the speedo is reading about 10 mph faster at 60 mph, where actual speed is 50 mph.

So here is the kicker, my engine,trans, are from a 1951 Canadian Dodge, this car may have come with 3.73 gears,3.90 gears, or even 4.10 gears, I have no clue which.

The speedo in my car is aof course a 47 Chrysler speedo, the speedo it self should make no difference, correct?

So I am still wondering why the speedo is that far out at 60 mph, it should be close, or if anything, reading under as I have reduced the gear ratio to 3.73, where the factory diff was probably a 3.90 or 4.10.

i do not think the diff that was originally mated to my engine and trans was a 3.54, they did not make them that way in 1951 for a Dodge car.

I have no probelm off the line with the 3.73 gears, but the 4.30 had a little more zip from a dead stop, which is natural of course.........Fred

[James_Douglas]

Don't assume your friends van is accurate. I had my Desoto checked at a local (closer to Tod) speedo shop and they told me that almost all new cars read 3 to 5 MPH high and some as much as 8 High.

Apparently that 50K mile warranty comes a little quick. Multiply that times tens of thousands of cars and they save a dollar or two.

My speedo was off and not linear. I have 4.1 gears.

20-30-40-50-55-60-70 (Vehicle)

19-28-38-47-51-54-63 (Actual)

James

[Plymouthy Adams]

Fred..the good news in your court is that you have two different speedo gears in your hot little hands...knowing what you do about this one..install the other gear and check against you buds van...the difference can be figured out quickly by the difference per tooth count per gear used...from there you will know what tooth count gear you will need...

[Jim Yergin]

Fred,

Last week I posted a response to your question about speedo gears. Here it is again:

Fred,

You might want to take a look at the 1936-1948 Plymouth Parts book that Chet has posted on his website (http://www.1948plymouth.info/). On page 314 of that book there is a chart for the various speedometer gears used on the 1936 through 1948 Plymouth cars and trucks. According to that chart, Plymouths used a 19 tooth gear with the following rear axle ratios:

4.1, 4.3, 4.375, 4.56, and 4.78

I assume the same would probably be true for Chryslers.

Also, the chart indicates that a 3.73 rear end would use either a 16 or 17 tooth gear (depending on tire size 16X5.50 (17 teeth)vs. 16X6.00 (16 teeth))

Hope this helps.

Jim Yergin

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[55 Fargo]

Tim, I do have a 19 t speedo gear in hand, but there is another 19 t speedo gear in my car it's not going to matter much. I will check it and see, do I have to drain the trans to remove this gear

[55 Fargo]

Hi Jim, have tried unsuccessfully to access this website you have mentioned, 3 times.

I had a 4.30 in the car before, it read upwards of 15 mph of actual speed, it either has a 16 or 17 t speedo gear possibly. But now I have a 3.73 rearend, it should be very close if it is in fact a 17 t gear, in my car now.

If it is the right speedo gear, my tries are 27.2 inches tall, what else could be throwing this out so far............Fred

[55 Fargo]

Just pulled the speedo gear, it is a 17 tooth, the right one for a 3.73 rearend, so what might be my problem, it I am reading at 10 mph over. This was easy to take out of the trans, did not even jack the car up, just reached under and unbolted it.Fred......... ps tires are near the same size as would have been on the car in the 40s/50s

[Plymouthy Adams]

install the 19 and see what it indciates...gotta start somewhere..

[55 Fargo]

The 7.10 15 is 28 inches in diameter, the tires I have on the car are 27.2, this would also put the speedo out by 2 mph. So this could be adding to the problem.

My buddys van also has slightly taller tires, his speedo could be out 2 to 3 mph, so maybe I was out only by 5 mph, I can live with that.........Fred

[Jim Yergin]

Fred,

Here is the chart.

Jim Yergin

[JohnS48plm]

There used to be a guy by the name of george Wellman who was on the forum. He made adapters that would go on the speedometer cable to correct the reading. He could make them to the correction that you needed. I haven't seen him on here for a while.

There is also a place that advertises these adapters in the back pages of Street Rodder Magazine.

Are you still out there George. JohnS

[55 Fargo]

Looking on this chart, with a 3spd trans, 3.73 rear end,27.1 inch tires, I should be right on the money with a 17 tooth speedo gear. Not sure what is the problem, other than if the trans driver gear is different.If I go the 19 tooth gear, and I will try this, as there are more teeth on it, this might slow the cable down and possibly slow down the speedo.

On the parts car, where I got the 3.73 diff, in the 4 spd semi-auto M5 trans, there was a 19 tooth speedo gear, and maybe it was there ofr a reason, not sure. As Chrslers were supposed to come with 3.54 gears not 3.73 gears. So later I will try the gear switch in the speedo and report back....Thanx yall

[Plymouthy Adams]

here is another clinker in the works Fred..though you do have a 17 tooth gear...which by chart may be right the second part of the equation is the gear on the tranny shaft..is it a 5 tooth or 6 tooth..this will make a difference..say about 3 teeth off in your count comparing one to the other same ratio and tire size..

[55 Fargo]

Thats true Tim, in that case I might need a 14 or 15 tooth gear, or am I thinking in the wrong direction..........Fred

[Plymouthy Adams]

I'm thinking your are thinking

[Young Ed]

Fred I think the one I put in for my 3.73 was a 14T. But that is TOTALLY from memory............

[55 Fargo]

Tim, was on a website, for every extra speedo tooth you add, it will lower the speed of the speedo cable 2.5 to 5 %, and will of course slow the speedo down. Will try the 19 tooth gear today.

[55 Fargo]

Ed, you could be right, the speedo driver gear could be different than mine, and/or your tire size. Just found out "rule of thumb" every tooth you add on the speedo driven gear, slows the speedo cable down and of course lowers you r speedo reading...........Yhanx Fred

[Plymouthy Adams]

I knew you had the 19 and that was why I suggested last night to install it and go for another speedo run with a chase car...get a modern car with correct wheels..they getting these real accurate now

[BobT-47P15]

Hmmm......lets see. When my speedo says the car is going 60 mph, the speedo on the wife's PT Cruiser says I'm only doing 45. Pretty big difference, but I just work with the 15 mph thing for now and don't worry about it.

The rear end in the car is from a 39 Dodge truck, or something in that era, and is geared lower than a P15. One of these days, will find a handy P15 rear and replace the current one. Should help some, anyway. Also have at least one, maybe two, other speedos I could put in when in the mood for a headstand under the dash. My odometer works, but is not right as I checked it against the PT during the Tulsa trip. (My gas mileage figure looks better

with my odometer, but not accurate of course. Is less in reality.) Tires are about the right size diameter being 6.70 x 15s.

Fred, I think you're getting close to the answer.

[Young Ed]

Bob the standard ratio for a truck was 4.11 same as a p15 I believe. Now the trucks did have 2 higher ones. The 4.3 and I think a 4.5 or something like that. Also a 3.73 is listed. Interestingly the 3.90 that I put in my truck isn't listed as an option. OOPS. Bob might be time to try and clean off that flat spot on the top of the diff housing.