Speedometer Error

[Don St Peter]

Ever since I installed a George Asche Overdrive in my 40 Plymouth P-9, my speedometer reads 8 to 10 MPH to slow ( ie actually doing 30 MPH and spedo registers 20-22). I know there are suttle differences in the reinstallation, like the tranny to speedo hook-up is different ( ie on left now vs right before ), however why the reading difference and can I correct the situation.

By the way, I love the car with OD, especially the " fast second gear" feature.

Thanks in Advance

Don

[james curl]

It could be the difference in the speedometer drive gears in the transmission. The overdrive cars usually came with a 4.11:1 rearend ratio and the non-overdrive cars with a 3.9:1 rearend. The speedometer drive gears must be the correct ratio for your rearend ratio for the speedometer to be correct. If I am remembering correctly the speedometer cable should be turning 1,000 RPM at 60 MPH for the speed indicated to be correct. If I am wrong I am sure someone here on the forum will correct my statement. Sometimes old men like me have CRS.

[Don Coatney]

If I am remembering correctly the speedometer cable should be turning 1,000 RPM at 60 MPH for the speed indicated to be correct. If I am wrong I am sure someone here on the forum will correct my statement. Sometimes old men like me have CRS.

Here is my CRS version:D

I think it is 1000 turns per mile irrevelent of speed but I could be wrong:o

[Norm's Coupe]

If your speedo is off due to the gears in the transmission, you can fix that without changing the gears inside it. There is a place in Michigan that sells gears you can mount between the transmission and speedo cable that will allow you to adjust the speedo, without changing the internal transmission gearing. Forgot the name of the place, but they advertise in the custom and street rodder type magazines. Maybe someone on the forum knows the name offhand and will post their name.

[P-12 Tommy]

I've got the same problem. My speedo reads 15 mph slower then what I'm doing. I've got 15s on my P-12. Could that be causing the speedo difference from going from 16s to 15s?

[Norm's Coupe]

Changing wheel size will change the speedo adjustment. That's why that place I mentioned makes the external gears to adjust the speedo.

[P-12 Tommy]

Thanks, Norm. Now all ya gotta do is put the name of the place that sells the gear thing.

Tom

[Norm's Coupe]

Thanks, Norm. Now all ya gotta do is put the name of the place that sells the gear thing.

Tom

Unfortunately, that's the part I don't remember. Must be having one of those senior moments. About all I can remember is, "I think" they are in the eastern part of Michigan. Not sure about that either. I do believe they had a website though. Maybe run a search for something like "adjustable speedometer gears", and it might pop up.

[TodFitch]

It could be the difference in the speedometer drive gears in the transmission. The overdrive cars usually came with a 4.11:1 rearend ratio and the non-overdrive cars with a 3.9:1 rearend. The speedometer drive gears must be the correct ratio for your rearend ratio for the speedometer to be correct. If I am remembering correctly the speedometer cable should be turning 1,000 RPM at 60 MPH for the speed indicated to be correct. If I am wrong I am sure someone here on the forum will correct my statement. Sometimes old men like me have CRS.

Here is my CRS version:D

I think it is 1000 turns per mile irrevelent of speed but I could be wrong:o

You could be wrong. But usually you aren't... And you are correct in this case too.

1000 RPM at 60 MPH works out the same (at 60 MPH it takes 1 minute to go a mile, so that will be 1000 revolutions/mile * 1 mile/minute = 1000 revolutions/minute).

So you are both right.

I find 1000 turns/mile easier to remember than 1000 RPM at 60 MPH. And I need all the help I can get to remember things. Just ask my wife.

[Frank Elder]

http://speedometershop.com/ratio.html

These guys have ratio adapters.

[DonaldSmith]

When my speedometer was rebuilt, the speed was set to match the odometer reading, that 1000 revolutions per mile thing.

Checking with a Garmin GPS, I found that the speedometer was reading 10 percent high, no fault of the speedometer, but a function of rear end ratio, tires, etc.

I found a ratio adaptor on line, 10 percent under, from Performance Automotive and Transmission Center, for 62 bucks. Other typical ratios are available from various vendors. For unusual ratios, speedometer shops can provide ratio adaptors with various combinations of gears.

The speedometer cable on the klunk-o-matic was tight to the floor pan anyway, and right at the edge of the permanent floor. I tried various configurations of the ratio adaptor and speedo cable to find the best layout.

I wound up cutting the removable floor pan and access cover and cobbling a sheet metal closer over the area. But now the speedometer is right on.