Speedometer reads 21% high

[48ply1stcar]

I have searched for speedometer issues but I can't seem to find the fix for this problem. After I stop the bouncing needle, thanks to many members and a fine documentary from a member.

I took my nephew for a ride recently, he held the GPS and told me what speed I was traveling at and I compared that to the speedometers reading. At 30mph the speedometer read 38 and at 55mph the speedometer read 70. That's a 21% difference at both speeds. Has anyone had this issue? And how did you solve it.

Thanks Harold

My 48 is my first car.

[TodFitch]

Sounds like the speedometer gears in the transmission are not matched to the differential ratio and/or tire size. Maybe someone put a rear end with taller gears for better high speed operation.

You can check that by seeing if the odometer calibration is also 21% off. If the odometer is off by the same amount as the speedometer, then I'd look to a speed shop to see about getting an adapter for the speedometer cable that will increase the turns/mile one the cable.

[William Davey]

To follow up on the last post. You need to carefully record your odometer reading, then use the mile markers on a local interstate to travel exactly 10 miles. If the odometer says you've gone 12.1 miles, it's your spedometer drive gear in the transmission.

However if the odometer reads 10 miles, then your speedometer head needs calibration. Any number of places do this - I had real good results with "Bob's Speedometer Shop" advertized in Hemmings.

Hope this helps.

[TodFitch]

To follow up on the last post. You need to carefully record your odometer reading, then use the mile markers on a local interstate to travel exactly 10 miles. If the odometer says you've gone 12.1 miles, it's your spedometer drive gear in the transmission.

However if the odometer reads 10 miles, then your speedometer head needs calibration. Any number of places do this - I had real good results with "Bob's Speedometer Shop" advertized in Hemmings.

Hope this helps.

He could also use that same GPS unit to do the 10 mile distance check... (California has a lousy mile post marking system. If you have great eyes and know what you are looking for you can used the posts that CalTrans puts up for their own use. But otherwise using something else is a lot easier here.)

[greg g]

why not use the same GPS to record the 10 miles???? Mine has a count down to destination or a count up of miles traveled. Seems to be a lot easier than tracking those oft incorrectly plased mile markers.

[William Davey]

I know some GPS devices like my Delorme (with software loaded on the laptop) will calculate actual road miles. My understanding is that some hand held GPS only calculate "crow flight" distance and might not account for the actual road miles traveled.

Maybe we're all overthinking this?

[Mopar-Boy]

I know this is a VERY old post but was it resolved?  Can the speedo gears in the transmission be changed?  I have a three speed manual (w freewheeling...) and my speedo is off almost exactly the same as this one.

[Plymouthy Adams]

one of the better solutions would likely be to call these folks...I got a corrector built (ratio adapter) for my bastard set up of MG rear axle, larger wheels, Datsun 5 speed and the Morris 1504 Rev Per Mile speedometer...she is dead on the money at all speeds...(per GPS) this one also needed custom cables for the different systems in play...excellent service by John.  

 

Speedometer Cables USA - Gear Box Adapters

 

Edited January 19 by Plymouthy Adams

[Sniper]

25 minutes ago, Mopar-Boy said:

I know this is a VERY old post but was it resolved?  Can the speedo gears in the transmission be changed?  I have a three speed manual (w freewheeling...) and my speedo is off almost exactly the same as this one.

 

Yes, however, good luck find the gears these days.  I don;t have a parts manual for that far back so I can;t help there but I suspect Plymouthy's answer is the one that will work best.

[FarmerJon]

58 minutes ago, Mopar-Boy said:

I know this is a VERY old post but was it resolved?  Can the speedo gears in the transmission be changed?  I have a three speed manual (w freewheeling...) and my speedo is off almost exactly the same as this one.

Paging @Semmerling

On the later transmissions the gears can be changed, for the '40-56 transmissions they run around $50. Maybe you can determine if they interchange?

 

[Doug&Deb]

Tire size makes a difference also. The original bias tires were tall and skinny. You can’t mount a large enough radial tire on the original wheel to match the height. 

[Plymouthy Adams]

set your vehicle up the manner you want it...tires, rear ratio..different tansmission, different speedometer head makes no never mind.....check the real (GPS)  against the indicated on the speedometer....tell the man just those two and your worries are over.....its just that simple....

[Ivan_B]

Just a thought: I am not sure how accurate the average GPS speed reading is. I suspect that one of those "Your Speed is" road signs is a better tester

Or, if you see the highway patrol, just ask them to clock you going buy. These guys keep their radars well calibrated, for legal reasons.

Edited January 22 by Ivan_B

[Plymouthy Adams]

I believe the current car industry for mods and rods have way more GPS speedometers offered across the counter than anything based on radar...if this offers any peace of mind.....I have compared my speedometer against the GPS and the random roadside radar.....clean sweep.....check check check

Edited January 22 by Plymouthy Adams

[Sniper]

37 minutes ago, Ivan_B said:

Just a thought: I am not sure how accurate the average GPS speed reading is. I suspect that one of those "Your Speed is" road signs is a better tester

Or, if you see the highway patrol, just ask them to clock you going buy. These guys keep their radars well calibrated, for legal reasons.

 

Just compare the GPS reading to a known good speedo reading, such as your daily driver.  I know my work truck reads 1 mph fast, probably legal reasons, lol.

[Ivan_B]

That's the reason I mentioned this. My phone is showing something like ~5 miles off, from the modern car speedometer (stock wheels/tires an everything).

[rallyace]

A GPS system breaks down your travel into a series of vectors. It will not recognize most changes in elevation. The calculation is based on a jagged line in one plane and will not be accurate. On the straight and level the GPS speedometer is quite accurate as is the distance but add in curves and elevation changes and the accuracy diminishes quickly.