Speedometer accurate? No worries now...

In the newest Goodguys Gazette magazine has an ad for a product alot of people have discussed needing... a "speedometer ratio adapter". Is your speedometer accurate? IF NOT, check these guys out! All you have to do is tell them what your speedometer reads at an actual speed & they'll mail you an adapter to correct it... easy install too. Instead of quoting their ad, here's a link to the web page:
http://www.speedcorrect.com/

I'd seen these offered years ago, but they finally ran an ad & thought i'd share...
 
you got to think about the current woes in the car market. these people have been doing these for the auto industry for a long time. I have a few for the gm trucks. but haven't played with them on one of the ambulances. depending on there price the gear from GM may be a lot cheaper. and changing on on a t-400 is as easy as putting this adaptor on. I thinking all you need is a one tooth smaller to bump it up the 5 mph it's slow with the 235 x 15 on the car. if you had a different tranny to worry about one of these would save you a lot of tear down.
 
Mine is off readin about 6 miles faster than actual speed and will use my GPS on the trip to inform me how fast I'm going. Neat product though.
 
I found that mine is 2 MPH by using the GPS.. Close enough for government work. Change of tire size, inflation, and tire wear will all effect the speedometer reading. Most times, this will only be a 1/2% variation of the actual speed. Not enough to worry about. Most police departments will give you a 5 MPH variance before they will stop you for speeding. Only time that I have ever seen them hold it exactly to the signs, is in school zones.
 
gps mph speed

I found that my coach is about 3mpg slower than the actual reading on the factory speedometer, using my GPS. Not enough for me to worry about. When I got rid of the L78x15 tires and went to 235x75x15 tires I knew there would be some difference. Just not how much. The ride and handling are much improved with the radials. Now if the makers would just price wide white wall radials like they do the everyday tires I would be a happy camper.
When I was part of the working class we had large tubs of speedo adapters to get the read outs within 3mph up or down on the new trucks. I don't remember what the figures we used were now. That happens when you retire.
Mike
 
Well, for comparison, does anyone know a source for '72 Cadillac speedometer gears? I had a leak around my speedomoeter cable and had the mechanic put in the speedometer gear and cable from my '72 MM parts car. The speedo in the '72 Superior hightop had been about 4-6mph fast and the odometer about 7% fast!! The cable from the parts car sttopped the leak but now the speedometer is about 10mph fast and the odo 10-12% fast! I find this unacceptable and want to fix it. The gear from the original cable is not interchangeable. That cable had a cylinder near the transmission w/ a red wire leading from it that had been cut. I suspect this was some kind of speed recorder for a time/distance recorder that fleet owners put in their vehicles to monitor usage. It would be nice to compare prices for this adapter unit and a correct speedometer gear.
 
I can possibly get you a gear, but I would need a close up picture of the old gear assembly, along with the color of the plastic gear. The color of the plastic determines the number of teeth to the gear. If all else fails, you can take the speedometer head out and send it to a repair shop. They can alter the magnetic attraction of the head and calibrate it that way. The best way to do this is to bring the car to a true 60 MPH and note the speedometer reading. You can determine the true miles per hour by using a GPS unit.
 
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