Technical I'm having speedo reduction issues

By the way, with a little more information, there's a lot of talent on this board to figure this stuff out. Year and make of OEM car (for speedometer)? Transmission? Rear end gears? Tire size?

You shouldn't start a thread bashing a vendor unless you fully present your case to the audience, and RIPOFF is a pretty strong word to use.

 

I was reading through this thread and just wanted to add my thoughts to what readhead said - I have seen speedos that were NOT 1000 revs per mile. Just throwing that out there in case someone gets balled-up trying to make an odd speedo accurate.

Ratrod72 - I hope you get your speedo woes figured out. I know it makes me nuts when you're driving along trying to do speed conversions in your head !

 

Speedo-reduction - I thought this thread would go an ENTIRELY different direction. The HAMB has changed.

 

My Pops always told me don't step into the middle of a pissin match unless you don't mind getting wet, well here goes.
I guess to start off I don't have a dog in this fight but maybe I an p*** on some information on how I was schooled on building speedometer gear ratio adapters by my dad and our shops great employee Tom ( our shop was the only place Tom ever worked from the age of 17 in 1959 until he retired 45 years later). We never build a ratio adapter when given only a speed reading, such as my buddies car showed this and I was going that. Or my GPS said this and I was going that.
As people have posted most American speedometers based on 1000 cable revolutions is one mile or 60 mph.
so that is the baseline that needs to be established first. The only way to do that is to run the vehicle on a measured mile. That's the way the Stewart Warner book says to start in determining what ratio to build it to. If you are turning 12/10's in a mile it's 20% fast so you build a 20% reduction box. Calibration problem solved right? Maybe not.
The second way a speedometer is calibrated is by hairspring tension and magnetism in the speedometer its self. If the speedometer is worn, bad jewel in the magnet, or just a loss of magnetism it can read slow or fast by different amounts at different speeds ( not a consistent amount at all speeds). That's why you don't build a box by speed reading alone.
Its just asking for trouble and a unhappy customer, who might unintentionally bad mouth you for a job they say wasn't done right. This is especially true when it comes to mail order jobs.
That being said I will take a GPS reading along with a measured mile reading and compare the two. If all is right with the speedometer they should be off by close to the same percentage. If the speed reading is off a significant amount compared to the measured mile, well then we might have two problems but we have to start by getting the baseline correct (the measured mile) then go from there to see if the speedometer needs work also.
GPS has been a blessing and a curse in my business. I use a GPS while running cars on the measured mile to see if the percentages match up. That's a great tool, but I have also had go arounds with customers who complain that their speedometers are off by 1MPH after I have done a calibration. Some people just don't understand the disconnect between state of the art satellite technology and speedometers that were calibrated usin math, gears, cables, springs and magnets.
The truth here for the OP probably lies in the middle. I would be lying if I said that I never made a mistake building a gear ratio adapter, you get started then the phone rings or a customer come in, you go back to building the box and you read the wrong column or start on he wrong end and end up doubling the percentage. It happens. But his figures could have been off also.
Here's a pic of all of the possible gear ratio's you can build. What could possibly go wrong? By the way ratio's are built reading the columns from right to left and left to right(it's a lot of ratio's}

 

^^^^^^^^^Great Post!! Very informative. Thank you!

Ray