speedometer cable synch

[garyt]

i have a 51 chevy truck with an original speedometer, with a 700r tranny, 350 engine, is there a way to get an accurate mile per hour reading ?

[OrrieG]

Change the gear in the transmission, easy to do without pulling it. some newer autos have a reduction box on the tranny that does it, older one have changable gears inside the tranny housing. I did not take the time to google 700r transmission speedo gear change, so if the newer transmissions are electronically calibrated the old school method below may not do you any good.

Get on the freeway and drive five miles using the mile markers or gps at 60 miles per hour (one mile per minute if you speedo is accurate) and time how long it takes in seconds. If you speedo is right on it will take 300 seconds (5 x 60). Take the number you get and divide by 300 to get a percentage you are over or under. Find the reduction gear and count the number of teeth. Multiply that number by the percentage you calculated above. That will tell you how many teeth your new gear needs to be. If you are lucky it will be pretty close to an even available number. If not you will need to calibrate your brain for the difference. Your local gm dealer will have the gears, they are color coded.

[1project2many]

You may also be required to change or adjust the sleeve which holds the driven gear, and maybe the drive gear as well. It's fairly easy to do and if the GM dealer doesn't have all the parts stop in at a local trans rebuilder as they usually have stuff in boxes.

If you have rear gear ratio and rolling circumference it's not too tough to figure out the gears from one of the gear charts online.

[mr48chev]

As Orrie said you can change the gears to correct the reading. The other way is to use a little gear box known as a ratio adapter that may also be known by other names. They are quite popular with the 4x4 guys who switch tire sizes. They are also used on Semi trucks to correct speedometers and odometers.

I've used this site quite a bit in the past to figure out what gears I need to correct a speedometer. http://www.transmissioncenter.net/sp...n_______va.htm
You should be able to buy or order the gears at a local GM dealer or transmission shop that has a good parts room.

If you have a hand held GPS go out and take a run down a straight stretch of road at a set speed say 50 or 60 mph. Then figure the difference between the GPS actual speed and what reads on the speedometer. That way you can figure the percentage it is off and what change needs to be made to correct it.

Example if you go down the road at 60 on the GPS and the speedometer reads 72 the speedometer would be 20% fast.

[1project2many]

You don't need a GPS if you can travel a marked mile at one speed. Use a stopwatch (many cell phones have one built in). If you can travel between 50 and 70 mph it's very easy. If you go one mile in one minute, that's 60 mph. 70 mph is about 50 seconds and 50 mph is about 1 minute 10 seconds. It's almost 1 to 1 with the number of seconds in that range. I used this method long before GPS was available and it's how I checked the speedometer in my '36 Plymouth when I put it on the road last month.

[garyt]

thanks guys, i know what to do now because of your help. love this forum the advice is priceless, thanks again gary

[mr48chev]

Originally Posted by 1project2many

You don't need a GPS if you can travel a marked mile at one speed. Use a stopwatch (many cell phones have one built in). If you can travel between 50 and 70 mph it's very easy. If you go one mile in one minute, that's 60 mph. 70 mph is about 50 seconds and 50 mph is about 1 minute 10 seconds. It's almost 1 to 1 with the number of seconds in that range. I used this method long before GPS was available and it's how I checked the speedometer in my '36 Plymouth when I put it on the road last month.

So did I but the GPS is so much more accurate and a lot simpler. We still have the stretch of straight road a few miles from the house with 5 miles of highway with accurately marked mile posts that is designed for the purpose by the state and has been there since I was in high school but trying to drive at exactly 60 mph for one to five miles with a bunch of just crawled under the fence last month farm workers driving the same road is more trouble than it's worth. With the GPS I've checked several spedometers in all of 15 seconds and didn't have to watch for other traffic while watching the stop watch or digital watch and watching the speedometer. Some times you just need to go with techknology and let it happen.