Speedo is off, how do I correct it?

So I have a 63 500 that had a motor and trans swap. I now have a C4 and would ***ume that the speedo cable connecting to my speedometer still has the original speedo gear that they just installed in the C4. My question is how do I fix this? Is it safe to say my ***umption is correct about just hooking up the stock cable to the updated C4? Does that even matter?

Thanks,
George

 

in short, the calibration is in the speedometer driven gear in the trans.

there are online calculators. you enter the trans type, the rear end gears and the tire size and it tells you which gear to install.

sorry i dont have the website, its in my comp at the shop. do a google search.

 

Real Men know that the only way to correct your Speedos is a pair of Board Shorts...

 

Have you actually hooked it up and driven it. You might try that and check it (various ways: drive in front of a friend in in a new car to a predetermined speed and compare, time yourself over a measured mile at 60, etc.). If its off, take it to a speedometer shop and have them correct it.

 

To know exactly what your driven speed is use a GPS, then do what Altered Pilot suggested.

 

You might try this place: www.transmissioncenter.net/.
Tel 1-888-877-1008

I put a SBC/TH350 in my 1948 Plymouth. When the GPS said I was going about 60 MPH, the Plymouth speedometer indicated 100 MPH. I contacted this place, and spoke with "Kerry". He figured out the percentage of error, and sent a ratio adapter to correct it. Nice little gearbox arrangement that installs between the transmission and the speedometer cable. It cost $70, and took about 5 minutes to install on the car. My speedometer is now reading right where it should.
I have no affiliation with this company, but just wanted to p*** on a possible solution. It worked out well for me.

 

X's 2

 

The way the auto dealers used to fix them was to dissemble the head and carefully turn the little wheels to the desired milage, making sure that you dont leave finger prints on the face of the dial.

(at least that what I heard from the older guys (;-) )

 

When I had the 4x4 shop we did a lot of gear swamps and we had the kit to make correction box's. Very quick and accurate solution. Ask around different shops or contact someone like in the previous post. You need to know the percentage of error. Check at several speeds and take an average.

 

start here

 

+1 We did the have a friend pace you thing 40 years ago but almost everyone has access to a gps these days in one form or another including on your phone.

Drive down the road at a set speed on the speedometer and check the speed on the gps.

Figure the percentage of difference and which way and then you can figure out what you need to change.

Here is a link to a good site for figuring things out on speedometer drive gears. http://www.transmissioncenter.net/speedometer_calibration_______va.htm

Big truck (semi) and a lot of 4x4 truck shops have the correction gear boxes that you can change gears in and put between the drive gear on the trans and the speedometer cable to correct the calibration. They aren't all that cheap but you can then go back in and swap gears in them again if you change gear ratios or rear tire size. You would just have to tell them what percentage of change you need in which direction.

 

For the do-it-yourself type...

Measure how far the odometer is off in a 10 mile stretch by the interstate mile posts. Reading the odometer there are 100 tenths in ten miles. EX: 9/10ths gained would be 9% too fast. The driven speedometer gear on the transmission end of the speedo cable would need to be changed. To slow the speedo down, you would increase the number of teeth on the driven gear. To make the speedo read faster, decrease the number of teeth on the driven gear.

Find out how many teeth are on your existing driven gear by removing the cable and speedo gear from the transmission. Make a trip to the dealer or your local pull-a-part and try to find a driven gear with the number of teeth you need.

Note: on some types of transmissions, be sure to get the little housing that holds the driven gear as that housing is how the gear is located in relation to the drive gear on the main shaft of the transmission.

 

Awesome thanks guys! I went on a cruise tonight with the wife and we drive past one o those automatic speed limit signs. I compaired what it said to my speed and it was ten miles faster. I'm going to do some of the things you guys recommended tomorrow on my way to work and see if it is accurate. I'll let you know!

Thanks,
George

 

Ten mph faster, at what speed?

Also have you checked the odometer to see how far off it is? someone provided good instructions for that...I always start with fixing the odometer reading first by playing with the speedo gears in the transmission, then make sure the speedo is calibrated to read correctly.

 

It was off at 38 mph, I'm going to do that odometer test tomorrow and see what happens. I'll be on the freeway for a few miles to I'll be able to check out the speed as well. I just downloaded at speedometer app so we will give it a try.

 

10 mph off at 38 mph is a heck of a lot of error...

 

If you need a gear ratio adapter we can make one up for you. Just let me know what your odometer reading is.

 

Most, if not all, new (ish) vehicles have varying amounts of 'speedo reading faster than actual' built in from the factory these days, which makes it nearly impossible to calculate the 'follow a mate' in a new car scenario..... I've done the drive cog in the trans calculation before with good results. Of course GPS will give you 'actual speed' which will be very handy....gotta love them techological gizmo's.

 

really? I've noticed they are really accurate on newer cars. Especially compared to the old stuff.

 

Well, I did hear that from a 'reliable' source but had noticed it myself when driving any one of the many Hyundai Getz's in our work fleet past the many digital 'This is your speed' gizmo's that our Police department likes to ****ter randomly around the countryside. Always seem to read 5km slower than my car is reading.. Kinda induces me to think 'ok, I can do 55km and still have 5 up my sleeve' (around town) Haven't had a speeding ticket in more than a decade, so .......could be dumb luck, or......

On that note I believe the larger sportbike type machines are very accurate.

 

Perhaps a hundu needs a little boost? I've only checked some chevys and a ford and a mopar...

 

Sounds good, I'll send you a PM once I do the other tests to be more accurate. Thanks!

 

Haha, gimme those Chev's, Ford's, and Mopar's any day......all we get for pool vehicles in this neck of the woods is coke cans on wheels with funny names like Bongo...Friendee....Vitz ....Spacio......Homy.....ad nauseum.

 

Ok so I tested the speed again on the freeway. Here's the breakdown, at 80mph on my speedometer I was actually doing 64 which is a difference of 16mph. So I test it again at 70mph on my speedometer and I was actually doing 56 which is a difference of 14mph.
I'm going to test the odometer today on the way home.

 

From your preliminary info, it's off by 20%. If you want to play with it you could pull the speedo cable out of the transmission, and see how many teeth are on the gear. If there's a gear available with 20% more teeth than the one you have, you could see if that fixes it. for example if yours has 15 teeth, you'd need one with .20 x 15 = 3 more teeth, or 18 teeth total. Sometimes the gear you need is not available and you have to use a correction box. gears are a lot less expensive

sometimes the speedo needle is in the wrong place, so the speedo is always fast by some number of mph....the odometer check will help you figure that out.

 

Do you have the old trans or can get at it? Take the speedo drive gear out and see if it will fit the new trans. If it does it should make the speedo as accurate as it was before.

 

Take it to your local transmission shop they will take care of it.

 

More information, I took a 10.1 mile drive to work using my navigation and my odometer read 13.7 miles.

 

Wow, that's off by quite a bit. 36%? But I probably would want to try the drive on a straight highway with mile markers...not that I don't trust "navigation" or anything like that

 

You can also try Rowland Speedometer Co. at (727) 410-6352. They make an adapter based on your input.