Speedometer not accurate

[cppursell]

Hi,

I have a 1967 short bed stepside Chevy pickup that I've been fixing up with my cousin. It's great, but I'd like to have the speedometer to be accurate. Right now, it's about 20% off (when I'm driving 50 MPH is says I'm going 60). The cause of this is almost certainly due to the rear end gears on the truck being 3.54's, when the truck came originally with 4.11's (according to the glove box).

My question to you guys is this: what steps do I need to take to make the speedometer accurate again? Change gears of some sort, or put on a third party adaptor of some kind?

Thanks for your help! Here's a picture:

[Richard8971]

Sweeeeeeet. I have to respect another member who has a '67 stepper! Welcome to the site from sunny AZ!!!

Honestly, I am not sure how to help you, my speedo isn't even hooked up!

I will try to keep this up at the top and somone is sure to chime in with the answer!
Don

[68gmsee]

Welcome.... Love those 67-68 stepsides. I sho would like to find one in good condition here in Texas.

Re: speed off. My 68 was off also and I went to the dealer and bought a driven gear (didn't have to change the drive gear). I used one with less teeth than what I had to increase the cable rpms. Luckily I didn't have to do more than that.

Do a search above and you should find several threads on it. I used "driven gear" but you can play with it to see what comes up. Here's a couple of sites.

http://www.transmissioncenter.net/sp...n_______va.htm

http://www.bgsoflex.com/speedo1.html

[Woody]

I found that 1 tooth on the gear that is on the end of the cable is worth about 3 mph. So if your speedometer is off by 10 MPH than you would need a gear with 3 more teeth, slowing down the cable speed. That should bring it close to what it should be. Try a old tranny shop they probably have them laying around and wont charge you much if anything for one.

[cppursell]

Originally Posted by 68gmsee

Do a search above and you should find several threads on it. I used "driven gear" but you can play with it to see what comes up. Here's a couple of sites.

http://www.transmissioncenter.net/sp...n_______va.htm

Thanks for that link. On the page there are these "GM / Dodge / Ford Ratio Adapters" that would seem easiest to install. As a matter of fact, looking under the truck to where the speedometer cable goes in, there is one of these installed already, although obviously it's not enough. If this is the way to go, then now I just have to find out what percentage the speedometer is off then buy the appropriate adaptor. Sound good to you?

Thanks for all your replies!

[68gmsee]

I don't know if you scrolled down towards the bottom for the gears that you need but if you notice they are color coded as to the amount of teeth on them. Usually you can go one number up or down without having to change the internal drive gear. IIRC mine was about 15% off-give or take.

I just happened to luck out and didn't have to do all the calculations. It's stlll not exact, but 2-3 mph off is okay for me. To start, take your gear out and count the teeth on it. Then see if you can locate a driven gear with less teeth. If you really want to be exact, do the calculations on the other site or maybe take it to a shop that does calibrations.

forgot to mention: the 350 automatic gears is what I used. Should be the same as yours.

[tim21391]

i have stock gears on my 72.. but dad and i put some beefier tires on mine..that are an inch taller and it threw it off 5 mph..could it be that to for this guy?

[d10s69c10]

yes you need to replace the gear inside your tranny with a larger (more teeth) gear.

other options would be smaller diameter tires. more than likely not a wise decision

another option that would be the simplest is to invest $150 into a garmin GPS. it will give you a pretty accurate speed. and has countless other little benifets.

[cppursell]

Actually, I do have a Garmin GPS already, and am using it currently for getting more accurate speed . Okay, so I either need to open up my transmission and change the gears, which I don't know how to do, or I can just buy an adapter that I screw right in and it accomplishes the same thing... I think I choose buying an adapter! Now I need to save up some cash.

Thanks for all your help! Now I'm on to fixing the backup lights, and getting the dash lights to work!

[msgross]

There should be a ratio on that box... I suggest that you take it off and see what happens, then order the correct box or gear...

[theyammieguy]

[QUOTE=68gmsee;3004162]Welcome.... Love those 67-68 stepsides. I sho would like to find one in good condition here in Texas.



About a month ago there was a sweet 67 stepside on Dallas craigslist.