Speedo gear with TH 350 help

[62lwbc10]

Hey guys I just had my late 70's TH 350 completely rebuilt by an old school builder and it is performing unbelievable well. I have a brand new speedo cable and brand new speedo itself/ The guy did the "best he could" with getting the right speedo gear, but Im still reading a good 20 MPH over my actual speed. I have a SBC Chevy (305) for a motor. Can someone who is running a similar setup please let me know what type of speedo gear you are using? Number of teeth? COlor? Any info would be most helpful. Im sure I have a stock rear as, with the TH 350, the motor is revving a high RPM on the highway at 60 MPH (when the speedo tells me Im doing 80!).

Thanks all,
Mark

[buggy5872]

Mark,
Check out this link http://www.transmissioncenter.org/GM...eter_Gears.htm
it list out the speedo gears and colors. Also look at the chart to determine what you need. You will have to find out your rear ratio and what gears are currently in the tranny.

[parkwood]

Originally Posted by buggy5872

Mark,
Check out this link http://www.transmissioncenter.org/GM...eter_Gears.htm
it list out the speedo gears and colors. Also look at the chart to determine what you need. You will have to find out your rear ratio and what gears are currently in the tranny.

nice link buggy....

[mr48chev]

That's one of the best links to use to figure out what you need. You can usually get the right gears at a local GM dealer or a lot of local transmission shops.

If you have access to a handheld GPS you can get an accurate reading of your actual speed at certain speeds and then figure out the % of difference and then figure out what changes you need to make on the speedo gears.

[62lwbc10]

Thanks All, I had a look at the link and its very helpful. Im wondering though what needs to be changed when the speedo is reading too fast? The drive gear or the driven gear, or both? WHen the speedo is reading fast, do you reduce the number of teeth on the drive and driven gears? What is the rule of thumb (if there is one)...?

THanks,
mark

[sqrlnts]

More speedometer gear information here.

[mr48chev]

Originally Posted by 62lwbc10

Thanks All, I had a look at the link and its very helpful. Im wondering though what needs to be changed when the speedo is reading too fast? The drive gear or the driven gear, or both? WHen the speedo is reading fast, do you reduce the number of teeth on the drive and driven gears? What is the rule of thumb (if there is one)...?

THanks,
mark

Reading too fast will take more teeth on the driven gear to correct.
It takes a bit of studying but the drive gears and driven gears are somewhat matched. If you go down the list each drive gear will match up to a few driven gears that are usually within a number of tooth range. Usually you can tell what drive gear you have by it's color.

Actually the two links that Buggy5872 and Sqrints gave go to the same website. That is the one I always recommend as it is easy to figure out and it is pretty well the go to speedometer gear site on the net.

HERE: Try this link, http://www.tciauto.com/tc/speedometer-gear-calculator/ Scroll down a bit. I think it is a lot easier to understand and figure out for Chevy Turbo 350 or Turbo 400 speedometer gear sets.