The Stovebolt Site Forums The Shop Area The HiPo Shop Tranny change/speedo help

Just purchased a 69 shortbed. Previous owner had dropped in a crate 350 and a new jegs auto tranny. The speedometer works but is not accurate. 85 on the speedo is actually 70 on the pavement.

I have done searches, but have not discovered any way to fix this. Is there an adapter or converter that will put my speedometer back to proper accuracy? I don't want to change the stock instruments unless I have to.

Thank you in advance for any help you can offer.

jim in WV

What kind of speedo is it? If it's mechanical, there may be tranny speedo drive gear adapters that can correct the reading. If it's electronic, you should be able to adjust the dip switches.

Are the tires factory sized?

Most of the late model GM auto trans. have a large driven gear for the speedo. You can jugel the gears according to number of teeth. Pull the gear out & the tooth count is usually marked on it, or count them. Get one that has about 2-3 more teeth. Check the odometer against the mile markers on the freeway. If you get the odometer correct the speed is usually right also.

Check tire size first. If the speedo is reading fast he proable has smaller diameter tires on it.Larry

"85 on the speedo is actually 70 on the pavement."
That should keep you from getting any speeding tickets.8~)
Cliff

Check with these guys - PATC.
They built me a ratio adapter for my 3sp OD in my 55 1st series.

They also seem to have a truckload of replacement speedo gears.

regards,
Leon

Thanks for the replies, the tires are very close to stock and the speedo drive is mechanical. I thought maybe the diff gears being different may have thrown things off. I haven't had a chance to get under the truck and check the drive gears yet, trying to get the wipers working so I can get her inspected and drive it every day.

Again, thank you for all the help.

jim in wv

Check the drive and driven gears in the transmission. Is it a TH400, TH350, TH700R4? You can find charts that tell you based on which drive gear you have in your transmission which driven gear you need, you can purchase the gear online and install it. Easy fix. If it's that far out you might need to change both gears, but that's still not too bad.

For years we fought this but now it's so easy it's almost a non-factor. GPS-GPS-GPS. It's dead accurate no matter the tire size or gear ratio. Right now you have too many teeth on the drive gear and/or too few on the driven. Stick the GPS in the truck and see what PERCENT the indicated speed is off from the real speed on the GPS. Google "speedometer drive and driven gears" and find what drive/driven combination will reduce the driven gear by that percent. The early small gears went by tooth count and the later big gears go by count and/or gear color.

Thanks again guys, I measured the distance between mile markers vs recorded milage calculated the percentage differance and bought an adapter from the company mentioned above. If that don't fix it I will look at gps speedometers. I'm not confident in my abilities to change out drive gears in the tranny output driver.

Thanks all,

jim

Be sure and let everybody know how it turns out.

regards,
Leon

Just a note, if you can't change speedometer gears I don't think you should be qualified to drive. It's extremely easy, anyone can do it.

Driven gears are easy, they screw in from the outside. Drive gears require dropping the trans and opening it up.