Speedo question?

[john02fxdl]

OK, I need some input. I just got my 70 stepper on the road and my speedometer is WAY off. I'm running a 350, th350 trans, 3.08 gear in the rear and 235/75/15 all the way around(approx. 29" tall tire). While I'm driving the truck, I think I'm doing around 30-35 mph but the speedo shows 100mph. What gives? I looked through the forums and guys have the problem with a 5mph off scenario. I'm off by like 70mph!!! Could my drive and driven gears be that far off? The trans was rebuilt and came out of an early to mid 80s chevy something. I bought the trans done already. It shifts great and the truck drives great, but I am stumped on this speedo. I looked at the sites on how to chose the drive and driven gear setup but I was wondering if anyone who is running the same combo I am could remember what color their drive and driven gears are in the trans. Any help????

[Longhorn Man]

ummm.... wow.
Bumpin this back to the top.
never heard of that kind of issue.

[68gmsee]

Originally Posted by john02fxdl

......... I'm running a 350, th350 trans, 3.08 gear in the rear and 235/75/15 all the way around(approx. 29" tall tire).

Same setup I have (had) in my 69 but it was only a few mph off. The speedometer needle is moved by eddy currents produced when the cable turns a magnet. You could have a speedometer problem and not necessarily gears. I'd check that first.

[mr48chev]

Probably the easiest way to check exactly how fast the truck is going is to use a gps. I use my Garmin 72 handheld as a speedometer on the highway at the moment. If you don't have one you most likely have a friend or relative who does. It's a whole lot easier than timing mile posts on the highway with a wristwatch at 60 mph.

There are several sites that give speed,gear ratio and tire calibration forms. I lost all of my links when my old laptop crashed again.

This site is one of the best I have seen for helping you figure out what you need. http://www.transmissioncenter.net/sp...n_______va.htm

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

They do sell the proper pieces but you can get them at a local transmission shop that has a good parts room or you should be able to get them at the dealer's parts counter once you know what you need.

You will have to pull the driveshaft and the tailshaft housing to swap out the drive gear but that is a simple process that doesn't take long if you have the right pieces to put back in.

[mr48chev]

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

TCI has a good site with lots of info
http://www.tciauto.com/Products/Tech...eedo_gears.asp

http://usa.vdo.com/NR/rdonlyres/952A...otingGuide.pdf

that should be enough info to help you get it squared away or thoroughly confuse you. I'm liking the tci site above better all the time.

[mr48chev]

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

TCI has a good site with lots of info
http://www.tciauto.com/Products/Tech...eedo_gears.asp

http://usa.vdo.com/NR/rdonlyres/952A...otingGuide.pdf

that should be enough info to help you get it squared away or thoroughly confuse you. I'm liking the tci site above better all the time.

Don't forget to check out the offerings from the GM performance dealers like Gilbert Chevrolet in Milton Freewater, Oregon. So far off the beaten path that they don't even have a path. http://www.sallee-chevrolet.com/BorgWarner/bwt56.cfm

[68gmsee]

I still say you should check the speedometer first otherwise you may be chasing ghosts.

If you can find an exact mile marker, look at the odometer to see if actually measures a mile. If it's off as much as you say it should show you traveling 7 miles instead of one.

In this case you would have to look at the gears. But, if it's closer to the mile than you are looking at the speedometer problem I mentioned.

Let us know what you find for future reference.

[ERASER5]

I vote a bad speedo. My F**d truck wacks out when it is cold. Speedo says I'm doing 80+ in 2nd gear, pegs by 4th, drops back after it gets warmer in the cab. I just deal with it, after all, it is a Furd.

[Stock72c10]

Whoa!!!! First things first, cruise down the road going 35 and have a friend take a picture of the speedo pegged as you cruise past a cop for bragging rights (or take it yourself if you have good life insurance). Now you can go about fixing it.

[mr48chev]

I'd agree with seeing what the odometer measures in a set distance. That is probably the best indicator of how far off the drive/driven gears are in the trans. If it is as far off as you say the odometer should read almost three miles for each mile driven.

You do have the stock speedometer and not an electric one from the aftermarket?

[john02fxdl]

Thanks for the input guys, I'm now thinking it's a bad speedo from what some of you have mentioned. I bought the cluster off of ebay because it had white faced gauges and that was the look I was going for. All stock gauges, just white overlays. I still have my stock, non gauge cluster so when I get a chance, I'll drop the speedo cable and hook it up to my old speedo and drive it. If it shows pretty close to actual speed, then I know the speedo in my dash(white faced) is the culprit. I'll keep you all posted. Here's a pic of the cluster.

[john02fxdl]

OK, hooked up my old speedometer to my speedo cable and drove around town to check my speed. It's reading the same as my new one in the dash, so now I know its in the trans, not the speedo. I knew my old speedo worked before I resto'd the truck, so now on to my question.

What color drive gear and driven gear are you guys running for this combo:

235/75/15 ( approx. 29" tall tire)
TH-350 trans
3.08 gear in rear

I could use the formula some of you guys use, but it would require me to measure the speedometer at 60mph and count milemarkers BUT if I do that I will be doing about 20mph on the highway, asking to get rear ended.

[turp mcspray]

If it's that far off you will deffinitly need th change the drive gear as well as the driven gear. If I can find it, I have an old super chevy mag with all of both gears and combos listed. I will look tonight.
FYI... when fine tuning a th350 or anything else with the small driven gear, changing the plastic gear by 1 tooth, is worth 1/2 of a tenth on the odometer.

[turp mcspray]

Have you had any luck with your speedo issue? As of yet I haven't been able to locate that magazine.

[chevy_mike]

Did you change the rear end ratio. If you had 4.11 gear before and swapping an axle with 3.08, you have to change the gear on the speedo drive in the tranny, otherwise your speedo will be off, even alot.

If you did not change anything and it was working correctly before you resto'ed, then I don't have an idea.