Speedometer gearing.

[Bryan98801]

First post so bear with me. Just bought a 76 scottsdale stepside. It has the original 350 but the P/O put in a muncie 4 speed (LO,2,3,4). Then he regeared the rear end to get better gas milage. I'm not sure what was going through his head when he did this. Anyways. In an effort to get it drivable for now I need to know how fast I'm going. He didn't bother to hook up the speedo after the switch. So what would I need to do to accomplish this?

Thanks.
Bryan

[Texasupton1]

I downloaded a speedo app on my phone. It was free. I just put the phone on the dash and compare its speed with what my factory dial says. At the 35-45mph range, I am off by 5 or so. At the 65-75 range I am off by 10. This will get you close.

[Bryan98801]

I've found GPS is not always accurate plus it is very slow updating most the time. I just need to know which little plastic gear to put in the transmission. If indeed it is the gear driven type in '76.

[ray_mcavoy]

Hi Bryan,

Here's a formula that I've used in the past:

Multiply the rear axle ratio by 20.2 and then divide the result by the tire diameter (in inches). That'll give you the drive-to-driven gear ratio that you need for the two speedometer gears in the transmission.

Next, pull the driven gear out of the transmission and take a look in through the hole to see if you can count the teeth on the drive gear that's on the transmission output shaft. Once you have the tooth count for that gear, multiply it by the ratio you calculated above. Round that off to get the tooth count necessary for the driven gear.

Note that the above calculations might give you a tooth count for the driven gear that is not available. That often happens when considerable changes have been made to the tire size and/or axle ratio. In that case, you'll have to change out the drive gear on the trans output shaft or add an external ratio adapter.

[76stepsidechevy]

hook the cable up and compare to a gps to see how far you're off. There are gear reduction/multiplication boxes that go inline with the speedo cable. Mine was off when I got it due to bigger tires and I was able to get a range box on ebay for 30 bucks and now its accurate. actually when I got the truck it already had a range box but it was stripped out and the speedo wasn't working at all.

[Bryan98801]

Ray-mind is completely blown now. haha.

76Stepside-do you have pictures of yours? I'm looking to go for more of a cruiser and less of a speed monster with mine. And now to convince myself to actually get a GPS.

[76stepsidechevy]

I can get some for you tomorrow. I'll look for a link so you can see what Im talking about.

[76stepsidechevy]

http://www.ebay.com/itm/17-RATIO-ADA...item3f11eabab3

heres what I used.

[ray_mcavoy]

Originally Posted by Bryan98801

Ray-mind is completely blown now. haha.

It actually sounds a lot more complicated than it really is. Here's an example that might help ...

Say your truck has 3.73 gears in the rearend and 28" diameter rear tires. Plugging that into the formula gives (3.73 x 20.2) / 28 = 2.691 as the drive/driven ratio of the gears you need in the trans.

Now, lets say you took a look inside the trans and found that the drive gear on the output shaft has 8 teeth. That gives 2.691 x 8 = 21.528 for the number of teeth you need on the driven gear. Of course, you'll have to round that off to either a 21 or 22 tooth gear.

[Bryan98801]

So that would go inline with the speedo cable yes? Trans side or gauge side? Course I would have to find the right percentage for my application.


Another question. Steering column. I need tilt on my wheel. I can barely squeeze under the steering wheel getting into the truck. What truck can I swap with to get that? Tilt with no automatic transmission shifter.

[76stepsidechevy]

trans side, it was an easy install. I think the trans needs to be pulled apart to change the speedo gears so I went with this option instead.

For the column, pretty much any other truck, blazer or suburban. Try to find one the same year or pretty close to yours so the wiring will just plug in. 2 wheel and 4wd have different steering shafts too so you might need to get one from whichever you have.

[Bryan98801]

Sorry Ray but Mr. 76stepsides process sounds a little easier. Now to borrow my dads GPS...

And mine is a 2wd. So I'm off to Seattle for a junkyard pull.

[hatzie]

I put together an Excel spreadsheet several years ago to do what-if scenarios. I compared several different tires, gears, and transmissions to what I presently had installed in my project.

It's setup to calculate RPM tables based on transmission, transfer case, and axle ratio.
About 1 page down on the bottom left it has a table of automatically calculated speedometer gears.
You can manually calculate ratio adapters if needed based on values from the speedo gear table.
You only need the tire size and rear end ratio to calculate speedometer gears. To properly populate the RPM tables will need the transmission gear ratios.
I attached it in a ZIP archive FWIW.