speedo questions

[spanky55amg]

Ok, so the truck came with a 3 speed. I assume TH350 since its a C10 Silverado. It has a 700r4. The truck does have 22's on it. 285/45r22 I compared them with 235/75r15 and thats an 11% difference.
I had my wife follow me yesterday. At 55, I was doing 73.

Could/does the 700r4 have a different speedo gear that will throw my speed off by 20% more?

 

[yevgenievich]

700r4 might have came from a truck with a different gear ratio, so the spedo gear would be wrong for your application. Just need to get a correct one depending on your rear end ratio

 

[74 Shortbed]

These will help figure speedo gears.

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

http://www.5speeds.com/calculators/SpeedoCalc.html

 

[spanky55amg]

Thank you, thank you!

 

[spanky55amg]

Well... looks like I have some work to do. The speedo cable needs to be replaced anyways and I planned on putting 20's on that were closer to the stock size. So I need to make sure that the rear end hasnt been changed, then I guess I can check the gear colors, assuming thats an end all be all to the gears. This is a whole bunch of math I didnt want to get into. Ha.

And to add on, since this is all in the tail shaft housing, that needs to come off anyways since the tail shaft is leaking. I assumed it was the bushing and might need to go to the transmission shop anyways. Might as well, right, bushing, seal, and now to get the gears figured out.

 

[gpmorgan]

I put some 410 gears in my 92 2wd with a 700r4 and my speedo was 20 mph fast. Now I have a th400 that I need an adapter for because my speedo is elec and my trans is mechanical. I use my tach for a speedo now until I get what I need for it. I put the 308 gears back in and I have a 17 tooth drive gear and a 39 tooth driven gear that's suppose to correct my speedo. We'll see.

 

[spanky55amg]

I put some 410 gears in my 92 2wd with a 700r4 and my speedo was 20 mph fast. Now I have a th400 that I need an adapter for because my speedo is elec and my trans is mechanical. I use my tach for a speedo now until I get what I need for it. I put the 308 gears back in and I have a 17 tooth drive gear and a 39 tooth driven gear that's suppose to correct my speedo. We'll see.

And at least you know what you have done. The PO of the truck has done a lot of ***** work. And I dont think it was him, I think it was the PPO. (previous previous owner). The guy I bought it from only had the truck for 2 or 3 months. The nice thing about Texas and getting the title is you can see how long the PO had the truck titled. Now that being said, titled doesnt mean he didnt own it longer, but Im guessing with the issues the truck has, he just wanted to dump it. Thats fine... Ill fix it.

 

[gpmorgan]

Yeah, it may take some trial and error, but you (we'll ) get it. There is always some unknowns when making changes.

 

[HotRodPC]

I put some 410 gears in my 92 2wd with a 700r4 and my speedo was 20 mph fast. Now I have a th400 that I need an adapter for because my speedo is elec and my trans is mechanical. I use my tach for a speedo now until I get what I need for it. I put the 308 gears back in and I have a 17 tooth drive gear and a 39 tooth driven gear that's suppose to correct my speedo. We'll see.

So are you using some kind of adapter to convert from the manual speedo to the electronic speedo? The electonic speedo should have a DRAC that you can change the ratio with and correct your speedo. To the best of my knowledge it's all about changing jumper pins on the DRAC chip to change the ratio. Some guys have even modified and made their own DRACs using Dip Switches to make it easier to make changes for quick gear and tire swaps all while keeping the speedo and odo correct.

 

[gpmorgan]

My factory in dash speedo needs an elec signal or ecu, one of the two. Was looking at a few units that go on the trans that work off of the cable hookup and convert it to a elec signal. Not sure which one I need for my 92. There are a couple of different type signals they send. Got to do some more research and come up with something. Do you know what type signal a 92 700r vss has? I looked up a reluctor gear and vss for a th400 but the company didn't have the vss. I would rather go that route. Do you have any info on that setup?

 

[spanky55amg]

Ok. Question. It looks like this tail shaft is going to have to get resealed sooner than later. So I might as well fix the speedo.

Is the gears for the speedo only dependent on final gear ratio?
Does he cluster not play any part in it?

 

[HotRodPC]

The cluster DOES NOT play a part. The cluster will be a standard ratio of so many rpms per mile and it does all that math by itself so no need to worry about that. Just let that do it's own thing and you don't worry about that. Just an FYI, IIRC, the ratio of your speedo is 1000 rpm per mile but again, no need to be concerned with that.

So what does matter is, your rear axle ring and pinion ratio and your tire height. As a rule of thumb, each tooth you add or subtract is 5%. So if you're speedo is 20% off, then you'd need to add/subtract 4 teeth (4 teeth x 5% = 20%).

Without me having to go back and re read all that, what transmission are you working with? What is your gear ratio? What is your tire size?

 

[4WDKC]

I put some 410 gears in my 92 2wd with a 700r4 and my speedo was 20 mph fast. Now I have a th400 that I need an adapter for because my speedo is elec and my trans is mechanical. I use my tach for a speedo now until I get what I need for it. I put the 308 gears back in and I have a 17 tooth drive gear and a 39 tooth driven gear that's suppose to correct my speedo. We'll see.

Ok. Question. It looks like this tail shaft is going to have to get resealed sooner than later. So I might as well fix the speedo.

Is the gears for the speedo only dependent on final gear ratio?
Does he cluster not play any part in it?

and tire size... my truck with the stock 342s and stock tires is very close to 410s with 35s

 

[spanky55amg]

Well I plan on going with a 20" wheel but with the tire combo, it will be within 1.0% of factory height (i think its like 0.7%). The tires on there now should be about 11.0% bigger than factory size, but they are coming off soon!
The transmission is a 700r4. I dont know the gear ratio, even if the RPO sticker showed it, I wouldnt be sure if someone swapped them out. So Im going to do the whole tire spin and driveshaft rotation check. But if thats all I needed to check, I should be able to figure out what speedo gear set I need.

(I read on another site that someone was bring the cluster into conversation about 700r4 speedo gears, so thats why I asked.)

 

[HotRodPC]

Wheel size won't matter a thing, it's the tire size so tire height is known that will matter. Only thing a cluster will matter for is it you have a total analog cluster with just a speedo cable, an analog cluster with a VSS signal converter for Cruise Control and ECM for TBI trucks, or Full Electronic Speedo used in later TBI trucks that people are looking for to use with LS Swaps where a VSS on the transmission is used.