Lifted '84 K10 speedometer

[Lewis]

On my '84 K10 that I just bought, I am having the 700R4 rebuilt and I have a question for you guru's. The Truck is running the original 203 transfer case with a stock 3.08 ratio rear BUT, the PO lifted it and added 35 x 12.50 R20 tires and wheels. I assume this makes the speedometer read incorrectly. I understand the speedometer is actually driven from the transfer case and not the transmission so do I change something in the transfer case and not in the transmission as I have done in 2wd vehicles? If so what do I change and where do I get the parts? Your advice is greatly appreciated!
Lew Shaw

 

[Joshua Keith]

There’s a Speedo gear in the transfer case that needs to be changed. The speedo gear is what your speedometer cable plugs into in your 203 transfer case. To figure out what Speedo gear you need, first you must see what drive gear you have on your 203 output shaft. You can then use (the drive gear teeth amount, your rear diff ratio, and your tire height) to figure out the Speedo gear needed to make an accurate speedometer.

Plug in your numbers here and use this calculator page to figure that out.

Speedometer Gear Calculator

Speedometer Gear Calculator. Fill in the blanks to find your needed gear! Need assistance? Call Jim at 1-888-877-1008! Available Mon-Fri, 8am-5pm Central

transmissioncenter.net

 

[Lewis]

Thanks Joshua! I have the 203 transfer case and not a 208 Does that make a difference? Sorry, I read your comments on my phone and misread the number. I see now that it is related to the 203. Your advice is greatly appreciated Where should I go to purchase the proper gears?

MY BAD, IT IS A 208 C !

 

[Joshua Keith]

I believe this works for the 203 and 208. I believe my 208 had the gray 15 gear so used that in my calculation. You may have to take off the tail housing to see what gear you have. Oh it doesn’t matter. eBay, Amazon…they’re pretty inexpensive.

 

[Joshua Keith]

Also one more thing. On my 208 there were multiple different Speedo gear housings. One for teeth with a range of like 20-25, 26-30, 31-35 and so on. You must have the correct housing for the correct gear.

 

[Turbo4whl]

There aren't as many gear ratios for transfer cases as there are for automatics. Many would have a ratio adapter to get the speedo correct.

You must be registered for see images attach

 

[bucket]

I want to mention a few things just so you are aware. If you do in fact have a 203, it is not original. A 208 would have been original. It's real easy to tell the difference, one is cast iron and the other is aluminum. If you don't want to cook your freshly rebuilt transmission, install a big auxiliary fluid cooler and a transmission temperature gauge too. It's real easy to overheat a 700r4 with large tires and tall gearing. The cooler will help keep the fluid temperature down and the gauge will tell you when you need to get your foot off the throttle and coast for a bit. Lastly, don't plan on using overdrive AT ALL with 35's and 3.08's, unless you are going downhill and have a tailwind.

35's, 3.08's and overdrive is an absolutely miserable combination for everyone and everything involved.

 

[Joshua Keith]

Here’s a video that’ll help ya.

xc_hide_links_from_guests_guests_error_hide_media

 

[Lewis]

Thanks to you all. You folks are amazing! I was mistaken and the Transfer Case is in fact a 208 C. If it is aluminum, it sure is heavy!
Bucket, thanks for the cooler idea. It has a brand new one that is almost half the size of the grille opening. Will that be enough for daily driving in Phoenix Arizona?

 

[bucket]

Thanks to you all. You folks are amazing! I was mistaken and the Transfer Case is in fact a 208 C. If it is aluminum, it sure is heavy!
Bucket, thanks for the cooler idea. It has a brand new one that is almost half the size of the grille opening. Will that be enough for daily driving in Phoenix Arizona?

Should be ok with a cooler that size. But you will still not have much use of overdrive, which is where you have to be careful. The problem is, driving around in OD with tall gears and tall tires, it's going to lug the engine. When it lugs the engine at normal road speed, not only will it hunt between 3rd and OD, it likely won't have the converter locked when driving in OD. Driving around like that will cook the trans fluid. Driving in 3rd will save your new trans' bacon, plus it won't feel like a gutless slug just trying to keep road speed.

 

[Hunter79764]

Good advice on the speedo stuff, I think you are headed the right direction. As for the transmission heat, I think the trans temp gauge is a good idea, as it will take a lot of guesswork out of it. Even if you just run a temporary for a summer to see where the limit is before it starts heating up significantly, that would be good information.

If it is getting hot as outdoor temps rise, I'd be looking at an auxiliary fan for the cooler. Aftermarket is fine, but there are some good OEM/junkyard options that you can probably find as well. And I'd also make sure to route to the radiator tank cooler then the air cooler, as the stock cooler will do a pretty good job of zapping the temps down to 200°, then the air cooler drops it further, and running bottom fittings to top fittings such that you push out any air in the system as it flows.

 

[Joshua Keith]

@Lewis Dont know what gear you need, but I found a blue gear 38 tooth, in my garage today if you need it, let me know.

 

[Lewis]

That's very generous of you Joshua. Not sure what I need yet.
Now, I am considering changing the gears in the differentials. Do you or anyone else have a recommendation as to what gear ratio would bring performance back to near stock with the 35" tires? The differentials are the original 8.5" 10 bolt.

 

[Bextreme04]

That's very generous of you Joshua. Not sure what I need yet.
Now, I am considering changing the gears in the differentials. Do you or anyone else have a recommendation as to what gear ratio would bring performance back to near stock with the 35" tires? The differentials are the original 8.5" 10 bolt.

I have 4.10's in mine and it is adequate with the 350/TH400/NP205 combo. You will have a taller first gear in your 700R4, so it would probably be OK for you as well. I just put 4.10's in my 2011 Suburban 10 bolt to replace the 3.08's that it came stock with. That made a huge difference in drivability and towing with the 6L80E that it has. I would definitely not go any less than 4.10's with the 35's and 700R4, you would probably be very happy with 4.56's or 4.88's as well.