Diff Question

[Druid5_0]

Hey guys, long-time lurker, first-time poster really. (Besides my new guy post) I have an 88 K5 Blazer with 2.73 gears in the rear, 700r4, np208.
My drive gear is a 15 tooth. My Driven gear is a 40 tooth. My tires are 33 but measuring for diameter come up at 31.

My question is, what would be my cheapest option to make this just a functional (showing correct speed) rig.

 

[Blue85]

You may need a ratio adapter. I think 15-tooth is the only one still available and to make that work on ~32" tires needs like 26 teeth, but you can only get down to 34.

https://speedometergear.store/shop/ols/categories/chevy-gmc-np208-speedometer-gear

Of course what you REALLY need are lower differential gears.

 

[Blue85]

So your speedometer is reading about 35% slow? You should be able to use a free GPS app to get your actual speed or do it old school passing mile markers with a stopwatch.

The closest adapter may be +38%. They aren't exactly cheap, so you should figure out where you're at first. https://transmissioncenter.net/shop/54ra2-gm-dodge-ratio-adapter/

Somebody also needs to mention that the combination you're running is pretty hard on the transmission. Drop to 4.10:1 and you're moving 50% more oil through the cooler. Not to mention getting in the powerband means you're putting down MORE than 50% more torque. That's a whole new vehicle.

 

[Blue85]

My question is, what would be my cheapest option to make this just a functional (showing correct speed) rig.

You can get an aftermarket GPS speedo for $60 and mount it wherever you want (wouldn't have to be in the cluster). Technically the cheapest option is a phone/tablet mount and run a free GPS speed app (Even in the unlikely event you have no device, you can get an old phone for next to nothing). Depends on what you're looking for.

 

[Wes Harden]

Speed hut makes a gps motor that drives a cable driven speedo. It's not cheap.

 

[Fastereddie]

Speed hut makes a gps motor that drives a cable driven speedo. It's not cheap.

That’s pretty sweet though!

 

[Weird Beard]

Abbott Enterprises - cable x

www.atrol.com

 

[bigjimmy82]

That looks cool

 

[scoots90245]

So your speedometer is reading about 35% slow? You should be able to use a free GPS app to get your actual speed or do it old school passing mile markers with a stopwatch.

The closest adapter may be +38%. They aren't exactly cheap, so you should figure out where you're at first. https://transmissioncenter.net/shop/54ra2-gm-dodge-ratio-adapter/

Somebody also needs to mention that the combination you're running is pretty hard on the transmission. Drop to 4.10:1 and you're moving 50% more oil through the cooler. Not to mention getting in the powerband means you're putting down MORE than 50% more torque. That's a whole new vehicle.

I did this gear swap it added 200hp
JK it felt like it though. Huge difference