speedo reading difference? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

Blue_71 · 07-08-2002, 03:44 AM

im changing my truck from a 3 speed manual to a 4 speed manual (SM465) and changing the rear gears to a 3:07 posi (beleive its a 3:73 now) about hwo off will my speedometer be?

Longhorn Man · 07-08-2002, 07:27 AM

enough.

heh heh heh

mrein3 · 07-08-2002, 07:46 AM

If you are putting the tranny in your 4x4 and you don't change rear end gears it won't be off. On 4x4s the speedo cable goes in the transfer case.

I'll guess since you are going from a 3.73 to a 3.07 you'll be off by 20% at every speed. There is very close to a 10% difference between all the factory offered rear end gears. 10% more than a 3.73 gets you to 4.10. 20% from a 3.73 gets you close to 3.07. With no torque converter slip in a straight stick it is just simple math.

I believe the NP205s speedo output was geared for a 3.73 rear end. This is based on the fact that I took a 3-speed manual/NP205 out and replaced it with a Th350/NP205. The new 205 came out of a truck with 3.07 gears. My truck has 3.73s. The new 205 had an adapter attached to the speedo output on it. When I installed the new 205 I left the adapter off and the speedo works within one or two mph of my GPS which is what it did before the swap.

BTW a GPS that can show speed over ground in mph is a GREAT way to check your speedo.

Blue_71 · 07-08-2002, 04:41 PM

thats going in the 71 1/2 ton 2wd. the 4x4 already has a SM465 and NP205, with 4:11 rear gears.

07-08-2002, 07:46 AM	#3
mrein3 Registered User Join Date: Feb 2000 Location: Center City, MN, USA Posts: 3,254	If you are putting the tranny in your 4x4 and you don't change rear end gears it won't be off. On 4x4s the speedo cable goes in the transfer case. I'll guess since you are going from a 3.73 to a 3.07 you'll be off by 20% at every speed. There is very close to a 10% difference between all the factory offered rear end gears. 10% more than a 3.73 gets you to 4.10. 20% from a 3.73 gets you close to 3.07. With no torque converter slip in a straight stick it is just simple math. I believe the NP205s speedo output was geared for a 3.73 rear end. This is based on the fact that I took a 3-speed manual/NP205 out and replaced it with a Th350/NP205. The new 205 came out of a truck with 3.07 gears. My truck has 3.73s. The new 205 had an adapter attached to the speedo output on it. When I installed the new 205 I left the adapter off and the speedo works within one or two mph of my GPS which is what it did before the swap. BTW a GPS that can show speed over ground in mph is a GREAT way to check your speedo. __________________ '70 cab, '71 chassis, 383, TH350, NP205. '71 Malibu convertible '72 Malibu hard top Center City, MN

07-08-2002, 07:27 AM	#2
Longhorn Man its all about the +6 inches Join Date: Jan 2001 Location: Hilliard Ohio Posts: 2,690	enough. heh heh heh



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