Help! What am I missing?

[Madbomber88]

I bought a rough, bone stock (as far as I can tell) 72 K20 with a 4-speed manual. It came with the Eaton HO-52 and 4:10s with a locker. When I got it (had it shipped, so wasn't able to check it out beforehand), it was revving around 3600 while driving 40 MPH...not good

Since gears for the eaton are basically non existent, I tracked down a 14 bolt with 3.42s and an eaton limited slip. Put that in this weekend, and now it revs about 3000 at 40 MPH. Am I missing something, or does this not make sense. The clutch is not slipping, and the tach is accurate. What else could it be, or is this normal? Was there any gear differences in the SM465s or NP205s? The number on the NP205 is C-99404.

I understand this is a truck, but I need to at least be able to drive it on the highway. What does everyone think? BTW if anyone wants the eaton rear, it is available and in very good shape.

here's a pic of the truck

[Oldtruckfanatic]

What gear is the transfer case in while driving that speed if it's in low range on the transfer case you'll rev like that. you should be able to go regular freeway speeds reasoably well with the transfer case in high range

[JimKshortstep4x4]

Nice looking truck!

Since you reduced the ratio by 17 percent approximately the rpms were reduced by a similar amount (.17 x 3600 = 612) so going from a reading of 3600 to 3000 would be expected.

The SM 465 is direct drive in high gear. The NP 205 has a low and high range.

If you are not in low range than I would wonder about the speedometer reading.

Jim

[RANDY COX]

Hmm,nice truck though

[Longhorn Man]

Either your speedo or your tach is off
That,
or you have some clutch slippage or it's in low 4X4... but I doubt you could do 40 in low 4X4.

If I remember correctly, at 3200 RPM with 4.10 gears, you should be doing 65 - 70.

[Madbomber88]

Ok, so I will concentrate on the transfer case. it seems to shift ok. I have not changed over the gears in the front to match the new 14 bolt in the rear. If I test the low range 4wheel drive without locking the front hubs, will I do any damage?

Someone a few months ago had mentioned something about a reducer that was used in the NP205s? I always thought a 205 was a 205...

[Oldtruckfanatic]

If the front hubs arent locked in you can try the low range but I still wouldn't drive it far...you should know right away if it's in low range or not. As far as I know the NP 205s were four position transfer cases 2 wd, neutral, 4 high, and 4 low. I'm not really a 4x4 guy but I'm sure one of these other guys can tell you if there is a reducer. Hope this helps out.

[Stocker]

Originally Posted by Oldtruckfanatic

As far as I know the NP 205s were four position transfer cases 2 wd, neutral, 4 high, and 4 low.

From rear to front -- 4Hi, 2Hi, N, 4Lo

[Stocker]

Originally Posted by Madbomber88

I have not changed over the gears in the front to match the new 14 bolt in the rear. If I test the low range 4wheel drive without locking the front hubs, will I do any damage?

Just make sure you don't have Warn Lock-O-Matic hubs (embossed on the twist dial) -- they will engage going forward in 4wd, even if they are in the 'Free' position.

[jhow66]

Your speedometer is off with the gear change.
MPH=RPM X tire diameter-divided by-gear ratio X 336
This should give you your true speed.

[4x4Poet]

You can definitely go 40mph in low range:

Tire Diameter: 30 (my guess)
Transmission Ratio: 1:1 (4th gear)
Diff Ratio: 4:11
TransferCase Ratio: 1.96:1 (NP205 low range)
MPH: 40
= 3608.9 rpm.

You can plug in these numbers with the actual tire diameter @ http://4lo.com/calc/gearratio.htm