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383 Stroker, TH350, NP205, D60/14BFF w/ 4.56's - best tire size?

handloader90

handloader90

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Hey fellas,

I've got a '78 K5 with:

383 Stroker
TH350
Currently has an NP203 but will have an NP205 installed withing the next month or so.
Dana 60/ 14BFF w/ 4.56 gears and Detroit in the rear.

I will drive it on the street to work, speed varying from 25 M.P.H. up to 80 M.P.H. but I still want that edge when I'm offroad.

What would you guys say the best tire size would be?
 
Depends largely on lift. 35s or 37s I'd say. Without o/d you're rpms are already high at 80mph. Larger tires are heavy but bring rpms down.
 
40's, or get an overdrive trans. 3500-3700 rpm is not an rpm you want to spend any time cruising at and that's what you're faced with if you choose 35's or 37" tires and need to drive at 80 mph. Something in the 40" range gets things down to an annoying 3250 rpm at 80 mph. If it were me, I would swap in a 700R-4 and pick the tires I feel are best for my needs, and then still be able to use it to go to work.
 
nutt7 said:
Depends largely on lift. 35s or 37s I'd say. Without o/d you're rpms are already high at 80mph. Larger tires are heavy but bring rpms down.
Click to expand...

For this truck I'm gonna run a shackle flip and some Alcan 4" spring up front. 35'so would fit in there perfectly but I've been know to trim some sheet metal to stuff bigger meats in there.
 
tRustyK5 said:
40's, or get an overdrive trans. 3500-3700 rpm is not an rpm you want to spend any time cruising at and that's what you're faced with if you choose 35's or 37" tires and need to drive at 80 mph. Something in the 40" range gets things down to an annoying 3250 rpm at 80 mph. If it were me, I would swap in a 700R-4 and pick the tires I feel are best for my needs, and then still be able to use it to go to work.
Click to expand...

Stuck with the TH350 for now, I have 2 SM465's sitting in the garage that I have to get rid of before I can bring another tranny in. Definitely leaning towards a 700R4, the on that I had in my '76 kinda scared me away from the 700's though, i don't know if it was just time for a rebuild or what but it shifted like s**t and I had the TV cable set perfectly... the TV cable was another thing that scared me too, it's gotta be perfect or you can toast your tranny.

I'm still on 1/2 ton axles for now, just waiting on some disc brake parts to finish up my 14BFF, gonna rebuild the kingpins on my 60 and I need brake lines and hard steel lines then I'll be rolling them under the truck.

At what speed does overdrive kick in?

Also, can you link me the calculator that your using so I can bookmark it?
 
I thought 37s and 4.56 went very well together with no OD.

At 70 MPH that setup puts you right around 3100 RPMs.
 
My brother's 78 K5 (pic below) has a reman 350 (runs good)/TH350/1tons with 4.56's and 37's. He still runs 75 down the highway just fine, and still has pretty good gearing offroad. He went from 12b/d44 with 3.73's and 33" tires to the 1tons with 4.56's and 37's and he said it felt very very similar as far as power and drivability on and offroad goes. And he just has cheapo 4" lift springs in the front and a 4" shackle flip in the rear is all (just a tad bit of fender trimming, but looks clean)

 
I had a th350 with 4.10's and 37's, buddy has the th400 with 4.56's and 37's and neither of us go above 70mph with the BBC's on the highway.

If you need 80mph you'll need OD
 
With 37's you will probably be running in the 3,500 rpm range at 80 mph (keep in mind that most tires that say 37 on the sidewall really don't measure that tall and you have some slippage with a non-lockup torque converter). I think driving with the engine at 3,500 rpm would get old after awhile. Sure, larger tires will lower the rpms but you also have to consider how fast you want to run on big tires. A true 40" tire would only drop you down a few hundred RPM but decent street tires of that size are very expensive and not sure I would want to run 80 mph on a set of bias ply Swampers!

Without having an overdrive transmission you need to compromise with either the highway driving or off-road driving aspect of the gearing. If the truck will be used daily to drive to work I would focus on setting it up for the highway. Overall I think axle gearing plays the biggest role in street manners and then you can rely on the low range in the transfer case to get off-road performance. It's not like the difference between 4.10 and 4.56 axle gears really makes a truck more capable off-road.
 
The speed limit on the road I take to work is 55 m.p.h., a lot of times I find myself going 80 m.p.h. I'll lay off the skinny pedal and go the speed limit :burnout:

From the advice I'm getting it seems like I should be looking into some 37's.

How does overdrive work? Does it kick in at a certain R.P.M. or speed?
 
1986 K20 Suburban with 383 stroker, TH400/NP208, Gearvendors O/D, 14 bolt with 4.10 gears, 35" Toyo AT2 Extremes.

70 MPH turning 2000-2200 RPMs. Approx. 12 MPG.
 
37's it is then. I'm gonna be sticking with TH350's for now due to them not being as touchy as the 700's.

I'm looking at getting a set of those 37x12.5x17 BFG T/A K02's and putting them on some Raceline steelie beadlocks. I really liked my BFG T/A KO's and I hear the K02's are even better.
 
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