best way to increase top end speed in a 1967 K20

[psychobilly451]

what is the best/easiest way to increase my top speed?
I used a gear ratio calculator but I must have missed something. It said my top speed should be about 115mph but its more like 50ish. Is the transfer case something other than 1:1 ratio in 2WD High?

[too much stuff]

According to your signature, i would say 50-60 mph would be working it. Taller tires, change your gears, or add an overdrive.

[tcb-1]

sm420...... that transmission was made for rock crawling, not hitting the century mark. 1:1 for top gear sound right.

your more than likey going to have to ditch the tranny or add an o.d. to get more speed.

but 115 mph in a 67 4by? you sir are living dangerously..... but I like the concept!

[Critter]

Story time...years ago when I was in high school and working on my 68 swb stepside, I had a lot of carb trouble. An old hot rodder was over at my dad's house and offered a little advice. He said "if you want that thing to run faster here is what you do...you take out the cigar lighter....and put a sports car under it..." I had to think about it for a little while then I got it. These trucks weren't made to go fast.

In all seriousness, your inline 6 and and heavy 4 speed aren't made for much more than that, but when tuned up it should be up around 60-65 I would think. Even though the sm465 is a 1:1 top ratio it is a big heavy sluggish gearbox, not to mention the transfer case is robbing a little horsepower as well. That along with the fact that your I6 is probably a little tired and worn anyway keeps your top speed down. Maybe you have already done it, but when was the last time it was "tuned up", spark plugs, cap, rotor, points, fuel filter, etc? Long story short, I don't think your transmission/transfer case/rear gears is the problem, its lack of HP to push you up in the higher speeds. Just my $0.02.

Oh yeah, one last thing, that speed calculator was probably basing its numbers on a more-or-less stock V8. Just a thought.

[BLAZERMAN]

Are you sure you have 3.73 gears? If stock I would think your truck would have came with at least 4.10s if not 4.56. The 250 is good little motor but I would think pretty underpowered for a 3/4 ton 4x4

[CPNEJR]

i would think that truck would have 4.56 with the 250 I6.

[psychobilly451]

how do I add an OD?

[oldblue1968chevy]

700r4 or other tranny..

[70cst]

Originally Posted by psychobilly451

what is the best/easiest way to increase my top speed?
I used a gear ratio calculator but I must have missed something. It said my top speed should be about 115mph but its more like 50ish. Is the transfer case something other than 1:1 ratio in 2WD High?

I am not sure what the answer is but with these trucks top speed is a non issue unless your doing 1/4 mile runs. Lets say you get up to 125mph ...how are you going to stop the truck should you need to? Handling a truck at high speed...good luck. Top speed goes with a muscle car like our trucks are made to haul Americas goods. JMHO

[El Jay]

Here's a tale, FWIW.

When my son bought the truck it was a 292/4 speed, with 4.10:1 gears.
When the motor went away it became a 350/4 speed.

When I bought it from him, it had become a 350/350, same rear gears.

Now, having given you all that information; I've never had it into the triple digits, but I have had it on the "bad side" of 90.

With non power, 4 wheel drum brakes, and added on "twithcy" power steering, I'll honestly admit it's not a fun ride!

These things just aren't made to go that fast.

You now have my .02.

Live long and prosper.

LJ

[70cst]

Originally Posted by El Jay

Here's a tale, FWIW.

When my son bought the truck it was a 292/4 speed, with 4.10:1 gears.
When the motor went away it became a 350/4 speed.

When I bought it from him, it had become a 350/350, same rear gears.

Now, having given you all that information; I've never had it into the triple digits, but I have had it on the "bad side" of 90.

With non power, 4 wheel drum brakes, and added on "twithcy" power steering, I'll honestly admit it's not a fun ride!

These things just aren't made to go that fast.

You now have my .02.

Live long and prosper.

LJ

El Jay...Good points and you show signs of wisdom.

[Skirkpat]

My dad used to harass me about my F-350 Superduty's I used to drive, said I was wasting gas. (he was right but that is not the moral of this story) We both had a good laugh when we jumped in his Acura Vigor to go get his new 32" T.V. from best buy.....We got to the car and I swear the box was almost as big as his car!!! (old school tube T.V.) It took him a minute, but he reluctantly asked me if I could go home and get my "useless" truck...

As Critter said, buy anything but a full size truck to go fast in, Thats not what they are for....

[psychobilly451]

I dont want to go 115mph, the gear ratio calculator said it should be the top end. I would be happy to cruise at 55-60 mph but not be at such a high rpm.

[Critter]

Originally Posted by psychobilly451

I dont want to go 115mph, the gear ratio calculator said it should be the top end. I would be happy to cruise at 55-60 mph but not be at such a high rpm.

I see what you are getting at. You have three reasonable possiblilities in that case.

1) Taller tires = slightly higher effective ratio = slightly lower RPM at highway speeds, not overly expensive.

2) Change to overdrive transmission either NV4500 manual 5 speed from an 1988+ 4x4 or 700r4/4L60E auto = Nice overdrive but you are probably talking $1500+ once you buy converters, crossmembers, wiring harness for the automatic, etc. The auto being the cheaper of the 2 probably. (for the record, I'm not sure how either of these would work exactly with the I6, maybe someone else can comment???)

3) "Higher" gear ratio. 1st confirm you do have 3.73s and what rear axle you have. You can probably make this swap for a few hundred bucks depending on new or used parts. But keep in mind you need to change the front axle gears to match if you want to use your 4 wheel drive in anything other a mud pit.

[vectorit]

Back before I tore down my 68 burb, I took her on a camping trip with the GF. She said her parents were going to want to meet us, as they were driving down the way we were.
Well, we pack up from our campsite at Convict lake. Head up 395 and who do we see movin fast? was her parents in their MB sedan, and the GF got upset that she wasn't going to see them. Since we were in my "old truck"...

Well, ahem - skewz me I said.

I'm already doin 70 or so up 395, and I just stand on it with secondaries a howlin. Yep, burried the speedo just as she looks over since we were catchin up to the Benz. I can't remember how long it took me to get up to that speed, but I do remember the burb performed like a champ and I was proud. But, dang was I scared and stupid for doin it at the time.

The parents were cool, and just laughed as they saw the big truck gainin on them so fast. He also confirmed the speed, since he was in cruise control doin 110.

I didn't have to touch the 4 drums to slow down, since the 4.56's and trans took care of slowing down to highway speed from there.

They still talk about that day even now.

At that time the truck had the same running gear it has today, just back then I had headers, intake, and electronic ignition.
Tires were 33-12.50-16.5, just as they are today.

I was in my 20's then, now I just know better...

[El Jay]

Originally Posted by 70cst

El Jay...Good points and you show signs of wisdom.

I may be crazy, but I didn't get to be this old by being stupid!

Originally Posted by psychobilly451

I dont want to go 115mph, the gear ratio calculator said it should be the top end. I would be happy to cruise at 55-60 mph but not be at such a high rpm.

With my 4:10's (corrected speedo) 70 MPH is just about 32-3300 RPM.

Originally Posted by vectorit

Back before I tore down my 68 burb, I took her on a camping trip with the GF. She said her parents were going to want to meet us, as they were driving down the way we were.
Well, we pack up from our campsite at Convict lake. Head up 395 and who do we see movin fast? was her parents in their MB sedan, and the GF got upset that she wasn't going to see them. Since we were in my "old truck"...

Well, ahem - skewz me I said.

I'm already doin 70 or so up 395, and I just stand on it with secondaries a howlin. Yep, burried the speedo just as she looks over since we were catchin up to the Benz. I can't remember how long it took me to get up to that speed, but I do remember the burb performed like a champ and I was proud. But, dang was I scared and stupid for doin it at the time.

The parents were cool, and just laughed as they saw the big truck gainin on them so fast. He also confirmed the speed, since he was in cruise control doin 110.

I didn't have to touch the 4 drums to slow down, since the 4.56's and trans took care of slowing down to highway speed from there.

They still talk about that day even now.

At that time the truck had the same running gear it has today, just back then I had headers, intake, and electronic ignition.
Tires were 33-12.50-16.5, just as they are today.

I was in my 20's then, now I just know better...

Yours were big & brass. Mine never were!
I've kind of gotten attached to the breathing, no pain thing!

[70cst]

Originally Posted by psychobilly451

I dont want to go 115mph, the gear ratio calculator said it should be the top end. I would be happy to cruise at 55-60 mph but not be at such a high rpm.

Originally Posted by El Jay

I may be crazy, but I didn't get to be this old by being stupid!
With my 4:10's (corrected speedo) 70 MPH is just about 32-3300 RPM.
Yours were big & brass. Mine never were!
I've kind of gotten attached to the breathing, no pain thing!

psychobilly451...now I feel better. What you want is to cruise about 60-70 mph with a lower rpm. Now that makes sense and now you show sign of wisdom. My initial take is you wanted to be able to wind your truck up for top end speed. You had me nervous for a while.

[spinning wheels]

Originally Posted by vectorit

Back before I tore down my 68 burb, I took her on a camping trip with the GF. She said her parents were going to want to meet us, as they were driving down the way we were.
Well, we pack up from our campsite at Convict lake. Head up 395 and who do we see movin fast? was her parents in their MB sedan, and the GF got upset that she wasn't going to see them. Since we were in my "old truck"...

Well, ahem - skewz me I said.

I'm already doin 70 or so up 395, and I just stand on it with secondaries a howlin. Yep, burried the speedo just as she looks over since we were catchin up to the Benz. I can't remember how long it took me to get up to that speed, but I do remember the burb performed like a champ and I was proud. But, dang was I scared and stupid for doin it at the time.

The parents were cool, and just laughed as they saw the big truck gainin on them so fast. He also confirmed the speed, since he was in cruise control doin 110.

I didn't have to touch the 4 drums to slow down, since the 4.56's and trans took care of slowing down to highway speed from there.

They still talk about that day even now.

At that time the truck had the same running gear it has today, just back then I had headers, intake, and electronic ignition.
Tires were 33-12.50-16.5, just as they are today.

I was in my 20's then, now I just know better...

No tach? Just wondering what you were pulling. 6k rpm? With 4.56 grears, that is a lot of speed. Plus, that is high altitude, 4k to 5k feet above sea level, and not flat either. Your motor has some cajones

[psychobilly451]

I have a SM420 transmission with a 1:1 4th gear, 3.73 or possibly deeper axles, 29 inch tires. if I put that into a mph/rpm calculator it says I should be turning aprox 2600rpm at 60mph.

After messing with the calculator I think I might have 4.56 axles or a messed up tach.

the tach is fairly new.

I guess I will get 33" tires and deal with 55mph at 2550rpm. I want to use this thing offroad for hunting anyway.

Is 2550rpm ok for a 6banger?

or maybe find a donor truck for axles, disk brakes, other dodads.

[Sinister]

I used to bury the speedo in my '72 all the time. Out on the highway, foot to the floor, these trucks do like to go fast.

[tcb-1]

Sin,
hopefully you don't do that anymore. we'd like to keep you around a little longer!!

[69halfton]

had my truck (383stroker/350/3.73 & 33 inch tires) up to 120 mph or so (im geussing, the speedo ends at 100, it was pointing strait down) and it didn't scare me. the truck handles very well i found.

stopping wasn't a concern, i was going up hill with a headwind and those bricks aren't very aerodynamic. the biggest thing to concour is horsepower as said above, usually gears dont matter too much...unless you are trying to save gas

in terms of getting your rpms down, id swap to a larger tire, i know ive swapped from a 33inch to about a 28 inch tire, and in doing the math, its the same rpm % gain as if i had swapped from 3.73 to 4.30's

in otherwords, its as noticable when driving as if i had made that gear switch. the same can be said going from a small tire to a larger tire, so I would start there. but keep in mind, sure it will lower you rpms, but that old I6 might not have the power to get you moving with the large wheels..

..decisions, decisions

[El Jay]

Originally Posted by 70cst

I am not sure what the answer is but with these trucks top speed is a non issue unless your doing 1/4 mile runs. Lets say you get up to 125mph ...how are you going to stop the truck should you need to? Handling a truck at high speed...good luck. Top speed goes with a muscle car like our trucks are made to haul Americas goods. JMHO

Originally Posted by psychobilly451

I have a SM420 transmission with a 1:1 4th gear, 3.73 or possibly deeper axles, 29 inch tires. if I put that into a mph/rpm calculator it says I should be turning aprox 2600rpm at 60mph.

After messing with the calculator I think I might have 4.56 axles or a messed up tach.

the tach is fairly new.

I guess I will get 33" tires and deal with 55mph at 2550rpm. I want to use this thing offroad for hunting anyway.

Is 2550rpm ok for a 6banger?

or maybe find a donor truck for axles, disk brakes, other dodads.

25-2600 should be just fine for a long life on the engine, even a little more wouldn't hurt it.

[El Jay]

Originally Posted by Sinister

I used to bury the speedo in my '72 all the time. Out on the highway, foot to the floor, these trucks do like to go fast.

I hope you don't do that anymore.

You're on my "friends" list & I'd hate to lose you, or anybody else.

[Sinister]

Thanks for the concern guys. I don't do that anymore. I put a rod through my oilpan driving like that.