74 k10 going 55 mph is at 3,500 rpm

[granthendrickson]

I got a 1974 k10 with a 203 transfer case, turbo 350 and a 355 sbc. The rear gear is 3.73 and at around 55 mph cruising the rpm’s are up to 3,500. That seems a bit high to me, does anyone know what I should do to the truck to get lower rpm’s at 55? I’m also getting 8mpg.

 

[Snoots]

Are you sure that TH350 is going into 3rd?
I've got the same setup and do about 2500 rpm. BTW, I'm on 32" tires.

 

[Goldie Driver]

What size are your tires ? Going up in size will reduce the RPMs - and, of course, throw your speedo off.

Here is an example- not sure if I remember the OE tire size correctly but they are only about 28 inches...

235/75-R15 vs 33/12.5-R15 Tire Comparison - Tire Size Calculator | Tacoma World

33/12.5-R15 tires are 4.15 inches (105.42 mm) larger in diameter than 235/75-R15 tires and the speedometer difference is 12.5%

www.tacomaworld.com

This one is cool, too, but accuracy unknown to me as I just stumbled upon it.

Spicer Engine RPM Calculator

Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.

spicerparts.com

 

[NickTransmissions]

does anyone know what I should do to the truck to get lower rpm’s at 55? I’m also getting 8mpg.

Swap in a 700R4 and cruise at 70 mph/2000 rpms or so.

 

[granthendrickson]

What size are your tires ? Going up in size will reduce the RPMs - and, of course, throw your speedo off.

Here is an example- not sure if I remember the OE tire size correctly but they are only about 28 inches...

235/75-R15 vs 33/12.5-R15 Tire Comparison - Tire Size Calculator | Tacoma World

33/12.5-R15 tires are 4.15 inches (105.42 mm) larger in diameter than 235/75-R15 tires and the speedometer difference is 12.5%

www.tacomaworld.com

This one is cool, too, but accuracy unknown to me as I just stumbled upon it.

Spicer Engine RPM Calculator

Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.

spicerparts.com

Y

What size are your tires ? Going up in size will reduce the RPMs - and, of course, throw your speedo off.

Here is an example- not sure if I remember the OE tire size correctly but they are only about 28 inches...

235/75-R15 vs 33/12.5-R15 Tire Comparison - Tire Size Calculator | Tacoma World

33/12.5-R15 tires are 4.15 inches (105.42 mm) larger in diameter than 235/75-R15 tires and the speedometer difference is 12.5%

www.tacomaworld.com

This one is cool, too, but accuracy unknown to me as I just stumbled upon it.

Spicer Engine RPM Calculator

Measure your engine’s RPM based on the ring gear and pinion gear ratio, speed, and tire height, with the Spicer Engine RPM Calculator.

spicerparts.com

Cool, my tires are stock size and are also brand new so I don’t want to change them out yet

 

[75gmck25]

My similar ‘75 K25 with 4.10 gears, np203 and TH350, and stock height tires (245/75R16E - about 31”) runs at about 2800-3000 rpm at 60 mph. I doubt I’m getting much over 10 mpg.

You should be able to fit up 33” tires with no lift or fender trimming on a K10, and that would help. Also make sure your vacuum advance is working, and try bumping up your base timing a couple of degrees.

 

[granthendrickson]

Are you sure that TH350 is going into 3rd?
I've got the same setup and do about 2500 rpm. BTW, I'm on 32" tires.

I’m fairly certain that it is, I can check next time I drive. My tires are about 28 inches

 

[SquareRoot]

I got a 1974 k10 with a 203 transfer case, turbo 350 and a 355 sbc. The rear gear is 3.73 and at around 55 mph cruising the rpm’s are up to 3,500. That seems a bit high to me, does anyone know what I should do to the truck to get lower rpm’s at 55? I’m also getting 8mpg.

That ain't right. I had 4:56 and was 3000 rpm at 60 mph. You're either stuck in 2nd or you ain't got 3:73. Or you have Mickey mouse wheels.

 

[granthendrickson]

My similar ‘75 K25 with 4.10 gears, np203 and TH350, and stock height tires (245/75R16E - about 31”) runs at about 2800-3000 rpm at 60 mph. I doubt I’m getting much over 10 mpg.

You should be able to fit up 33” tires with no lift or fender trimming on a K10, and that would help. Also make sure your vacuum advance is working, and try bumping up your base timing a couple of degrees.

Thank you, when my tIres wear out I’ll buy bigger ones lol

 

[granthendrickson]

That ain't right. I had 4:56 and was 3000 rpm at 60 mph. You're either stuck in 2nd or you ain't got 3:73. Or you have Mickey mouse wheels.

Okay, I don’t believe I’m stuck in second but I very well could be if it seems that off

 

[SquareRoot]

Okay, I don’t believe I’m stuck in second but I very well could be if it seems that off

I guess your speedo could be off but that would be obvious by the flow of traffic blowing by you.

 

[squaredeal91]

My question is why do you think you have 3.73 gears? it Sounds more like 4.56s or your only getting first and 2nd gear.

 

[gmbellew]

use this to check what your RPM should be for your setup. it's the most accurate and comprehensive that I've found.

Gear Ratio Calculator

 

[SirRobyn0]

Are you sure it's really 3.73's? That sounds like 4.56's to be at that RPM, unless the tranny isn't shifting into 3rd.

 

[GTX63]

With the discription you gave, your truck is about 1200rpm too high with that set up at that speed.