How fast can I cruise?

[Smokey burnouts]

72 k5 350/th350 on 35's with 3.73 gears.

Still haven't gotten around to putting a tachometer in the truck(trying to save up and redo all the gauges at once)so not knowing the RPMs how fast should I be able to go with the turbo 350?
When I'm cruising at 55-60 it feels like I'm almost out of pedal but I imagine the truck should go faster than that right?

Thanks

[truckster]

I have the same setup except for 33 inch tires, and I can easily cruise at 75. Your larger tire diameter should give you slightly lower RPMs.

How is your torque convertor? Could it be slipping?

[Smokey burnouts]

Originally Posted by truckster

I have the same setup except for 33 inch tires, and I can easily cruise at 75. Your larger tire diameter should give you slightly lower RPMs.

How is your torque convertor? Could it be slipping?

Might be something I need to look into.
Now is that an actual 75? I ask cause with my 35's and my uncorrected speedo 55(on gauge) is actually 65(gps)

[truckster]

My speedo is corrected, so when it indicates 75 the GPS says 75 as well.

[GMC-YA68]

I was running around 2700-2800 RPMs with TH350 and 33s w/ 3:73s at around 75mph.

Your sig says 10 bolt rear. Are you sure you are running 3:73s?

Check your fuel hoses at the tank. I've seen multiple stories where they have been kinked, rusted, dirty, and only allowing "X" amount of fuel to the engine...

[87Skier]

My torque converter is crap. At highway speeds it slips about 400 rpm, so even though my gearing is at 2200 at 60, I end up pulling about 2600-2700 at 60, but I still have no trouble doing 80.

Doing the math, at 60 your truck should be around 2150 rpm. Even at 80 it should be pulling less than 3000, assuming that the torque converter isn't slipping.

x2 on checking the fuel system.

Also, if you need a cheap tach to hook up easily buy one of these: http://www.ebay.com/itm/New-Digital-...76cfdd&vxp=mtr

They are kind of slow at updating but are so easy to hook up I can deal with that.

[Smokey burnouts]

GMC- based on the "jack up truck spin tires and count drive shaft rotations" I have 3.73 gears or are you getting that 3.73 not common in 10bolts? Could be 3.55 maybe.

87- that tach just get hooked up to distributor or to coil? You feel it's pretty accurate though?

[87Skier]

It is within a 100 rpm. I checked it on my other vehicles with tachometers.

To hook it up you just need to wrap the wire around a spark plug wire and either zip tie or electrical tape in place. No need to mess with getting into the distributor or coil. It is also self powered so you don't even need to hook up to the battery.

[cdowns]

I used to have a stock 2wd blazer and had no problems cruising it 80-90 on an all day trip on the highway

[az1970k5]

Originally Posted by truckster

My speedo is corrected, so when it indicates 75 the GPS says 75 as well.

What's the best way to calibrate a speedometer? Mine is 16 mph over

[truckster]

To do mine I changed the speedo gears on the transmission. I did some math to come up with the previous ratio and then counted the teeth and figured out what I would need. The parts guy had no idea what I was doing, but I just had him grab a bunch of gears until I found the ratio I needed. Unfortunately, it's been long enough that I don't remember what that ratio was.

[cleszkie]

Blazers with automatic trannys came with 3.08 gears in 12 bolt axles. Only the manual versions got the 3.73 gears from the factory. But if you have a 10bolt rear, it has been swapped in and this information means nothing in this case.

[truckster]

Originally Posted by cleszkie

Blazers with automatic trannys came with 3.08 gears in 12 bolt axles. Only the manual versions got the 3.73 gears from the factory. But if you have a 10bolt rear, it has been swapped in and this information means nothing in this case.

I know you know your stuff, and I hesitate to disagree with you, but Chevrolet Heritage Center shows the 72 Blazer with a 350 CU engine had 3.07 as standard with manual or automatic, with 3.73 and 4.11 as options on the 2WD and 3.73 as an option on the 4WD - that's what mine came with.

https://www.gmheritagecenter.com/......let-Blazer.pdf

But to Smokey Burnouts, are you sure it's a 10- bolt? That would mean it's from a later truck, and someone had to cut and weld spring perches and shock mounts, which doesn't seem (to me at least) worth the effort for a 10-bolt.

You really ought to pull the rear cover and see what your gear ratio is. There will be two numbers stamped on the ring gear. Divide the larger number by the smaller number and that's your ratio. And your differential will thank you for the fresh gear oil after all these years.

[asolde]

I have a 72 cst 350/350 auto with 373 12 bolt rear end 31" tires and I can go 65mph.

[bouncytruck]

I have only heard of 3.07 and 3.73 in [stock] 4wd blazers. One of my previous Blazers had 350/TH350/NP203 combo with 4.10 gears on 35" tires (none of it was stock). It felt like top speed was around 65 (but it was too scary to drive faster than that).

If you have a 10 bolt, I'd double check that your front and rear gearing matches before putting it into 4wd.

[cleszkie]

Originally Posted by truckster

I know you know your stuff, and I hesitate to disagree with you, but Chevrolet Heritage Center shows the 72 Blazer with a 350 CU engine had 3.07 as standard with manual or automatic, with 3.73 and 4.11 as options on the 2WD and 3.73 as an option on the 4WD - that's what mine came with.

Well, I'm not too proud to admit that I'm wrong. Sound like there were auto trannys with 3.07 gears from the factory. Maybe I should have said that all of the Blazers with automatics that I have seen had 3.07 gears.

[mcmlxix]

Maybe this will help...
http://www.ringpinion.com/Calculators/Calc_RPM.aspx

[Smokey burnouts]

Looks like it's a basic 10 bolt to me

[Smokey burnouts]

Is there any reason I NEED to swap a 12 back in there? If so won't 70-85 blazer rear ends fit right in there?

[truckster]

Yep, I count 10, too. Strange. But in answer to your question, no, there's not a reason you need to swap a 12 bolt in. In fact, when I blew up my 12 bolt, I considered using a later 10 bolt until I was able to find a decent 12 bolt. I think GM started using the 10 bolt in trucks in 73, but I'm not 100% positive about that. (Edit: Hemmings says they used 12 bolts in trucks up through 1981.)

Hopefully the rear gears match the front. If not, you'll know it as soon as you shift into 4WD, because they're going to start binding pretty badly.

Were you able to figure out your ratio? Also, you say it feels like you're almost out of pedal at 55-60. Is the engine screaming at that speed? You might want to check the throttle cable and make sure it's adjusted properly.

[Smokey burnouts]

I haven't pulled the cover and checked yet, but feel pretty confident it's 3.73.

Hard to hear the motor with the wind and the 35's humming. I pushed it up to 70 cruising and seemed to do fine. I'll look at throttle linkage and stuff see if anything looks off. It all might just be simple fact that the stock motor (180HP) is just too weak to do what I expect. I drive a 450+hp Cummins as daily driver so prolly just used to lil more nuts under hood.