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83 blazer gearing set up?

K5Titan83

K5Titan83

1/2 ton status
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okay so i got this truck like 3 or 4 years ago when i was still in highschool and it was my dailey driver, i fixed it up to the best i could while still keeping it drivable well iv gotten a new dailey driver and im wanting to start on building my blazer, well something iv always noticed on my truck is it drives at (what i think) higher RPM's than what it should be driving at. at 55-60 MPH my tach reads right under 3000 RPM. i dont think this is right. now i know not much is stock on this truck the guy before me had gutted it and put it all together ass backwards so im still trying to sort all that out lol but anyways..

Heres the setup:
400 small block chevy built up a little, with a 400 turbo transmission(the road ripper from certified transmission if this matters?) and i believe an NP208 transfer case. and the gearing in the axles is 3.42 which i think was stock on an 83? and it has a 6 inch skyjacker lift on 35X12.50 tires.

id just like to know is this normal for trucks with only 3 gears? i imagine the axle gearing is a little high but all the website gearing/rpm calculators iv been to are saying i should hit 3000 rpm till like 70-75 MPH with this setup im just confused if anyone could tell me whats going on that would wonderful
 
It does sound a bit high on the rpm's at that speed with 35" tires..(I did not do any calculating)--my old GMC with a TH350 and 4.10 gears wound up to about 2800 rpm at 60 mph with 245/75/16 tires,which are roughly 31" in diameter...

You sure its shifting into third gear?..or that the axles were not swapped or re-geared to a lower ratio than 3.42's?..only way to be sure would be to pull a diff cover and look at the numbers or count the teeth ..
 
yeah iv had the cover off i had to replace the wheel seals and thats when i learned they were 3.42. and yeah i know its getting into third gear i can feel it shift twice.plus this is the second transmission iv had and with the last one it was riding the same way, this transmission is brand new less than 1500 miles on it probably, i heard that the transfercase could be the issue? like it could be causing a lower gear ratio or something? im not really sure what the guy had said about it. but i thought the gearing would just match the transmission? correct me if im wrong im still learning about all this haha
 
The transfer case is 1:1 in hi range on all of them,so unless you have it in low range the case wont affect the rpms..

If the tranny had a high stall speed torque converter it might allow a increase in rpms ,mostly at take off though,not so much at cruising speeds..

Maybe the tire sizes "say" they are 35",but I'd measure them and see if thats true--many tires sizes vary among brands,even with the same markings..the 35" height rating might be an estimate,not a true measurement..
I learned this the hard way when shopping for used tires--one day I spent an hour digging 4 235/75-15's out of the pile at a junkyard,and I was just about to pay the owner for them when he asked me "do you care if those tires aren't all the same sizes"?...

I said I had taken care to pull out all 4 with that size marked on them--but he lined them up beside one another and only two of them were close to the same height...two goodyears,one "wrangler" and another goodyear--there was one B.F.Goodrich and one "Futura" tire that were a lot smaller,like 2"!..if I had put them on my truck it would surely have had problems the first time I locked the hubs in..he explained different brands aren't always the same!..
I ended up going home with two 225/75-15's and one 215/75-15" that were all very close to the same diameter...(within a 1/4" or so)...and the truck drove fine in 4wd..

A lot of tires for lawn tractors,especially the "AG" style tread ones ,aren't the sizes they have printed on them,especially ones made overseas...they are often on the "small" side of the actual size marked on them...I assume car and truck tires aren't much different too...thats why they want you to buy all 4 of the same size,brand,and tread pattern...
 
okay i went out and measured with a tape measure i guess the true height is 33 inches tall, i have mickey thompson baja mtz. i guess its normal then for the tires to be that way? so does this change things at all? i might go try the rpm gear calculator again too i cant imagine it would be all that different though?
 
Have you verified the speedometer is correct? If it has stock gears and larger tires it's actually going faster than what the speedometer states, assuming it was never corrected. Also if the transfer case and transmission were taken from another truck the speedo gears were setup for whatever gears the donor truck had.
 
Yeah i used to different gps to show my speed becuz my speedo is way off. So i know my speed is okay. Maybe my tach is off? But It definetly does run high i can tell by the engine sound. The front end needs re-aligned but it shouldnt be that bad.
 
33" and 3.42 is about 2100 rpm at 60mph. Even if the Torque converter was unlocked, it shouldn't be more than 2300 or so. Two theories:

-you don't really have 3.42. Some mistakenly think the ratio is stamped on the ring gear. What's actually stamped there are the tooth counts, so you have to divide the big number (ring teeth) by the small number (pinion teeth). Or count all the teeth.

-Your tach is wrong. What kind of tach is it? Many are designed for 4, 6 or 8 cylinder. If you have the switch set to 6, it will read 33% high. So 2100 RPM actual would read 2800 on the tach. That's pretty close to your description.
 
years of wrenching I don't trush cheep parts store low buck gauges. I run bone stock or autometer stuff . the tack might be bad.

I run a truck with th400/205 not that it matters but 35" tires ( act 33ish ) and 65mph I am turning 3000 rpm . and this is with 4.10 gears.
 
Okay blue85, I have counted the teeth and did the math it is 3.42 so you might be right the tach could be bad, I don't know what brand it is, couple years ago I was at the shop my mom works at and it was laying there and asked if I could have it the guy said hed never used it just let it sit so high chance that's it then. I might try just buying a new tach and see how that goes, I kinda always thought it was the tach it does sound like the engine is running high but not that high so I'm ganna see about getting a good one.

Also could it by chance have to do with lack of horsepower? Just curious becuz when I built this engine not knowing much about things yet I bought one of those econocams hoping for a little fuelmilage since it was my DD. (I regret it now of course) but just a thought that maybe its just not got enough power?

And I really appreciate the help from you guys many thanks!
 
Before buying a new tach, look for the little 4,6,8 switch on the one you have. It might have been pulled from a V-6 vehicle and you never looked.
 
okay il do that before just incase, would the switch be on the inside of the tach ? i think when i got the thing it came with other lights i could put in it for different shift color lights cuz i dont think theres a switch on the back of it. i could take a picture of it tomorrow and show you but how do i upload pictures onto here?
 
find out the brand if you can and maybe model .

its a switch / set of switches / dial / even wires to clip to set them to 4/6/8 cyl modes .

some external and some inside threw a service hole. and some are off set and not direct inline with this hole.
 
Omg, I cannot believe I never noticed before that there was a switch on the back of the damn thing. I'm just beside myself.... damn. Thank you guys very much very helpful never ever would have thought of this... sadly I still need as new tech. On the back of the thing it shows the switch but only for v6 4 cylinder and 3 cylinder. Damn the luck but anyway thank you guys very much cannot believe I never noticed that thing, just beside myself lol
 
Well I got one of the little equuis techs mounts on the steering column like a 2 or 3 inch and I like to watch the tach anyways.but thanks though. I really appreciate all the help from you guys!
 
If you search online you might find a way to make that tach read right by using a resistor ,and using the 4 cylinder setting perhaps..
The 4 cylinder setting would read I think twice
the actual RPM of a V8,you could just use that setting and remember to cut the reading in half--and impress your friends with its 8,000 rpm redline!..:D
 
I did this with the tach in my dash. I drew a 6000rpm scale to go on the stock fuel gauge location. Then I put Sunpro 8000rpm tach guts behind it, with the switch set to 6 cylinder mode. A tach is a frequency to voltage converter. You could change a resistor or 2 and make it work for 8 cylinder, but it would be hard to figure out without a schematic of that tach.
 
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