I have a 1987 K5 Blazer all stock except 4 inch lift. I recently aquired a set of 1 ton axles from a 87 K30. The 1 tons have 4:56 gears. My question is what size tire would be best for me ? Keep in mind this is a daily driver that see's a lot of hwy usage. 65 mph speed limit common. But interstate speeds of 75 mph seen almost everyday. I'm looking for decent power and mpg's. Needs to be able to run down hwy at 70 mph without over reving engine. Planning on a aggressive tread M/T tire. What size tire do I need ?

 

here is a cool website you can use, you can put in what gears, transmission, ratio and tire size and itll give you your rpms at certain speeds. it looks alot more complicated then it is

Gear Ratio Calculator

depends on what transmission you have. if you have over drive its gonna be a different story then a non over drive.

i got 411 gears 33s and no over drive and highway driving is basically impossible unless you wanna fart around at 55-60 in the right lane. at like 70 im pushing 3500rpms.

 

Stock auto tranny. Think it's a 700

 

with ur 4:56 and 4" lift i wouldnt go much bigger than a 33" tire. ul still be able to hit highway speeds cause 4:56s are really designed for 33" tires. that chart will show u that. when i had my 4x4 toyota, i had 32s under it with stock 4:11 and to reach highway speeds with good gas milage the chart recommended 4:56s. on my s10 blazer i have 5:39s with 39" tires and i can reach 70 mph running 2800 rpms, but i have od. i dont run it at those speeds cause u can watch ur tires go bald....lol; i just wanted to see if i could.

 

Currently running 32's and at 65 mph I'm turning 2200 rpm.

 

perfect. if u went with the 1 tons u could run 33s maybe even 34s and still maintain highway speeds and low rpms. 33s would be best though. ezr to find and better priced. bfg km2s are one of my recomendations. awesome all around good milage tire.

 

I'm running 4.56's in an 86 with a 700R4 and 37's. My speedometer is spot on, no adjustment needed.

Why would you need a 60/14 if you're only going to be running 33" tires.
I would strongly recommend running 35's with 4.56



Note that this gear chart shows the rpm's for a given tire size while using a 1:1 ratio.
Since you are using a 700R4 there is a 0.7:1 ratio to figure in. Plus remember that your torque converter will introduce a few revs into the equation. I'll be conservative and say 200rpm for the TC slip.

With 33's and 4.56's you'll be at 3018rpm at 65, figure the OD 2113rpm plus the slip and you're turning 2300 at 65.
If your tires actually measure 33" then you're fine, but they are typically an inch or inch and a half smaller than advertised... so you're going to be another 50-100 rpm higher, now you're at 2400.
Do you want to drive around @2,400 just to do 65?

Stick 35's under there with the same calculation and you'll be at (2845*0.7) +200 +50 + 2,200rpm.

 

Wasn't planning on 33's. Currently have 32's on the 10 bolt axles and after doing the lift they look way to small. Was asking what size tires to get. Have a set of 1 ton axles with 4:56 gears and don't want to regear them. What size tires will give me about same rpm range going down hwy as my stock 10 bolt and 32's ?

 

So you think 38's would be a good choice ?

 

i dont think 38s would fit under ur k5 with only a 4" lift, and not a good idea if u wanna keep ur same rpms and such. 36s is as big as i would go and thats pushing it. id do 35s with the 4:56 and ul like it. satisfied in all your areas, well, on ur blazer anyhow. bwahahahaha