Hey guys/gals,

im just curious if you keep your trucks under a certain speed. Ive been keeping it under 55 mph, just curious is it safe to go any higher? The guy at the tranny shop told me 55 is the max. Do you have any better rules of thumb?

also im looking for some interior pieces...ie head liner. any ideas where i find one??

thanks!

a great place to find any pieces for these older truck, is LMC, i got one of their free catalogs and it is amazing, good prices too...do you know your gear ratio's? depending on those you might be able to go faster.

you can go much faster than 55 depending on gear ration and tire size. The th400 is not an overdrive, but the 6.2L can spin pretty fast. The tranny should have nothing to do with it. You just have to watch the rpm's of the engine. The sweet spot for the 6.2 is somewhere between 1800-2200 rpms.

It depends on the gear ratio and tire size. The THM400 is direct drive (1 to 1 ratio) in drive range. I limit my speed on my pickup to 55 - 60 as much as possible. With 4:10 axle ratios and small diameter 245/75 tires, that seems to be where the engine is the happiest and suffers the smallest drop in fuel economy. The diesel delivers it's best economy at lower rpm's. Engine wear goes up with rpm too. The slower you can cruise the engine, the longer it lasts.

My old k3500 with 4.10 and a th400 would do 82 mph. I ran 60 most of the time

i can push my 87 up to 100kms if i really have to, but with the sm465 i don't like to push it, as its a granny 4 spd.

As stated earlier, knowing the axle ratio and tire sizes will help you determine what speed you'd like. There are several RPM calculators online that would tell you what the RPM would be at a given speed (without taking torque converter slippage into account)

See sig for a gear / tire combo example: My truck cruises the freeway at 60 MPH fine. It also cruises at 65 without much effort. (It just sounds a little worse!)
But by 70, the beast sounds like it's gonna die at any moment. Yet my brother-in-law (PO) had it up to 75-80 regularly, so I figure the engine saw 3000 RPM a time or two...

Take a look at this simple calculator
http://www.csgnetwork.com/rpmcalc.html

Simple example: 55mph, 31" tires, 3.73 diff, 1:1 final drive on trans = 2223 rpm

i got my truck up to 140 kmh but my engine ran outa power on me i have 3:73 axles i think? and a 4 speed overdrive i cant rememeber the name NP 833?

Hmmm...87mph - that's not bad. The standard OD ratio is 0.7:1 but I think the wind drag would mean you would never get within 10% of the rated redline of 3600rpm.

I've wound my '85 burban up to 85mph - she felt like there wasn't much left and I'd guess I was running close to 3000rpm. That's as fast as I want to go, driving a building on wheels.

wait till i get a turbo ill wrap that speedo all the way around to 0 again

My 87 with the 700R4 is happy at 55-60, unhappy at 65, and distressed at 70. It would probably do 90 but at 60mph I'm getting well over 23mpg.

Hey guys/gals,

im just curious if you keep your trucks under a certain speed. Ive been keeping it under 55 mph, just curious is it safe to go any higher? The guy at the tranny shop told me 55 is the max. Do you have any better rules of thumb?

also im looking for some interior pieces...ie head liner. any ideas where i find one??

thanks!

My K5 Blazers with T400 and 3.08 axles can do 80 MPH and be tolerable.
My 3/4 Suburban with T400 and 3.73 axles sounds like it's going to blow up at 75 MPH, but can do it.

wait till i get a turbo ill wrap that speedo all the way around to 0 again


Don't get going too fast, when it blows up and coasts to a stop it will be a long walk back to pick up the parts.

ahh it will be fine in overdrive doin 120 it sounds like its only doin max 2000 RPM so igot lots to go

thanks for the replys, looks like 55-60 is the money spot. this might be on another thread, but im having some trouble finding a tach taht will work on a diesel? I think the engine is around 2000 rpm, but its always better to know.

thanks again

they sell them at autometer they clamp on the alternator and then you have to tweak it so it reads the same as the engine idle then your tach works fine i have one that the previos owner installed but it doesnt work and i cant figure out why

My 87 with the 700R4 is happy at 55-60, unhappy at 65, and distressed at 70. It would probably do 90 but at 60mph I'm getting well over 23mpg.

x2

I just did a 600 mile highway run, trying to keep the speed at around 60-65mph. Average consumption was 22mpg - not bad at all.