Top speed with 4.57 gears? Can I drive 70MPH? I think I'm doing something wrong...

[66Submarine]

I think I must be doing something wrong...my truck doesn't seem to "top out" at 32MPH like it's supposed to.

Cruising 65-70MPH with 4.57 gears, SM420 trans,...

Going about 70 per GPS in most of the clip (IIRC)...I normally try to go something like 62-65MPH most of the time.

Perception is a funny thing, isn't it?

[joedoh]

big tires! gotta be something like a 265/70/15 if 3500rpm and 4.56s

[TJ's Chevy]

You must be running 110 octane and nitrous...or yer tach is lying. Heck I got mine up to 4200 in 4th and was still climbing. 3.73 gear. 30 inch tire. Little lighter truck. My 64 GMC should have either 4.10 or 4.57 and I have 31's on it...maybe I'll get a tach and see if it'll get up to 75.

[vince1]

Time to get the right speedometer drive gear.

[jtrichard]

if the GPS said 70 average then your speedo is a little fast (smaller tires then OEM?) and i would say you have a V8 tach on your L6 that would put it about 875 RPM slow

[TJ's Chevy]

Originally Posted by jtrichard

if the GPS said 70 average then your speedo is a little fast (smaller tires then OEM?) and i would say you have a V8 tach on your L6 that would put it about 875 RPM slow

That inline would NOT cruise at 4300 rpm. lol Now put in a different cam and lighter pistons and that'll change.

[CdM]

this made my morning!! haha. I run 4.56's with my 39.5's on my 64 chev. Perfect gear for me. But like yours my speedo doesn't seem to get it.

[66Submarine]

Originally Posted by joedoh

big tires! gotta be something like a 265/70/15 if 3500rpm and 4.56s

Yeah, 235/85R16 @ 32" and 660 revs per mile

Originally Posted by TJ's Chevy

You must be running 110 octane and nitrous...or yer tach is lying. Heck I got mine up to 4200 in 4th and was still climbing. 3.73 gear. 30 inch tire. Little lighter truck. My 64 GMC should have either 4.10 or 4.57 and I have 31's on it...maybe I'll get a tach and see if it'll get up to 75.

I don't think you should have any trouble getting up to 75MPH. Just don't blow up that obscure V6 or get arrested while making your land speed attempts...lol

Originally Posted by jtrichard

if the GPS said 70 average then your speedo is a little fast (smaller tires then OEM?) and i would say you have a V8 tach on your L6 that would put it about 875 RPM slow

It's a trans from a '66 1/2 ton SWB truck; the odometer reads about 10% fast and the speedo reads faster than that. +875RPM would put me at something like 4,400RPM@70MPH with theoretical 5.70 rear gears...lol

Originally Posted by TJ's Chevy

That inline would NOT cruise at 4300 rpm. lol Now put in a different cam and lighter pistons and that'll change.

Actually, it should run at 4,300RPM. It wouldn't be super happy there, but it would (or should) do it. 4,300RPM @ WOT is actually the upper spec for propping a Mercruiser 165. The pistons are listed at 20.84 ounces in the GM books, which is pretty much the same weight they give for a 283 piston (20.3/20.42).

Originally Posted by CdM

this made my morning!! haha. I run 4.56's with my 39.5's on my 64 chev. Perfect gear for me. But like yours my speedo doesn't seem to get it.

Glad you liked it.

[factorystock]

Originally Posted by TJ's Chevy

My 64 GMC should have either 4.10 or 4.57 and I have 31's on it...maybe I'll get a tach and see if it'll get up to 75.

GMC V6 3/4 tons came with 4.10's standard 4.56 optional, GMC inline 6 3/4 tons came with 4.56 standard. V6's are like diesels, high torque at low rpms. Not designed for excessive rpms, high rpms will destroy a V6. If you want higher speeds, change the ring and pinion to 3.54's.

[66Submarine]

Originally Posted by factorystock

GMC V6 3/4 tons came with 4.10's standard 4.56 optional, GMC inline 6 3/4 tons came with 4.56 standard. V6's are like diesels, high torque at low rpms. Not designed for excessive rpms, high rpms will destroy a V6. If you want higher speeds, change the ring and pinion to 3.54's.

Yeah, I have no personal experience with them, but I'd be hesitant to beat on one like I would a 250/350/454, etc. I'd kind of expect them to live at or below rated power RPM, but with how hard to find/expensive parts are I'd try to go easy on them.

On a different note, I'm hoping to see an increase in fuel economy and a decrease in engine speed real soon...

[joedoh]

thats funny, I have a 3:73 and I would like to swap in a 4:30 because of the 31" rear tires. I have overdrive but its better to leave in 4th (1:1) unless I am over 75mph, my little 4 banger makes peak torque around 3300.

[TJ's Chevy]

Originally Posted by factorystock

GMC V6 3/4 tons came with 4.10's standard 4.56 optional, GMC inline 6 3/4 tons came with 4.56 standard. V6's are like diesels, high torque at low rpms. Not designed for excessive rpms, high rpms will destroy a V6. If you want higher speeds, change the ring and pinion to 3.54's.

Highest recommended RPM for the 305 was 4000 and even that is pushing it. The highest recommended rpm for my 292 was 3600 but I have gotten it to do 5100 though I kept it there for a very short time. Top rpm on an old 5.9 cummins is 3000. lol Talk about a lugger but man some serious torque.

[TJ's Chevy]

Originally Posted by 66Submarine

Actually, it should run at 4,300RPM. It wouldn't be super happy there, but it would (or should) do it. 4,300RPM @ WOT is actually the upper spec for propping a Mercruiser 165. The pistons are listed at 20.84 ounces in the GM books, which is pretty much the same weight they give for a 283 piston (20.3/20.42).

Oh it'll do 4300 rpm, but I was meaning it isn't meant for cruising there. Cruising speed for a stock inline is 2000-2500. But I've had mine at 3500 as well for awhile. 2500 is around 60 for me so I'm happy with that. lol I've had mine up to 4000 in 4th and still climbing....but on windy roads....yeah... lol Free way time!

[66Submarine]

Originally Posted by TJ's Chevy

Highest recommended RPM for the 305 was 4000 and even that is pushing it. The highest recommended rpm for my 292 was 3600 but I have gotten it to do 5100 though I kept it there for a very short time. Top rpm on an old 5.9 cummins is 3000. lol Talk about a lugger but man some serious torque.

I almost mentioned the 5.9 in regards to piston weights...now that's lot of weight to be slinging around with a 4.72" stroke at 5,000RPM!

Originally Posted by TJ's Chevy

Oh it'll do 4300 rpm, but I was meaning it isn't meant for cruising there. Cruising speed for a stock inline is 2000-2500. But I've had mine at 3500 as well for awhile. 2500 is around 60 for me so I'm happy with that. lol I've had mine up to 4000 in 4th and still climbing....but on windy roads....yeah... lol Free way time!

Yeah, 4,300 wasn't really where they intended it to run at cruise, but 2,000-2,500 is really more of a modern contrivance...something around 3,000 seems to have been a common deal at a highway cruising speed.

BTW, I got the 4.10's installed yesterday, so I go 66-67MPH at 3,000RPM now...

[TJ's Chevy]

Nice, Little slower acceleration now, but better mileage at least and still plenty low for that 250.

[blazer-k5]

5500 RPM how big is the flame rolling out of the exhaust ports. ahahahah

[TJ's Chevy]

Originally Posted by blazer-k5

5500 RPM how big is the flame rolling out of the exhaust ports. ahahahah

Couldn't tell...the headers were still on.