55 mph rpm?

[boosted12a]

tell me if this sounds right... at 55mph im taching exactly 2,000 rpm. 2wd 3 spd manual, 390 with suposed 3.25 gears. it just seems kinda high to me. my tires are 30X9.5, not that that will make a difference in what the speedo says. i eventualy want to make her into a street truck with smaller diameter tires, but if its already taching that high ill need to make some more changes. im also kicking around the idea of putting in a C6, ive seen them on ebay for 900$ that are suposedly built......

[heep70]

http://www.csgnetwork.com/multirpmcalc.html

[390Nut]

2000 rpms isn't really that high, actually, and yes, that sounds close to normal. I'm running a 4 spd with 3.50 gears and run around 2350-2400 at 65 (indicated). These engines were designed long before the 55mph speed limits ever existed, and 70mph on the open highway was the norm. Granted, these are trucks, but they still had to get around, and the only difference between a truck 390 and a car 390 is the compression, really. Even with different cams, they still used the same valve train, and that's where the biggest issues with higher RPM's exist.

And yes, the rear tire size DOES make a difference. It plays a significant role in how fast everything turns to move you down the road, and the reading you get on the speedo because of it.

[72hiboy4x4]

everyone I've talked to says the C-6 eats about 10% of your power in slippage and inefficiency.
my 460/c-6/4.10 gears/ smallish tires gives me about 3800 rpm at 70. I hate that!
If I can find the parts I am going to yank the C-6 out of my f350 and put in a stick. hopefully a 5 speed, but I don't think I will find one that will stand up to my towing usage.

[flyboy2610]

Hiboy, the real culprit in your set up is probably not the C6, but the 4.10 gears. Replace them with some 3.25's and I think you'd be much happier.

[72hiboy4x4]

but then I couldn't tow 15,000 lbs up the hills around here!!

but really I have heard numerous times that the C-6 is a power robber. can't remember where right now, but it was one of the towing forums I frequent.

[spartman]

Mike I have a 4 speed out of a 67 F-100 that was behind a 352.

I don't have the stick shift for some reason so I cannot tell you what version it is.

You can have it for shipping if you want it.

[72hiboy4x4]

Thanks, but I can't give away the 3 I already have! :lol:but I want a 5 speed. and I don't have all the stuff for that truck (its my '88 with a 460) to do the manual conversion.
the '72 has a NP435 (with a lot of spares!) Gotta have that low gear for towing with it, too!






*anyone want a NP435 or 2??*

[rjewkes]

No Ig ot one coming with the 75 if it wasn't a decent amount of work to swap it to manual i would.

[spartman]

[quote="72hiboy4x4"]Thanks, but I can't give away the 3 I already have! :lol:but I want a 5 speed.

Fine see how you are.

[boosted12a]

cool. the tire diameter wont affect the speedo reading, but it will affect the "actual" speed you are going. my speedo may be off, i dont know what tire size was originaly on the truck so i could be going a few mph fastter than what the speedo says..... any way, i feal better knowing that this is prety much normal. thanks
jake

[1970F250]

i have a 390 with a c6 and 4.10 gears and im at 2300 rpm at 65 mph.

[1970F250]

oh yeah. on 33 inch tires.

[CJ,s 67 F100]

That link to the calculator is cool but the formula for figuring rpm's in high gear with a 1 - 1 final drive is gear ratio x mph x 336, divide by tire height = rpm's Example
3.25 x 70 x 336 div by 29 = 2,630rpm's
3.50 x 70 x 336 div by 29 = 2,838 "
3.75 x 70 x 336 div by 29 = 3,035 "
4.10 x 70 x 336 div by 29 = 3,325 '
You can change tire size up or down and make a lot of difference
3.25 x 70 x 336 div by 33 = 2,311 rpm's

[1970F250]

update: i looked at my tag and it says 3.54. don't know where i got 4.10 from