tach reads 2850 at 65mph

[Albertan]

I thought I had a 3:73 due to the high RPM. I wanted to change the gears to 3:07 so the truck would run down the road at 65 with about 2200rpm. When we took the axle apart to rebuild it, I found the pinion had 14 teeth and the crown had 43 teeth. (14 into 43 = 3.07)

The guestion I have is that something does not make sense to me. Using some of the gear ratio calulaters on the net I should be running at 2200 RPM. I have p235R75-15 tires. The truck has a 402, turbo 350 tranny and running down the road at 65 the tach reads about 2850.

[busterwivell]

I don't know how tall a tire that is, but I've got a 4:10 in mine, and 55 MPH is 2500 RPM with the tires I'm runnin'. I've got the formula tacked to the wall in my shop, let me know the tire height and I'll compute for you, if you want.

Oh, incidently, I've got a 700R4 behind my 402.

[Albertan]

The tire hieght is supposed to be 28.9 inches. (side wall is 6.9") But I will check when I get home tonite.

[71-longbed]

heres a couple i used before i bought new tires

http://www.dakota-truck.net/TIRECALC/tirecalc.html

http://www.speedworldmotorplex.com/calc.htm

[69 longhorn]

the dia of your 235/75/15 tire should be closer to 28", or maybe a fraction less. the calc i use , puts you at around 2500....you need to figure a little loss on the converter. try this one http://www.smokemup.com/auto_math/index.php

[Albertan]

Yup, using those calulaters says I have 3:73's, But the actual pinion is 14 and the crown is 43, that works out to 3:07.

Like I said before something is not working out??

I wanted to stay away from the 4 speed automatics, as I would not be able to pull at 2200 rpm. I have enough power with the big block. If I used an overdrive trany I would have to pull in 3rd (at a much higher RPM) as to not cook the tranny.

[69 longhorn]

A little off on the tire dia, & /or the speedo can toss a wrench in the formula. calc says 100 mph @ 3000 in OD , on my longhorn.....in real life, the speedo reads about 106......i could be a little off on tire size, to calc the speedo? the best way to figure tire diam , is to chalk the tire @ the bottom, & the floor @ the same spot.....then roll the truck ahead & chalk the floor when the mark on the tire gets straight down. measure the distance tween the 2 lines, to figure the "rollout", & you can find the true diameter of the tire by doing the math 69L

[CaperCat]

What kind of tach are you using to get an accurate measurement? The factory one, unless it has recently been recalibrated, is usually just for "show".

[69 longhorn]

Originally Posted by CaperCat

What kind of tach are you using to get an accurate measurement? The factory one, unless it has recently been recalibrated, is usually just for "show".

Yup, that is another factor that could be a tad off even with my autometer tach, i figure that if I am within 100rpms at figured speed, i am pretty close...... L

[busterwivell]

BTW, Albertan, welcome to the board. You are not totally correct about automatic overdrives. I had my 700R4 built by a guy who knows transmissions. I told him I wanted to be able to tow a 6000 lb trailer (uphill) with mine in overdrive. It's been in for 2 and a half years with no problems. It can be done.

[Protrucks]

I have not driven my truck since last summer, but I know mine is off due to it is not a tack out of a V8 truck. I did not know it at the time I put it in, but it turned out to be out of a 6 cyl truck and it is way off. I just can't remember how far off it was since I live in Michigan (The Great Road Salt State) it has not seen the road for quit some time

[Albertan]

I just checked the tire size, it is closer to 28 inches. That makes things a littel closer ,however it is still not working out to be 3:07. The tack maybe out and I will also check that. It seams to be very close to the one I used to set the timing etc in the shop.

Tell me more of how to built a 700r4 to pull in 4th gear. Up here in Canada the tranny shops say it cannot be done!

I have two newer trucks that have over drive. The '94 350ci I only pull in 3rd unless I have a light load. This truck is on the first tranny and has 280,000 miles on it (yes miles), but is getting tired.

The 2000 Chev I pulled in 4th, because the sales man said "all you have to do is push the tow haul button in". Well that cost me two trannys before someone told me the tow haul button only changes the presses up too a speed of 50 mph.

I pull an enclosed cargo trailer hauling a Model T, I don't think that is much of a load.

[farside847]

Are there any good threads that go over the advantages and options
for upgrading the rearend? Im a little new to this, but I think upgrading
the gears on my truck can help me keep up with traffic....

[busterwivell]

I don't know how to build transmissions, but I go to the guy who does. I've had 3 built by him, and they all have done EXACTLY what he said they would. My friend has had four. He also refused to build ONE of them because he said it would not hold up. I had a high horse, high torque motor in one, and it was fine for years till I sold the car. The one in my truck, I know he has upgraded 4L65 and 4L80 type stuff in it. I went to him with my 1 ton 71 longhorn bigblock truck (over 4000 lbs) and told him I wanted a transmission that will pull a 6000 lb trailer with no problem. He built it. I talked to him about 6 months after he built it to tell him how happy I was, and told him I downshift to "D" when I pull steep hills. He wanted to slap me! He said he built the trans to do what I told him I wanted to do. His words were "That transmission knows when to downshift, and it will." I drive it in overdrive all the time, and sometimes, it will downshift at a full load uphill, gather a little steam, and upshift again later. I couldn't be happier. I know it's a long way from you to here, but I'll bet, if you want, he'd tell you, or your tranny builder, how he has done the one in my truck

[Albertan]

Thanks BUSTERWIVELL, Sounds like I will have to find a tranny guy that is willing to built up a 4 speed auto.

I am still confused why my RPM's are so high with a 3:07 axle. I figured if I could get the RPM down to 2000-2200 at 65 mph , I would gain a few more miles per gallon. Right now I get 13-14 miles per canadian gallon (about 5 of your quarts).

[67ChevyRedneck]

Just my opinion, but if you're gonna use a 700R4 to haul a heavy load I'd put a 3.73 in your truck. My truck has a 700R4 with 3.73 and I get around 15-16mpg highway. She turns about 2400 rpm's @ 75 mph. If you haul a lot you're not gonna see a big mpg increase anyway.... put some taller tires on your truck to reduce the rpm's. All new chevy trucks that have the "towing package" have the 4l60 OD and a 3.73 rear. IMO a 3.07 is too tall for OD AND towing. I'm going to put a 3.07 in my truck, but I don't plan on towing anything with it. If you want a good OD tranny I would check out Bowtie Overdrives. They offer many differnt trannys for different appications and their prices are VERY reasonable.


LINKY

[vfrdude]

I had 3.07 gears in my truck when I had the 700R4 installed. The RPM's at highway speed are way to low. Going to 3.73 gears made all the difference in the world. I don't think you'll be happy with 3.07 and an OD trans. Maybe 3.42's????

[Russell]

Hey, Albertan, where are you at?

If near Edmonton, look up Personal Transmission on the East Side of the city. Russell (not me, lol) does excellent work. We had him rebuild the 700r4 in the 92 we used to have, which towed a large 5th wheel camper regularly, and it held up really well.

[Albertan]

Say VFRDUDE, what were the RPM"S at highway speed with your 3:07 4speed auto set up??

The truck has a 402 big block, built with torque in mind. I believe this engine will do just fine at 2000 RPM @ 65MPH pulling the Model T. Like I mentioned before it's not a heavy load that I pull.

Also Thanks RUSSELL I will call Edmonton and see what they have to offer. In Calgary I keep getting referred to the race shops. The race shops don't offer a warranty and you pay a lot more.

[CHEVY & GMC FOREVER]

Check your tourqe converter for slippage cause that's what it sounds like to me....

[Albertan]

I did not know the torque converter would slip that much, with out showing any other problems. Do I have to take it to a shop or is there a way I can check it?

I have a stall converter that came with an other turbo 350 I have. Would that work in my truck? I beleive the fellow told me it was 3000 stall.

What does a stall converter do? How do trhey work?

[vfrdude]

Albertan -

I don't have a tach, but the calculators available had RPM at 65 with 255/60r15 tires, out it right at about 1735. You're 402 would handle it better than my 350. Using 3.42 gearing, it would have put in at about 1935.

I was divided between 3.42 and 3.73, finally opted for 3.73 and I'm really happy with the set up.

[Albertan]

whoa!! 1735 is way too low! The poor 350. Yes your engine must have been lugging quite bad.

I wish I could trust the calulators, the one I used said just over 2000rpm. with my tires.

Any way my truck runs good now , but as CHEVY AND GMC FOREVER suggested , I am going to have the converter checked.

[JBstor]

Hey Albertan!

Is the TH350 original to the truck? Tourque converters ( unless they are the newer, lock up kind) are basically, one fan attached to the engine, and one to the trans, all encased in a hollow curling rock shaped casing. The fan from the engine blows oil onto the fan hooked to the trans, and makes it turn. This will not change unless the bearings go south inside, but if that happens you will usually know about it.

How long have you had the truck? I used to get guys come in after getting a speeding ticket and try to buy different speedo gears (the ones in the trans) in order to change them and then take the vehicle in to have the speed verified, all to beat a speeding ticket.

The cheap way to check is to drive beside a buddy with a real new car (digital and all that) Verify different speeds with a couple of cell phones. After you know the speedo is close, then get the tach checked.

If you are thinking of changing Trannys, you could always switch for a TH350 with a lock up converter, and simply attach a toggle switch to the lock up. The lock up is a little clutch they put between the fans to lock them together at highway speed. You gain a couple of hundred RPM (lower) when not towing, and have a normal TH350 when you are.

HTH
JB

[JBstor]

OOPs, you also asked about a stall converter.

If I read that corrctly, you mean a high stall converter.

Here's how it works.

A stock converter, the "stall" is, lets say 700 rpm. That means when the engine "fan" in the converter reaches 700 rpm, the oil blowing on the trans "fan" starts to move the car.

A high stall is usually made smaller, and they bend the "fan" blades. So what happens is that the trans "fan" doesn't move that car on a 2500 stall converter until the engine reaches that rpm.

High stalls are used usually in racing, to get the engine speed up (into the real power range) before the car launches, sorta like revving the engine and dumping the clutch on a standard trans.

Clear as mud huh?

JB