What gear ratio would you run?

I will soon have enough money saved up for a posi for my 66 C10 and this is the time to get a new gear ratio. It has a 455 olds putting out around 300-350 horse with a 3 speed turbo 400 behind it. I have never messed with gear ratios before so I am a bit clueless. I need a bit more passing power on the freeway and I will always take +mpg but cant afford to lose any and before anyone asks I don't know my current ratio but everything inside the axle housing is out of a 70s or 80s 2WD chevy truck (previous owner) if that tells you anything. I really appreciate any help you guys can give me.

 

3.23

 

If your location is G.V. Calif., does G.V. stand for Grass Valley? If so, call JAWSGEAR in Sacramento at 916-922-5297. Good people. Fair prices. JAWSGEAR has built two chevy rear ends for me.

 

I like 3.73. But I'm running tall tires and a 4spd. auto.

 

You are going to have to know that you have now to dial in a good ratio for your needs.Jack up your truck,put it in gear,mark your driveshaft and your tire.Next spin your tire one full turn and count how many times the driveshaft spins.Thats your ratio.If you count about 3 1/2 turns you have 3:55.If you want better top speed(less r p m )go to 3:23.Look up gear ratio calculator ,there is a couple of them on the net.

 

you have to find out what you have first , as Gm made several gear ratios available for truck in the 1970s and 80s , depending on the chassis and motor/trans and tire packages the owner wanted when he ordered it it can be from 2:73 to 5:38 , most trucks run a mid 3 series gears , around here in the chicago area most of the autos 3 speed trucks have low 3 series gears . those are what we call putters as they putt as they go down the street , one of my old c20s had 4:30's in it but had 16.5 tires on it , as it was a commercial used truck . you can do wha dan said or drop the rear cover and get the numbers off the gears themselves . general rule of thumb is 2000 rpm for the interstate at 60 mph cruise 1800 minimum , with a 29.5 tall tire that would be a 3:08 , if you tow it varies with the ammount of weight your pulling , a car trailer with a car ( apx 5500 pounds total , 3:73 's

 

Gear ratio calculators are great, be sure to use one that you can plug your tire diameter into.

Bob

 

Got a tach to know your cruise rpm?
CAn't afford to lose any mpg? Changing to a numerically lower rear ratio for better passing power is very likely to cost mpgs.

I'd think a decent running 455 Olds with TH400 and it's helpful torque converter would have decent passing power even without a downshift.
I might go thru the exercise of determining the rear axle ratio (mind the differential action if 1 wheel is on the ground), but then I'd spend some time mapping out the ignition curve and playing with the carb

 

You must have a steep gear if a 455 needs help in passing gear or really tall tires. You need to get the ratio and tire size posted on here. Then every one will figure out what you need. 3:70's is a good all around gear. If you are running 31" tires or larger you might want 3:90 or 4:10. The lower gear might even give you better gas mileage. You lose mileage if you lug the motor too much.

 

check calculators at: project33.com

 

My '85 C-10 has a 2.73 from the factory. If that's what you have, go something like 3.08, maybe 3.23 and you'll probably be okay. Don't be tempted to go too high, though: I have a 3.42 in my '53 Chevy and it turns 3,200 rpm at 75mph, which is too damn high for my taste.

Seriously, though, with a 455 Olds you should have all the torque in the world. Is it running properly? Does the trans kick down as it should? My '85 with a mild 350 in it does fine on the highway with the 2.73. I wouldn't race anybody, but I've never felt like I needed more power to pass on the highway?

 

http://www.csgnetwork.com/multirpmcalc.html here is a calculator.

An auto trans will pull a taller gear as opposed to a manual, but many factors come into play. Many say that the sweet spot of RPM is 2000 or just abit above, however, I have driven many newer cars that run 12 to 18 hundred RPMs on the freeway. Sweet spot?

 

The response is likely a bit better w/the newer electronics. I'd guess that the 3.23 would let the 455 open right up, the 3.08 would come on as it had 2-3 secs to turn. Pure estimates, in that I ran 3.08 for years.

 

I don't think the advice offered above would correctly determine the existing gear ratio.

Assuming an open diff.

Jack up one rear wheel just clear of the ground.

Mark the driveshaft at a convenient point.

Mark the wheel and the ground at a convenient point.

While watching the driveshaft, rotate the free wheel exactly twice until the marks on the wheel and ground align and stop.

The number of turns of the driveshaft equals the axle ratio.

Once you know this you can decide whether to go lower or higher.

If you do not know the axle ratio, you could swap in a new rear and have exactly the same! What use would that be?!!

Mart.

 

Depending on tire size a 3.23 gear should give you some jump with a 28-29" tire. But, a 3.08 will do good with a smaller tire say in the 26-27" range. In a larger tire you may want a shorter gear.

 

There is not a magical arbitrary number of 2000 for a sweet spot.
The builder decides the sweet spot which is close to the first turn on the torque curve. This is where the engine begins to reach high volumetric efficiency.

 

Your turbo 400 needs the kick down solenoid to receive 12 volts when the throttle is near WOT. Everything will sort of work with out it being hooked up but the difference is similar to being shoved by a 100# cheerleader VS a 335# linesman with it hooked up.

If your olds is running properly, I think a 3.08 will be a good gear because there's plenty of torque to pull it. My 307 sbc belly button with a turbo 400 and 308 gears will completely obliterate the tires if I stick my foot into it.

 

Even low performance smog Olds 455's put out so much torque you won't have to worry about passing power, trust me.
I think 3.42 is it

 

Yeah, honestly, you don't have to (and shouldn't) go overboard on gearing for a not far from stock 455 Olds. They were made to haul 5000 pound boats around with 2.56 gears, and make loads of low end twist. Same with the big Pontiacs; I had a friend who drag raced a '76 T/A with a 455/TH400, and he made his best times with a 3.08 rear.

 

I've run everything from 3.08's to 4.56's on the street, and about the nicest was 3.42's behind a 455 Buick/TH400. You might need slightly taller tires with 3.42's to gain the mpg you are after.

If you have a Q-Jet, you should get great mileage, plus tons of power when you open up the back barrels. Sad to say that's when your mileage goes into the tank. If there's a possibility of using an O/D trans, your mileage will jump right away. Most of the newer ones seem to run at .70 in O/D.

 

3.55 or 3.73 off the top of my head. Big blocks are more tq, and Gear ratio is also related to TQ not so much HP......

Still many questions, like..... Cam range, and tire height....

My gear choice is what I would run with a 30 inch tire and a mild cam, and a converter either stock or with a slightly higher stall.....

I am in total agreement with 2000 as a sweet spot, as long as the converter is locked or flashed..... otherwise its just makin heat....

300 rpms is the rule for gear changes.....

 

I'd go 3.08. It will give you about the best fuel mileage and when you go to pass the governor in the trans will let it kick down to second when you floor it at 60 or so. Provided you have the electric kickdown hooked up.

 

I knocked together a 300 net horse 455. At that power level you have to peak around 4,000 or so.

275 HP from about 3,000 to 4500 so that's the meat of the power range.



With a 235/70 - 15 rear tire and a 2.75 gear here is what a downshift at 65 MPH and accelerate to 92 MPH looks like. Of course more info = better advice. BTW, a 65 MPH downshift with a 3.73 = 4600, top in 2d would be like 70.

 

I will check the ratio tomorrow then get it on here also no kick down previous owner put on a $10 foot shaped pedal then welded lots of washers on both sides so it aint coming off easily, will the kick down work with that pedal?

 

with a th400 the kick down is electric , its a switch that mounts on the carb throttle assy you can find them in a salvage yard , e -pay or B&M sells a kit ( cheaper to find the factory carb style ) it really don't make a difference on which style you use , olds cars used them on the pedal assy under the dash by the fire wall and they had to use the oem pedal and linkage and you have to drill holes in the firewall for wires , I know a few guys who have them mounted elsewhere , one has a pushbutton mounted on his steering wheel , another on the floor ( old starter switch ) , so they can kick it down without having to hammer the gas to do it ( for quiet fast get aways or smoking the tires while rolling )

I checked the gear ratio on my O/T 96 3500 with a 7.4 vortech 290/410 tq , the gear ratio in o/d is 3:08 d/d 4:10 and it will move my 6900 pound pick up with ease with the 31" tires ( 245/75 /16) in O/d without kicking down , so a 3:08 should not have any problems with the torquey olds motor , a 2:73 would be ok if you had the kickdown hooked up but might hurt milage as the motor would be lugging (putt /putt ) .

heres a complete unit with wires for 49 bucks if you want to get rid of the bigfoot pedal http://www.ebay.com/itm/Chevelle-SS-Monte-Carlo-etc-Gas-pedal-with-kick-down-switch-/111154962643

 

Acquire a subtley built 700R4, a 9" Ford that is wide enough, run 3.73's with 31 in tires. My 32, which of course is lighter, gets 24-25 mpg at 70 mph at 2050 rpm's. Ford 9"ers are still plentiful, at least here in Texas. Good luck.

 

This will come in quite handy
http://bowtieguy.proboards.com/index.cgi?board=axles&action=display&thread=21

It probably has a g80 gov lock carrier- great for sloppy mud and no traction ice lock up because its kinda weak. Not good for tire smoking launches or High performance use. You'll have to look inside eventually anyway.

The epay link provided by stimpy is exactly what I'd use, but there are plenty of other ways to skin that cat. Try a temp wire to see the difference.

 

X2 if you drive on the freeway.

 

This only works if it's a posi. If it's an open rear you have to double the turns to get the ratio.

 

A lot is going to depend on your cam profile. With a stock cam, you're going to want a low RPM cruiser to utilize all of the low end grunt. I run a short tire, so I went with 2.73s. Like you, I'm using a stockish big block and non-OD trans with a 1:1 final drive ratio. The highway cruising is magnificent, effortless passing with no drama cruising at 80 mph. Not to mention less fuel consumption, less engine wear, and running about 5-10 degrees cooler. The downside being that the car has lost a lot of it's jump off the line and the fun 1-2 chirp that I used to get.