Rear Gears for gas mileage

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norton

norton

Doing the best that I can
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I really would like to improve my gas a little, without touching the engine. What are the best gears to run in an 8 3/4, limited slip diff. I'm not even sure what gear ratio is in it now.
This is all that I know, it's a 742 casing that's has Richmond gears and Auburn brand, limited slip unit. Any advice would be appreciated.
 
You need to find out what gear you have in there now. Also, what's your engine combo?
 
I have a 3:23 in my Swinger with a 27" tire and a stock 318-2 barrell and I get 17.5+ on the highway.
 
toolmanmike said:
You need to find out what gear you have in there now. Also, what's your engine combo?
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This is what I know, a 1970- 340 engine 30 over, street thumper cam, j heads 2.02 intake and upgraded1.94 exhaust valves. Also has an, Air-gap dual plane intake with a 600 cfm Holley 4 barrel carb. The trans is a 727, with reverse valve body and a 2800 stall B&M torque.
How do I determine what gears are in the rear now ? I'm as dumb as a stick when it comes to this stuff.
 
Mark your tire and yoke. Count how many times the yoke turns to turn the tire one revolution.
 
With that engine combo you won't get great mileage regardless of the rear gear. What kind of mileage are you getting now?
 
Sounds like you have a tuning problem more than anything else. The rear gear might be making things worse, but you should be getting better mileage than that unless you're running 4.56's and I would expect you'd notice that.

The '68 340 in my Duster is .060 over, 9.8:1 compression, 308 heads with 2.02/1.60 valves stage II ported and flowing 264 cfm @.500", has a lunati 60404 so .513/.533 and 234 int./242 exh. duration at .050, 750 Holley double pumper with mechanical secondaries, eddy air gap rpm intake, 20* initial timing with 34* all in mechanical (no vacuum advance), 4 speed and 3.55's with 26" tall tires. Engine should be around 400hp, but I haven't had it on a dyno.

I've done as well as 15mpg on the freeway, and typically I get around 12 mpg range around town. It was down in the 8-10 range, but that was before I hooked up my wideband AFR gauge and improved the tuning. Considering I don't run any vacuum advance and I've got a 750 DP on it there should still be room to improve a little even from my numbers.
 
That cam you have in it is probably the biggest reason for bad fuel mileage. It's designed to idle like a race car so it isn't very efficient. If you want it to thump expect bad fuel mileage. Tuning, as 72BlunBlu said can make a big difference too. If it were me I'd put in a decent cam that'd actually make good power and idle a little lumpy, something like a Lunati 60403. I ran the next size cam bigger than that in a 360 with 3.55 gears and got 14 mpg.
 
Thanks for all the information guys. I will start with a good tune and possibly look at changing out the cam. I`m also going to check out the gear ratio, as I'd like to know what I have.
 
So I, marked the tire and yoke and the results are, for every complete revolution of the tire, the yoke turns 3 1/2 times. With this information, can someone tell me my gear ratio.
 
Please forgive me for the stupid question that I'm about to ask. I hear people talk about gear ratios all the time, I hear numbers like 331s, 355s and 373s and a bunch more. So here is my question, what is the difference ?
Mr Gorsky said:
If both rear wheels spun in the same direction, 3.55:1.
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Thank you Mr Gorsky. I did make sure both wheels turned in the same direction before doing the count.
 
How many times the drive shaft turns for one rotation of tire
 
The rear differential (amongst other things) acts as a torque multiplier.

You have an 11 tooth input pinion gear attached to the yoke, and a 39 tooth output ring gear attached to the axles (3.545 output revolutions, for one input revolution). This ratio is rounded up to 3.55:1.

The higher the number (3.55 in this case), the higher the torque. This helps the car accelerate quicker. The trade off is a lower top speed in any particular gear (you reach redline quicker), and higher engine revs at a particular speed (compared to a different ratio, like 2.71:1).

You can equate this to your push bike. Big rear sprocket, more acceleration, less top speed. Small rear sprocket, less acceleration, more top speed.
 
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in a nut shell your maximum or most efficient lo speed torque must be your cruise rpm
and that is determined by your camshaft.I have seen guys go to overdrive and get worse mileage because they cruz at a very inefficient rpm...gears have the same effect.
there are other factors conspiring against you like the air gap that because it is cooler does not
atomize fuel properly.Big valves,lumpy cam make good power at high rpm but not at cruz speeds and that 727 is a power robber.
..Using vac advance,tuning and operating at 190-195 degrees all can help.
 
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halifaxhops said:
3.23 are the best for both worlds
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Not always. The tire height affect the final drive ratio just like gears do. So to say 3.23's are the best of both worlds can be accurate or not depending on what rpm your engine performs well in and what tire height you have. Another factor is if you have an overdrive trans or not.
 
I always have an extra center with 2.76 gears, open or sure grip for road trips. Not sure what rpm you would be turning and how it would work with your cam and torque convertor. A Performance Indicator, or vacuum gage, is a good tool to learn to drive for mpg. Highest vacuum for a given speed.
 
I have 3.55:1 and 26.1" tall tires. Fun around town...not so fun on the highway!

The ultimate would be like @66fs and have a separate center with a highway gear for road trips.
 
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