[trifivefan]

I have a 9" Ford rear end with a turbo 400 tranny. Around town this set up works great, but on the freeway the engine is working way to hard, the gas is being guzzled and the noise inside makes it hard to think. I know that I could go to a 700R4 to get the overdrive. However, is there is cheaper route by changing out the gear ratio that would give me a highway gear and lower the rpm's on the freeway? I am not sure what the ratio is--a buddy of mine counted the splines on the axle and thinks it is a 2:69 ratio. Does this sound right?

[joe350s10]

how many rpms are you turning at 55 mph?

If your engine sounds like it is revving too high at highway speeds I doubt the rear end ratio is a 2.69. Probably more like a 4.11 or a 3.73.

With a 2.73 ratio my truck turns @ 2000 rpms at 55 mph with my 265 75 r15 tires.

The best way to determine your gear ratio is to count the # of teeth on the pinion and ring gear...then divide the # of teeth on the ring gear by the # on the pinion. this will be your rear ratio.

[Centerline]

You don't determine the gear ratio by counting the splines on the axle. To get a rough estimate put the rear on jack stands and rotate the passenger's rear tire one complete revolution and count the number of times the driveshaft rotates. As an example.... if the driveshaft rotates a little over 4 times for one revolution of the tire then your gear ratio is probably around 4:11. If it rotates three and a half times then your gear ratio is around 3:50.

The inexpensive way would be to go to highway gears somewhere around 3:23 or so. I suspect your current gears are near the 4:00 range. If so that will make a big difference in fuel mileage and rpm on the highway. However acceleration will suffer somewhat around town as a result.

[trifivefan]

Thanks, Centerline. Are transmission repair shops the best places to go to for this work?

[farna]

Rotating the tire only works if the opposite tire is held still. If the opposite tire rotates in the opposite direction, divide the tire rotations by two for every rotation of the driveshaft. If the opposite tire rotates in the same direction, you can use the exact number of tire rotations. If the rotation is the same you have a limited slip, opposite is an "open" (standard) differential.

Removing the back cover and counting the number of teeth on the ring and pinion gears, then dividing, is the only sure way. If the pinion has 13 teeth and the ring 43, the ratio is 43 divided by 13, or 3.30769. Round to the last two decimal points for a 3.31:1 ratio.

Tire size affects engine rpm greatly! Most V-8s should cruise at around 3,000 rpm at 65-70 mph (2,000 at 55 mph is a good target though). Go to www.4lo.com and play around with the gear/speed calculators. There's a tire size calculator to find the diameter of your tire also. On the gear calculator enter "1" for transfer case ratio since you have no case (1:1). This will show you how changing tire size affects the ratio and what to change. I wanted to run a 65 series tire on my car, but by running a 70 series of the desired width I got a better cruising rpm, which means better mileage.

[Centerline]

Originally Posted by farna

Rotating the tire only works if the opposite tire is held still. If the opposite tire rotates in the opposite direction, divide the tire rotations by two for every rotation of the driveshaft. If the opposite tire rotates in the same direction, you can use the exact number of tire rotations. If the rotation is the same you have a limited slip, opposite is an "open" (standard) differential.

Quite correct. I assumed he was talking about a posi unit.

[trifivefan]

Farna:
Thank you for the great information. I will access the web site you mentioned

[madamstn]

Are you just looking foe a low gear or axle change. Ford used to make gear for a 250. If you dont want to change every thing, see if your trany was in pickups. Then you can have the made. Differential numbers are not exact what they call it, depends on several things, one being how close another axle is to it. The numbers on the gears are real. better count the teeth. Get the axle code off the tude, if it has them I don't about a ford. Usually or passenger side stamped into the top of the tube. There is no way to disscus gear theory here mark

[Siggy_Freud]

I went the other route and installed a 700r4 in my truck. I used to cruise at about 3100RPM on the highway and now I am at a nice 2300-2400. You can get lower depending on how/when your converter lockup comes into play.

[madamstn]

I have a few questions after reading a bit. You have a ford 9 in. I should go look at a picture, but is there a back cover an that axle. The entire front section comes out ,doesn't it, Or am I thinking about another axle-I don't know a lot about ford axles. If it does, it's easier to change axle gears. They used to make a 250:1 gear set. Need some low end torque to run that. Gm makes 2.73,3.08 3.40,353,373, then I don't know, but there will be others4.10 -4.11 some where in there.I have run 2 trucks with 2.73- really poor if you use a truck. 3.08 is alittle beter ,340 and 353 or there about is good, I had 2 70's trucks with 3.73. Generally ny choice,but with gas going up, running lower is good. I have run Dana Spicer 3.07, 3.73. 3.92. and a scout with 3.97- the only time I have seen that one. Point being, you can about anything you want US made. Need to decide what you want first. Go find a petersons book or similiar.4 wheel drive mag., If you want to smoke your tires at a stop light, you will find it hard to do at 3.08. I am looking for a transmision, 4speed, unsyncronized low gear . I have a couple of old tough 3 speeds with a tough low gear, but I realized I have to change some because of fuel ; and they have been obsolete for 20 years. Every thing I need has to be made. good luck m

[Deuce]

Originally Posted by trifivefan

I have a 9" Ford rear end with a turbo 400 tranny.

A 9 inch Ford has no removable cover on the rear end...



The entire center section comes out the front.....




Ford offered ratios, from the factory, ranging from 2.50 to over 4.86
A 3.25 or a 3.50 is a good ratio for decent street use....and some highway driving. A 3.00 is also a fair gear.....if you have decent power....to pull it ...

2.75 and 3.00 gears are very plentiful ........at least in my area.....and run $100 or so..for a decent used center section.....

They are easily installed with just a little common sense and a few common tools......

.

[madamstn]

Good info. I remembered the 2'50 in a ford' froma mag article with a 455 Pontiac pulling 425 hp a 5000 rpm, and huge torque. I have everthing except the 455 block-plenty of 400's just no big bore. They are nice to work on from what I have watched. My lodal junk yard owner bugs me periodically, for racing stuff, but I don't want' to part with with the thngs he wants. If I were doing this Iwould set 2 up. 250 and upcloser to 400. I like Spicers, 44's mostly, but the price around here is 75, Might be me-he gits a lot of referals and I buy a bit. I see more GM 10's so I remember them better. I am running 2 old trucks around the farm, etc, one 273 and the other 373.350 TH and 350 ci engines, about stock on those. The 2.73 is really stretched off the highway. Off the road, the fuel economy is not that much better- depending on when everything else kicks in. Making some assumptions I would say I would recommend the 325 ballpark depending on whose axle and tire size. I originally stated to go read a book, decide on a goal. I wood still say about the same I assumed he didn' have a posi. My 273 is OEM posi , It does close the gap a tad. What do you think. Today's Ford man, and all. Mark