Choose rear gearing

Topic: Choose rear gearing (Read 1623 times) previous topic - next topic

Choose rear gearing

November 04, 2007, 11:38:27 AM

While this car isn't a race car, I have a 93 Cobra top end to put on my new motor that is to be finished balancing this week. Since both the E7 and GT40 heads/intakes push the torque curves further up the rpm band, I was trying to figure out which rear end gears would work best for highway economy. Since the motor would be more efficient up higher, would say 3.08 or 3.27 gears do better at 60-80mph with an AOD than the stock 2.73's? The rear that will be put in currently has 3.55's.

I may very well swap these heads out with gt40p's at a later date after I get some headers that will work on the P's installed on the current setup, all in helping keep the performance the same but bumping up efficiency a bit. Of course, that only depends on what I can pull off with the current motor build. From what I've read, adding a supercharger (motor is built with it in mind) can give a slight bump in efficiency at highway speeds so if I can pull off 28-30mpg highway when taking trips, I'd be happy. If not, the heads will get downgraded.

So what gears?

Choose rear gearing

Reply #1 – November 04, 2007, 06:53:06 PM

I've always said I think 3.55's are the best "all around" gears.Good for having fun,and getting some decent mileage.My own opinion of course.

Choose rear gearing

Reply #2 – November 04, 2007, 07:07:35 PM

The highest gear you can go and still have the speedo be accurate with your current aod is 3.27s with a 21 tooth speedo gear(highest ford makes).
If you get a mustang aod you can go 3.73s(my current setup)and still have the speedo accurate with a 21 tooth speedo gear.
If you don't care about having your speedo accurate then i would say 3.73s are the best all around gear.
At 2500rpm on the highway i can do 120-125km's an hour.
However if you already have the 3.55s or are getting them for free or cheap then by all means use those.
Speaking from experience(running a supercharger and 3.73s with a h/c/i/ swap on a 5.0ho)you won't get 28-30mpg on the highway.
I currently get about 400km's per tank on the highway with the smaller turbocoupe tank which is 75 litres i think compared to the regular bird's 90litre tank(don't quote me on the sizes).
As for efficency,yes the sc makes the engine more efficent but hp uses fuel no question about it and sc's will use more fuel than an N/A setup.

Choose rear gearing

Reply #3 – November 04, 2007, 11:27:06 PM

Hmmm, the speedo gears make a good point. Guess bigger tires would fix that :p

I'm really skeptical about going with anything above E7's and maybe even the larger intake due to the less efficiency down low as again, this isn't a track car. I figure I'll give the motor a try how it is and see how it goes, keeping E7's and an HO intake on standby if mileage ends up being horrible at <25 highway. Anything's better than the SO really. I'm not sure if I could push 9psi non-intercooled with 8.8:1 static compression on the 92 octane around here, but it'll get at least 6psi easily to give a good 40% power boost when desired.

On a related note, I seem to go from fuel spilling out the top to 20 miles into "low fuel" on 16 gallons of gas, worried to run it any further. I've never had to drive a full 500 miles one way, but frequently make 145 mile trips to and from Seattle on 4.5-4.7 gallons each way with the SO running at 70mph average (there's 60mph and 70mph speed zones/limits).

Choose rear gearing

Reply #4 – November 05, 2007, 11:31:12 AM

Quote from: dominator;186210

The highest gear you can go and still have the speedo be accurate with your current aod is 3.27s with a 21 tooth speedo gear(highest ford makes).
.

You can use an aftermarket AOD gear for the T-bird/Cougar AOD that has 23 teeth and is pretty accurate with 3.73 gears (~2 mph off);) . That's what I use. I can't vouch for long term reliability as I only drive my car ~1,500 miles a year. Mine has been fine for 2 years though.

If you already have 3.55 gears use 'em. You really won't notice much of a difference going from 3.55 to 3.73 gears.

Choose rear gearing

Reply #5 – November 05, 2007, 07:10:07 PM

The 23 tooth's don't last long(extra thin)had one once,lasted 5k max.

Choose rear gearing

Reply #6 – November 05, 2007, 08:24:50 PM

Quote

Seek
but frequently make 145 mile trips to and from Seattle on 4.5-4.7 gallons each way with the SO running at 70mph average (there's 60mph and 70mph speed zones/limits).

Are you saying that you went 145 miles one way and only used 4.5 - 4.7 gls for that leg ??? That works out to 32.2 mpg and you did that with a 5.0LV8??

Choose rear gearing

Reply #7 – November 05, 2007, 10:36:58 PM

Quote from: 1BadBird;186354

Are you saying that you went 145 miles one way and only used 4.5 - 4.7 gls for that leg ??? That works out to 32.2 mpg and you did that with a 5.0LV8??

Its appears to be down to about 30mpg now ever since I had to go back to my mechanical fan (repaired controller is on its way, need to pick up a new MarkVIII fan as mine quit spinning) and my thermostat appears to be stuck wide open. I frequently enough make that trip (family) to get 4.8 to 4.9 gallons used per direction on most occasions. Once I get off the freeway and drive 5 or so miles through stoplights, it shoots up to ~5-5.2 gallons used. Round trip I always expect to see between 10-11gallons used. I have noticed that not using cruise seems to net me another 1mpg. Is speeding up and coasting back down randomly more efficient?

Of course, this is only driving 65-75mph the entire trip, none of the 32mpg (my best was 35mpg for a 73 mile stretch at 60-65mph) was achievable if/when I get in the mood to zip around, dropping the average of the trips to 28-29. Other than a worn out/noisy rear end, the car's comfortable to just make a trip in and cruise at normal speeds without slamming the throttle after each corner or to pass all the other drivers. 5 Layers of sound deadening mat, 4 layers of 1/8" Ensolite closed cell foam, and air gaps filled with 1/4" closed cell foam sheets around the passenger compartment help take it that extra step.

Lastly, this is going off the tripminder's fuel usage readings but calculating the distance manually/gps as my speedometer reads ~8% slow. I have found tripminder fuel usage readings to be accurate to pump readings over a month period of time I kept track of it when it was first installed.

So yes, I have seen 32mpg frequently, but I'd say that when I just want to get there, I get ~28-29. My tripminder itself reads 28.5-29mpg and only 135 miles for the trip. Like I said, I'd be happy with high 20's as in just under 30. On the highway, it doesn't matter as much whether you have v8 or not, the torque down low will help pull the car. I average 21.5mpg driving to and from work on a daily basis, I'd say 60% highway, 40% stop and go or so.

2 years ago I was getting 24mpg highway and 19 average before throwing HO intake parts on there...

Choose rear gearing

Reply #8 – November 05, 2007, 11:23:55 PM

wait...everything I can find states that the 7 tooth drive gears were used up until 89 when they switched over to 8 tooth? Which is it on the 88's stock aod? I know mustang information of the most prevalent on the net, but it seems to stand true for ALL aod's...Is it only due to the fact that it came with 2.73's from the factory?

Choose rear gearing

Reply #9 – November 06, 2007, 07:38:21 AM

Pretty sure it's an 8 tooth.
Mustang aod's where 7 tooth and stick's where 6 tooth.
My old 88 had the 8 tooth,my now stang tranny has the 7 tooth.

Choose rear gearing

Reply #10 – November 06, 2007, 09:12:17 AM

Wasn't there a 2.73 Mustang with an aod long ago? Any idea what it had? Just curious.