3.27 or 3.55 swap?

[DD86SVO]

Bud Morton enlighted me on this area. He states that when the revs are above 3150, the computer thinks it's in boost mode. So I'm gathering that the computer richens up the mixture at that point. Other than that, I haven't yet done an MPG calculation on the 86 I currently own, but my old one got around 25-26 MPG.

Tony

I know this has been brought up several time. My question is for someone (me) that is only go to use the car as a daily driver for a little over two years, and travels on roads where the posted speed is 70. Is it worth changing the gears to a lower one due to 70 puts it in the suck down my gas category. Still keeping the 3.73's.

[BrianO]

I did a similar swap since I drive 80 miles a day for work..mine came with 3.73's and 70mph was right at 3k rpm's which was borderline open loop for fuel. I could get no better than 23mpg.

I put the stock 3.45's back in the car and at 70mph my rpm's are about 26-2700, but I normally drive 65-68 and my best mpg to date is 30. I'm happy with that.

3.55's are very close to 3.45's..in fact I think the driven gear is the same for both.

[DD86SVO]

3.27's are two low i guess. anything around 30mpg would be ideal for me, seeing how nothing i have owned has been above 20mpg (2003 4 cam v8 with 4.10's).

Thanks BrianO

[BrianO]

I'm just glad to be able to drive more than 260 miles on a tank.. LOL.. I currently have 389 on this tank..filling up tomorrow.

[DD86SVO]

I currently have 389 on this tank..filling up tomorrow.

Are you on empty? That would be around 32mpg, no matter what thats good.

[BrianO]

I'm guessing I have about a gallon left..I drove home on faith.. LOL the needle is well past the E to where there's a space between them both.. haha

it would be more like 27-28mpg as I got on it a few times in traffic.

[SVOeric]

www.svoca.com/gears
that will give you an idea of what 3.27 gears will do to rpm at speeds
I decided to go with 3.45's too. I'm afraid 3.27 would be low enough my lil 4 wouldn't hold speed without boost.
altho I don't have mine in yet.....

[DD86SVO]

I didn't even think about the rearend size (7.5 vs 8.8). So i would have to go with the 3.45's. I forgot to mention that I have an 84 trans with 3.73's.

On my way home today I stayed below 3000 except maybe once or twice and no turbo. When I got home it was still the same as it has been 21-23mpg. If its not the gears then what is the first cheap thing to look at first?

www.svoca.com/gears
that will give you an idea of what 3.27 gears will do to rpm at speeds
I decided to go with 3.45's too. I'm afraid 3.37 would be low enough my lil 4 wouldn't hold speed without boost.
altho I don't have mine in yet.....

Eric, I tried that, and it put me at 70mph at 3100rpms.

[SVOeric]

did you select the SVO transmission at the top of the form ?
and enter your tire size?

[DD86SVO]

did you select the SVO transmission at the top of the form ?
and enter your tire size?

The top choice is for the trans right? I picked 84
Then at the rear ratio 3.73
Tires 235/40/17's

[SVOeric]

With the 84 transmission, I show
79 at 3000 with 3.27 and stock tires.
75 at 3000 with 3.45 and stock tires.
77 at 3000 with 3.27 and your tires.
73 at 3000 with 3.45 and your tires.

[BrianO]

I've compared Erics chart with the spreadsheet I've used for years and Erics is spot on.

I don't see much of an improvement with just those gears on your car...in my opinion if you plug in the 86 T5 tranny that is world class and use 3.45's and your tire size, that knocks the rpm's down to 2600 for 70 mph. I think I used a 93 WC T5 in mine..sorry, forgot to mention that part earlier. the gear ratios are similar to the 86 T5, but not exactly the same.

[svobud]

If its not the gears then what is the first cheap thing to look at first?

Besides a scan of the EEC to see if it's throwing any codes? A tune up using Ford Motorcraft parts, check your TPS setting and a new O2 sensor ...

[DD86SVO]

With the 84 transmission, I show
79 at 3000 with 3.27 and stock tires.
75 at 3000 with 3.45 and stock tires.
77 at 3000 with 3.27 and your tires.
73 at 3000 with 3.45 and your tires.

I don't know why I keep entering 3.73 when i was tring for a differnet ratio. I get the same as you.

[Pat_in_L.A.]

I've seen contrary info recently after running for a month with the EEC Monitor factory service tool plugged into the car. This is on an 86 with a PE.

I have yet to see closed loop at anything over 2900 RPM. There are two paramaters that govern closed/open loop in the ECU. The low load limit in the ECU appears to be 2900 and high load says 3100. I get rich/lean commands from the O2 sensor up to 2900. After that, even on downhill coast up to 3100 no load, it is in open loop.

Try to stay below 2900 and see what happens.