gear selection help.

[Guest]

[69 Merc]

It's that inefficient 5000 TC doing it to you...

When I did the gear swap years ago it only had a 2500rpm stall T/C ...

[85_GT]

So you are saying that the city red lights launch hurts the mpg that badly compared to a higher cruise rpm mpg killer?

Yes.  

Your red light acceleration will put you into fuel enrichment, no?  The less gear, the more time you are in it.

[69 Merc]

Thanks, I learned something good today!               

[95GTspeeddemon]

i got 17.5 mpg with my big ol everything 302 on a 70mile trip with most being highway.

[MeanMussy]

As mentioned, a gear swap is more than just bolting the things in.  You'll want to measure backlash and dial in the mesh pattern.  Here's an in depth write up:
http://www.corral.net./index.php/tech/drivetrain/52-88q-gear-installation

As for which gear, if you're trying to keep it streetable (in town and highway), then I would suggest 3.73s.  Coming from the stock 3.27s, it will still feel like a new car. 

Back when my car was stock and had an AOD (and was still in one piece), I swapped my entire rear end for one out of a T-Bird Turbo Coupe.  I went this route for a couple of reasons.  First, I wanted different gears but didn't have the tools, the gauges, the garage space or the know how to swap gears and not end up with a whining, grinding disaster.  The T-Bird rear end already had factor installed 3.73s installed in it.  And, it had rear disc brakes.  And it bolted right up to my Mustang.  I only recently pulled it out (it's on my garage floor taking up space, for sale, looking for a new home) when I decided to go crazy with my suspension and a 5 lug swap.

After installing the rear end, it was like I bought a new car.  The fun factor was back.  And, I could still take the thing on the highway and not hate it.   I found my highway mileage dropped a couple of MPG, but at the same time my city mileage (when I had the discipline to keep my foot out of the happy pedal) went up about the same.  It was about a wash. 

I'm sure it would have been a little quicker off of the line with 4.10s.  But I didn't want to turn 3000 RPM going down the highway.  For me, the 3.73s were the right balance of performance and streetability. 

[Haystack]

My thunderbird with 2.73's and an aod was right around 1400rpm at 65mph and 1600rpm at 75. I averaged about 27mpg highway. Doing 70mph in the 75 zone and 60mph in the 65 zone got me to 30mpg and speeding up to 80mph in the 75 zone ended up with about 25mpg.


Speed has just as big a factor as your gears, but it should be matched to your drivng.

[FordRacing250]

So you are saying that the city red lights launch hurts the mpg that badly compared to a higher cruise rpm mpg killer?

yes. I can drive on the freeway at 90+ and get 13mpg or better, me just cruising in the city i will only get 10.
It takes more power to get up to speed then it does to maintain it. Driving on city streets and going red light to red light kills mileage.