![]() I'm not making a drag racer but i would like to get it so it gets a little respect performance wise. my tires are 245/60/15's with a diameter of 26.6. i've changed gears in vehicles before, but nothing this large. While modding the rear end up to a point can make it peppier at lower speeds, I don't think it's advisable to push it too far. Instead they chose a stock engine/trans/rear gear configuration for good overall performance - and the Bullet Bird was sold as a 'performance' car even if it really did not perform all that well by then current standards. So they had them and could have used them in the Thunderbird. but we need to consider that when the engineers decided to put a 3.00 rear end in the Bullet Bird as stock that Ford was also putting in 3.55, 3.70 and higher rear ends in some of their other (lighter) models. I understand about wanting to be able get the car up 'out of the hole' and into second gear reasonably quickly. I'm personally leaning towards 3.55 for my build. That's why I feel 3.7 is about as high a rear gear as one should put in this heavy car. At street speeds you would still be over-revving the engine quite a bit with a 4.10 rear end. Overdrive might drop that 3200 RPMs at 65 MPH down to about 2400 which would put you back in the heat/friction ball park - but *only* at about 50 MPH or higher. We're not just talking about fuel usage here, we're talking about putting 36% more friction and heat on those internal moving engine parts all the time. That's a whopping 36% more continuous revolutions per minute than stock. Kick it to a 4.10 gear and now the engine is spinning about 3200 RPMs all the time. Go to a 3.70 gear and the engine now turns about 2900 RPM - that's a 24% increase in continual RPMs. and with 16-inch rims and a tire diameter around 28 inches, stock transmission and 3.00 rear gear, at 65 MPH the engine is turning about 2350 RPM. I took a look at the chart you provided a link to last week: ![]() Overdrive isn't good for much for anything else, although it will drop engine RPMs by about 25% - but only at speeds over about 50 MPH. ![]() The only reason to put in an overdrive transmission is if you *are* concerned about better performance and mileage at highway speeds. where I wrote about the potential problems of going any higher than 3.70 on a car that will be used primarily for driving rather than popping off the line race-style cruising. Actually, we had a good discussion of this very thing less than a week ago in this thread: ![]()
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