new gears, different car

I recently swapped pumpkins on the 8 inch. I went from a 2.79 to a 3.00, both are open, stock units. I did not expect the change would be that much, but it is much more than expected. I am thrilled. It drives like a totally different car. It pulls well in all gears, and I can putt along in 4th or 5th w/o lugging the motor. I have the 200/6 with a T-5 from a 4 cylinder mustang (3.97 1st gear, and a .085 OD) This was a 6/8 drivetrain conversion, so a stock 8 inch w/2.79 gears was installed along w/ T-5. It ran pretty good like this. I then purchased a CL pumpkin, w/ 3.oo gears, and did the swap. I think my gas mileage will greatly increase, as it is very happy cruising w/ very little throttle input. It also pulled good to 90 on the highway in 5th. I did not expect to see much change since these 2 ratios are so close together. The car had a 3.2o gear with the 4 lug set up. I never had much seat time due to many other issues with carb, ignition, suspension, & steering. But now it just purrs along. When I had the 2.79, I had to keep the rpm's up (read: keep my foot in it) and I had to shift alot. Lo speed cruising-25-40 mph was difficult.5th gear was also difficult, as I had to be going about 60mph to shift into it w/o lugging the motor. Now it is smooth and cruises in 5th from 45 mph and up. Time to start tracking gas mileage again.

Your in-town mileage may increase, but your highway mileage will decrease a bit with the 3.00:1. Glad it worked out for you!

I know many aren't excited about 3.oo gears in an open unit, but I have found very little info regarding this conversion/swap that I have done with the 6 cylinder motor and the T-5 (4 cyl gearing). I hope others may be able to use the information if they travel down this path. Their are a few 6 cylinder die hards on this board. I try to read all I can from them.

And thanks for the reply Mid, you are right about the mileage on the highway. I have mostly in-town driving, but the cruise on the highway seems as though I may actually see an increase in mileage. I know this doesn't sound right, but I think it may. When I had the 2.79 gears, I had to really stay in the throttle to get the motor up to enough rpm to shift into 5th. (usually over 60) So I was at 1/2 throttly or more alot of the time. I had to stay on the throttle as the car would actually begin to lose speed in 5th if I had a strong headwind or an average hill. This necessitated a down shift, run the rpms up again and shift back to 5th. Now I can be in 5th at 45-50. I can cruise at 65-75 with very little throttle input, far less than 1/2 throttle, maybe 1/4 or less. I will have to test this theory and report back. Prior to gear change, I was getting approximately 20-23 mpg, sometimes less, depending on driving style. I hope to see a little increase but that is with REALLY conservative driving. If I putt around town like my Grandpa, I think I will do really well. That's the hard part. It's so fun to drive, it's hard to go easy. I am liking this 68 coupe we have built as father/son project.

The trans gears setup and rear ratio is always calculated/computed to its engine torque specs.
Thats why you did feel a great difference in the cars agillithy.
Way to go .

I agree, now it's time for a Maredsous!

I went from a 2.79 8" open to a 3.55 8" open rear end. It made a considerable difference in my 1/4 mile e.t.s, and a huge difference in my gas mileage. Gas mileage went from so so to abysmal. I get around 10 mpg around town and 13 mpg highway. I am, however, a lead foot. The car probably pulled down 15 - 17 mpg when it was a stocker with a 2100 carb. I don't really know because the carb, intake and cam were the first things to go after I bought the car. I've no idea what the 1/4 mile performance was with the 2V, but it was running in the high 15s prior to the rear end swap. The rear end alone got me down into the low (15.04) 15s. The best e.t. I've run in its present configuration is 14.308. Not exactly heart stopping, but respectable and it required a second cam transplant, GT40P heads, Motorsport roller rockers and a Road Demon 625 cfm carb to get there.

coupster said:

When I had the 2.79 gears, I had to really stay in the throttle to get the motor up to enough rpm to shift into 5th. (usually over 60) So I was at 1/2 throttly or more alot of the time. I had to stay on the throttle as the car would actually begin to lose speed in 5th if I had a strong headwind or an average hill. This necessitated a down shift, run the rpms up again and shift back to 5th.

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That brought a smile to my face. When I worked for Ford Aerospace I bought my wife a 74 Pinto Squire station wagon as a lease car turn-in. Cute little car with just about all the options available in 74, including A/C. The car was a 2300 cc 4 cyl. 4 speed and, if you were cruising in 4th gear on the highway, when you turned on the A/C the car would noticeably slow down. After we had our second kid she demanded a 4 door wagon, so I sold the low mileage Pinto to a kid who promptly stuffed it between two utility poles and totaled the car. The wife got a black 81 Mercury Zephyr Villager wagon as a replacement. We all sort of hated that car.

Yes, sometimes 200 more RPM can make a big difference especially oat lower (2,000-ish RPM) area of the torque curve.

Everyone does gears near the end of long list of mods, but honestly they should almost be first. I mentioned on a recent thread that we got my son a 351W powered 69 Cougar.. I can tell he's less than impressed with the acceleration. With the FMX 2:40, 1:47, and 1:1 in front of a 3.00 open rear, he's got terrible gearing for acceleration. I'm nudging him towards an AOD and 4:11's to wake the car up but with the overdrive still have what is effectively a 2:75 rear ratio for highway.

gsxrken said:

Yes, sometimes 200 more RPM can make a big difference especially oat lower (2,000-ish RPM) area of the torque curve.

Everyone does gears near the end of long list of mods, but honestly they should almost be first. I mentioned on a recent thread that we got my son a 351W powered 69 Cougar.. I can tell he's less than impressed with the acceleration. With the FMX 2:40, 1:47, and 1:1 in front of a 3.00 open rear, he's got terrible gearing for acceleration. I'm nudging him towards an AOD and 4:11's to wake the car up but with the overdrive still have what is effectively a 2:75 rear ratio for highway.

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Look into a stall converter for the FMX. My son's 69 Mustang SportsRoof was a 351W with 2.75 open gears. We swapped in a set of 3.73 traclocs and a 2400 rpm converter and it made a huge difference in the car's acceleration performance. Unfortunately, the car is really not fit for highway driving any more.