Hi, I have a ‘63 Mercury Comet. It has a 200 inline 6 and a c4 automatic from a 66 mustang. It also has a 3.50 stock rear end and 13 inch wheels with 155/80 tires. My issue is the car tops out at only 50mph at what feels like a high rpm. Unfortunately I don't have a tach. Can I get a more reasonable speed out of it with bigger wheels and tires?
The limiting factor is your 3-speed transmission and 3.50 gears. You want to get in the neighborhood of 3.0. or get a 4 speed transmission. There's a limit in going bigger wheels/tires to increase speed and wouldn't recommend doing that.
Thanks, ill look into re-gearing it
You will get a higher top speed but it will take you a lot longer to get there.
Well, it's no speed demon, but it should get up to 75 or 80. Yes, 14" wheels would give you more/some top speed, I'd get one on loan from a junkyard and see if it clears fenders front and back. But perhaps your torque converter isn't converting/bands are slipping.
https://www.automobile-catalog.com/auta_perf1.php
https://www.automobile-catalog.com/auta_details1.php
Thanks for the reply. Anyway to check the torque converter? Ill try adjusting the bands as well
If you give it more throttle, does it rev higher BEFORE it downshifts? (Which you should be able to hear) then the trans is not tightly sending power to the rear end. If the revs don't increase before the downshift, you need to look elsewhere. Are you sure it's shifting into top gear? Perhaps the person who put the Mustang trans in didn't connect the gear selector correctly,
The linkage is definitely suspect. Ill play with it and check for those when I take it back out. Thank you!
Probably not much more. Those tires are close to the original 6.00-13 in diameter. What was the rear end gearing and tire size of the Mustang you got the trans out of?
It would be helpful if you could hook up a tach temporarily to see what's going on.
It has the stock 3.50 gears. Not sure what the donor car had in it
Using an online calculator with those tires (22.8") and 3.5 gears with 1:1 at the trans you'd be turning over 3600 RPM at 70 mph. (Actually worse than that with automatic due to torque converter slippage.) You need lower (numerical) gearing for highway driving but that will of course mean slower acceleration.
https://purperformance.com/p-29669-rpm-calculator.html
Thanks, ill look into re-gearing it
well that might get you some more top speed, but it's a trade-off. You will lose some power accelerating from a stop.
What you really need is more speeds in the transmission. Maybe you can add an overdrive unit.
Perhaps some different gearing could help get a higher top speed but acceleration would be worse. I am not sure of the stock gears but top speed should be around 87.
It would be a big help if you could find out the gear ratios within the transmission. Bigger wheels will go faster, but they also have a higher moment of inertia, translated as the "effort" needed to start them spinning. Bigger wheels have more weight further from the axle, which makes them harder to turn from the same size hub. It negatively affects acceleration, but the wheel being larger in circumference means the car will roll further in one revolution of the wheels.
To compensate for this, the manufacturer can play with final drive ratios and transmission ratios, to a lesser extent. As an example, my '99 Ranger has a 4.10:1 final drive ratio, it's a 4×4 with Ford's factory larger wheels. The 2x4 trucks have 3.73s or 3.55s in them because the wheels are smaller than mine. They get better gas mileage on the highway because they cruise at lower engine speeds. Mine pulls more effectively because a 4.10 magnifies torque to a higher value.
A genuine gearhead can play with these ratios as well, if they know what they're doing. It doesn't sound like whoever messed with yours knew what they were doing. If the Mustang transmission is geared low, with a 3.50 rear end it will have a whole lotta low end torque, which a muscle car would want for acceleration, but it won't have much top speed. If you know the tranny model, you can find the gear ratios. They're easy to multiply ratios with the final drive and then compare with the original, if you can find specs.
Drivetrains are actually deceivingly complex, as you're figuring out.
@justin-shepherd - You got your numbers backwards. The higher the number, the lower the gear ration. A 3.55 or a 3.73 gear ration will give your better cruising but lousy acceleration. A 4.10 will give you better acceleration but lousy cruising.
That's what I was getting at, I'm not sure where you're saying I miss-spoke, maybe you can quote it? Reducing RPMs with a 4.10 raises output torque (lower gearing, gets someone moving easier, inefficient at high speed). Raising RPMs on a 3.55 or 3.73 lowers torque from a 4.10 and raises top speed (higher gearing, less torque for acceleration, or pulling power, in my case).
Doc is correct.
Justin's statement " with a 3.50 rear end ... it won't have much top speed." is incorrect.
Thank you all for the input. I will check into the things you have suggested and will provide an update on what I find