Hello Jim,

The "standard" differential ratios for Fiat 600D was 4.88 (8/39). The standard 4th gear is .896.

The top speed of an automobile, in any gear, is dependent on the rolling circumference of the tires, the differential ratio and the ratio of the gear that you are in.

Here are some speeds in MPH (Kph), at 4000 and 6000 RPM in fourth gear, using a 185/60X13 tire, based on the various differential ratios available.

Diff ratio 4000 6000

8/43 (5.38) 53.69 (86.40) 80.53 (129.60)

8/39 (4.88) 59.19 (95.26) 88.78 (142.87)

9/41 (4.55) 63.48 (102.16) 95.23 (153.26)

9/39 (4.33) 66.71 (107.36) 100.06 (161.03)

As you can see the various differential ratios make a big difference in top speed. One problem however is that this assumes you have enough horsepower to pull that amount of gear. Lets face it, to do 160 Kph you need some serious horsepower to push a Fiat 600 along that fast. The Fiat 600 is not what you would call aerodynamically svelte. Likewise, the standard gearbox has a HUGE hole between 3rd and 4th. Add to this that 4th gear is an "overdrive gear", then you can easily see there is a further "mechanical disadvantage" working against you, in attaining anywhere near that top speed.

If we change the gearset in the transmission to a close ratio one, say with a 1.04 fourth gear, what happens to our top speeds?

8/43 46.26 (74.45) 69.38 (111.65)

8/39 50.99 (82.06) 76.49 (120.10)

9/41 54.69 (88.02) 82.04 (132.03)

9/39 57.40 (92.38) 86.21 (138.74)

As you might have guessed, the top speeds have dropped up to 14 miles per hour. More significantly with the close ratio gearset, the shift points are now spaced closer together, to where there is only a 1200 RPM drop from 3rd to 4th. If we assume that the motor delivers peak torque at approx 5000 RPM, then if we use 6500 as a shift point in third, then the engine will end up at around peak torque in 4th gear, giving us a good chance of using the available horsepower.

Assuming that we have 70+ horsepower to work with (a standard 1050 motor) then with a 9/39 differential gear and a close ratio gearset, then you will be at 5500 RPM for a 70 MPH (112 KPH) freeway cruising speed. I consider this to be a fairly good combination of components. BTW - It would not be unusual for a 1050cc motor to turn 7500 RPM or more. I would consider that your car could easily do that. At Scuderia Topolino we dyno street motors to 7500 RPM and race motor to 8500 RPM.

BTW the ratios of the 4 speed close ratio gearset are 3.38, 1.75, 1.20, 1.04 As you can see the gap between gears gets smaller the higher the gear ratio, as the amount of transferable torque at the wheels decreases due to less mechanical advantage in the gear ratio.

Yes Jim, I would say that you have an "optimistic" speedometer. You can solve this easily with an in-line conversion gearbox. You might want to contact Gails Speedometer Service 187 E 16th St, Costa Mesa, CA 92627 Phone: (949) 646-9120 . He has done work for me in the past and can organize the bits. He can also measure how far out the speedo is. Alternatively, one of the best speedometer repair/renovation shops is Palo Alto Speedometer, 718 Emerson St. Palo Alto, CA 94301-2410 Tel: 650-323-0243

I hope this helps.

Regards

Paul Vanderheijden

www.scuderiatopolino.com