Contour Enthusiasts Group Archives Forums Modifications Transaxle Technical Discussion - Gear Ratios

I'm not too familiar with how these things work. I just bought GT3 and have been messing around with the gear ratios to get my car to accelerate slower or faster, have more gear to go faster on oval speedways. I'm especially wondering about final drive. This has eluded me for some time. If I increase my final drive, what are the effects?

So if anyone would like to chime in on a nice technical discussion on gears and ratios in the new transmission forum, feel free to show us your wisdom! Thanks folks.

A lot of this comes more from experience than from theory.

In very general terms, the lower the gear (higher numerically) the greater the torque multiplication but also the lower the top speed. For the sake of discussion, you will get better acceleration from 4.00:1 final drive gears than you will from 3.00:1 final drive gears. Accelaration will be even better with 5.00:1 final drive gears, but you may need to shift through three or four gears just to get across the street.

If you have enough horsepower, your top speed will be higher with 3.00:1 final drive gears than with 4.00:1 final drive gears. If the gears are too tall (too high, or low numerically) you will run out of horsepower long before you reach engine red line.

A lot of gear selection has to do with matching the power curve of the engine with driving conditions. sometimes even a slight change in gearing can make a big difference in having the engine in it's best part of the power band accelerating out of a curve on the track. That is where experience and experimentation comes in.

This should get your discussion going.

How about some more input?

Does no one want to talk about ratio spacing between gears and the number of gears in the transmission?

There must be more out there. Anyone have some good links on gearing?

I've never driven anything with more than 20 forward gear ratios. Big Jim said it perfectly:

In reply to:

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A lot of gear selection has to do with matching the power curve of the engine with driving conditions. sometimes even a slight change in gearing can make a big difference in having the engine in it's best part of the power band accelerating out of a curve on the track. That is where experience and experimentation comes in.

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If you have a motor with a flat torque curve, you need less gears than if you have a engine with a sharply peaked torque curve. So number of gears and ratio's depend on the length of the track, the sharpness of the curves, weight of the car, hp and torque curves of the engine, rolling resistance and aerodynamics of the car.

Look at Indy cars. They all look alike. Yet someone wins and someone comes in dead last. Small differences in gearing, engine and of course skill, make the difference.

Ratio choosing would depend on a lot a factors. Do you have a specific question on which vehicle/motor/condition you would be selecting ratios for?

Well, from talking to Terry I know that there are at least three ratios available for the MTX-75. The tallest is the Zetec gearing. Duratec gearing is the middle of the road. The lowest gearsets available are for the diesel Mondeo cars in Europe. I don't know the actual spacing or the final drive ratios...I'm sure Terry can provide that off the top of his head. I personally had considered installing the diesel gear set, but the expense stopped me.

Personally, I think the gearing in the MTX-75 (Duratec) is pretty good, with the exception of 1st gear. I personally think it runs through too quickly - you're barely off the line and it's time to get into 2nd already. But once I get there, shifting at redline puts me right at the start of the sweet spot in the RPM range in the rest of the gears.

My biggest gripe is that I MUST shift into 4th gear within 100 feet of the finish line in a 1/4 mile race. If there was a way to avoid that (say, run 3rd out for the entire 1320 feet) I would be quite happy. I'm not sure what RPM I'd be turning in 3rd at 90 - 94 MPH, but if a chip can't get me there, the next place to look (when I hit the lottery) is in the gearing, I suppose. Either to run 3rd all the way out or make shifting from 3rd at 7000 RPM put me at a higher RPM in 4th gear.

I don't really know anything about gear selection; mostly thinking out loud here. It is going to be quite a while (KNOCK ON WOOD) before I take the trans out again.

Originally posted by Big Jim:

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Accelaration will be even better with 5.00:1 final drive gears, but you may need to shift through three or four gears just to get across the street.

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I can attest to that. I have driven a Mack Dump truck with a Fuller 18 spd. transmission. Can you say shift? Man, with this transmission you literally have to shift 5 times to get across the street. Another I've driven is a '78 Peterbuilt with a 14 speed Roadranger and a 2 speed rear end, thats 28 gears


I think a reason for auto manufacturers switching to transmissions with more gears in them is to allow them to use low ratios on the lower gears to help with acceleration, but then more gradually work up to high ratios to maintain lower engine RPMs at cruising speeds. The larger number of gears allows the engine to stay more within it's power band while shifting. This way when one gear is ran to the upper end of the power band, the revs don't drop off too much when shifting to the next gear. Generally, the the RPM range in which an engine makes the best power is also the rev range that it runs with the greatest efficency.

The optins Vs std gear stagger,1st thru 5th are 'close ratio' gear sets.The object being that each shift keeps the engine in its max power band or 'on the cam'....

that makes more sense.

Terry,
Can gears be mixed between the MTX-75's? For example, Duratec 1-2-3 with Zetec 4-5 ?

Brian Dors
99 SVT