At 11:32 AM 4/4/05 -0600, Dick Horn wrote:
"A club member has an MGB axle I can get cheap, real cheap, if you know what I mean!"

Hey, that's my kind of buy.

"How much rpm drop can I expect, especially on the highway in 4th?"

Gear ratios are in the front of the workshop manual.
Original ratio for the 1500/1600 cars is 4.300/1
Original ratio for the 1600-MK-II cars is 4.100/1
Original ratio for all (not special) MGB is 3.909/1

Changing from 4.300 to 3.909 will drop the engine speed proportionally, like this:

New ratio Old ratio

X original engine speed = new engine speed.

At 75 mph it would look like this: (3.909 / 4.300) x 4400 = 4000

The ring gear still has 43 teeth.
The pinion gear will have 11 teeth rather than 10.
So the new engine speed is 10/11 of original.

That 400 rpm drop may not sound like much (only 10%) but it has the potential to improve fuel mileage by about 5% (if you don't take to driving faster).

On the flip side, if you keep the engine speed constant the ground speed increases like this:
(4.300/3.909) X 75 = 82.5

This is more like what is most likely to happen. The car will seem to run right along with the same original engine speed with seemingly about the same effort as before, but the ground speed will be 10% faster. So on the expressway you may find yourself shifting one lane to the left and keeping up with the faster traffic.

The engine speed in the MGA never bothered me much. I was quite used to cruising at 75-80 mph all day long with engine speed of 4400-4700 rpm (even towing the little trailer). Without the trailer I could make runs in the mid 90's at about 5500-5600 rpm. My car had 335,000 miles on it before I changed the gear ratio.

What finally caused me to do it was when I had warmed over the engine a little, and it would easily blow right past red line in top gear. 6000 rpm = 102 mph with the 4.3 gearing. With about 100 HP in the fresh engine I one day found myself pacing along with another car at 6400 rpm (about 108 mph) with still a little room to go. Changing the ratio to 3.909/1 puts the 6000 rpm red line at 112 mph. The ratio change also reduces torque at the rear wheels by 10%, so I'm pretty sure it won't go that fast now. But it will still make at least 105, and will do that all day long at 5625 rpm.

I suppose the most important part is that it now moves along comfortably with traffic in the fast lane in the mid 80's, where I didn't really feel like keeping up very long at that speed before.

Immediately after you change to the lower ratio you will likely notice a slightly slower accelleration in 1st and 2nd gears. You quickly learn to rev the engine a little higher before shifting, keeping it in a lower gear just a little longer, which makes up for the drop in torque at the rear wheels.

At 6000 rpm 1st gear will make 31 mph instead of 28.
Low gear is sort of a stump puller anyway, so you will never miss the torque coming off the line. It will still spin the tires in low gear if you get on it strong enough.

At 6000 rpm 2nd gear will make 51 mph instead of 46
This allows me to run autocross more in 2nd gear without having to upshift. In fact now it will make 59 mph at 7000 rpm in 2nd gear.

At 6000 rpm 3rd gear will make 82 mph instead of 74.
Now you can use 3rd gear for passing slower traffic if you like.

This is not all for free of course. The trade off is that the top speed will be reduced slightly due to reduced torque at the rear wheels in top gear. When you encounter some steep hills you may feel it bog down a little, but that's what the lower gears are for. If you have to downshift it will still do 75 mph uphill in 3rd at 5500 rpm (where it used to do only 68). A fast street camshaft will generally give you an additional 10% torque (or more) at road speed, so the warm cam and the reduced gear ratio go together quite well.

On a side note, the die hard expressway cruisers may not be so concerned about top speed as they are about creature comfort and good fuel mileage. These people might prefer a lower final drive ratio in spite of the fact that it may reduce the best possible top speed. For a really nice change with very little compromise (other than initial cost), you can install an overdrive unit or a 5-speed gearbox with overdrive 5th gear and direct drive 4th gear. This can allow you to retain the original final drive ratio for highest top speed in 4th gear when you really want it, and also give significantly reduced engine speed in 5th gear for the creature comfort on the long road with improved fuel economy.

For the racing type folks, there are other reasons for changing the final drive ratio, and they often do it repeatedly between weekends when moving around to various race tracks. The "gospel according to speed" requires the best possible accelleration to achieve the highest possible speed in the space allowed. That allowed space is usually the length of the longest straight run on the race track. The car will be in top gear when cruising at the highest speed.

Changing the gear ratio in the final drive does two things. It changes engine speed relative to ground speed, and it changes the available torque at the drive wheels relative to engine torque. For achieving maximum accelleration you need the highest torque a the drive wheels, to which end you would want to increase the final drive ratio. But the magic red line on the tachometer (wherever you chose to paint it) dictates the highest speed allowable for the engine. The trick then is to use the highest gear ratio possible while not allowing the engine to run past red line at the fastest point on the race track. So on a short track where the car will not be moving so fast, it is advantageous to use a higher ratio final drive for improved accelleration. But for a longer track, especially with longer straightaways, the final drive ratio must be lowered enough to prevent over reving of the engine. To wit, it is common to swap out the final drive gearing between race dates.