Changing ring and pinion gears

[12jslater]

How easy would it be to change the differential ratio for a slight lower over all gearing, 1st gear seems very short in a seven and useless 2nd gear hill starts are easily possible so I was looking at getting a bit more top end speed, just being able to sit at 45mph with a lower engine speed than what feels like a million rpm.

I'm not looking for any more speed just lower revs when I travel long journeys.

Hope to hear your thoughts

[JonE]

presumably you are already on standard 19" wheels?

[12jslater]

It'll be 17" as she's a ruby

[JonE]

I wonder if anyone has tried running 19" with a 4.00 section tyre on a Ruby. I bet it would end up being superslow though and not giving you appropriate revs at top end, as the body is so heavy. Perhaps counterproductive...

[Ivor Hawkins]

I think I’m right in saying the rolling radius of the 3.50 x 19 and the 4.00 x 17 is pretty much the same, so upping the final ratio would be the best option.

I stand to be corrected if I’m wrong!

[Stuart Giles]

When you say lower, you presumably mean numerically lower, so in ratio terms higher? The ratios fitted in Ruby axles were 8:42 (5.25:1) or 8:41 (5.125:1) in the later cars. You could fit a 9:44 (4.9:1) from a 1931 car, with a bit of jiggery pokery. But many people find that the 4.9 gearing is a bit tall for everyday top gear driving in anything other than a light special.

[Bob Culver]

For a small car the gearing is not especially low, although it does sound that way. Conveniently thought of as mph per 1000 rpm. The A30 was 12.7, Renault 750 13.5, Ford 10 13.7. Mayflower, SV Minx, A35, 100E, , MG 1250, Victor 1500, Mini Consul, were all under 15. Even the 80+ mph 1500 Jowett Javelin only 15.5 None were intended for motorways.
Have you ever tried a Fordson van or truck at 11.4 with the engine inside the cab! I suspect having driven one I could qualify for a hearing aid under Accident Compo.

The Seven 5.25:1 on 19x3.50 14.75. early cars 4.9:1 15.83 .(One effect is that 2nd in early cars closely matches 3rd in the 4 speed.)

[andrew34ruby]

How easy would it be to change the differential ratio for a slight lower over all gearing, 1st gear seems very short in a seven and useless 2nd gear hill starts are easily possible so I was looking at getting a bit more top end speed, just being able to sit at 45mph with a lower engine speed than what feels like a million rpm.

I'm not looking for any more speed just lower revs when I travel long journeys.

Hope to hear your thoughts

With the gearing set up to give lower revs at speed I think you'd find there just isn't enough power. At the slightest incline you would be running in third gear much of the time. Forever dropping down a gear.

Remember you need power to maintain speed, and power is turning force multiplied by revs. You need revs to keep you going.

I find it easier to drive at 35, with resultant less revs, but think it would be better with a slightly lower gearing so changing into third less often.

jslater, do you live in a very flat area?

[Howard Wright]

Hi 

As has been said changing the back axle ratios is possible but might not get you what you want.

According to Bill Williams book a car with Ruby wheels and axle ratio should run in the mid forties mph when the engine is turning at 3000 rev.  Even reducing the back axle ratio to an early box or Chummy ratio will only reduce the revs by a couple of hundred.

Changing the gearbox ratios for ones more to your liking is possible. Talk to Andy Bird http://austinsevengearboxservices.co.uk/ This won’t change the revs in top mind.

Probably the easiest route is to quieten the engine and cab.  Check for rattles and clatter from the engine, gearbox and back axle. Then put some sound felt under the carpets and bulkhead. 

Cheers

Howard

[David Stepney]

Jake,

Assuming that you have bought  the three bearing Ruby you mentioned earlier, it should be running on 4.00 (or 4,50) X 17 rims and tyres. 4.00 X 17 tyres rotate at approx 800 revs per mile, giving 14.29 mph per 1000 engine revs is top and 4.50 X 17s at 785 revs per mile, giving 14.56 mph  per 1000 rpm.

3.50 X 19 rims and tyres rotate at 775 revs per mile, giving 14.75mph per 1000 rpm.

So, not much variation. Assuming the standard 5.25:1 diff, 45mph represents about 3000 rpm in top.

Austin Seven engines always sound busy: my RP (on 5.25:1 diff and 3.50 X 19 rims) is just this at 40 -45 mph, but has covered a lot of miles running at this sort of speed. A well sorted engine is really quite robust.

Hope this helps and gives some perspective on engine speed.