4 speed to 5 speed fuel economy

[General Lee]

Hi,

I have a 4 speed box on my 1.2 carb saloon at the moment planning on changing it in the near future to a five speed. I mainly do motorway miles 50 a day at roughly 70-80 mph. Currently i get about 300 miles from my car with a full tank-to empty, can i expect more mpg switching to a 5 speed. If so how much roughly.

Thanks Nathan

[nova_saloon]

thats sounds about right 50 miles to the gallon is what the haynes manual says i think. proberbly looking at a few extra miles, cant be sure to be honest till ithe gearbox is in there.

[Welsh Dan]

You're getting an average of around 32mpg, I used to get an average of around 38mpg on my 5 speed 1.2 carb hatch.

[General Lee]

You're getting an average of around 32mpg, I used to get an average of around 38mpg on my 5 speed 1.2 carb hatch.

so maybe roughly an extra 40 miles per tank i might be able to achieve.

[mowgli]

4 speed 1.2 has a 3.74 diff.
if you stuck a 3.74diffed wr box in, it would struggle to pull top gear. if you got a 3.94 top gear, then you'd be getting about a 10% over gearing on your current box, so you'd get maybe 5-8% better fuel mileage.

I ran a 1.4 with the 1.2 4 speed box, and drove it hard & still got 35mpg on all types of driving. maybe a trip to the garage will save you big in the long run.......

[nova_saloon]

lol i was well wrong. i know there spose to get 50mpg but how much does a nova tnak hold cause i think i got that bit well wrong? otherwise it would of worked out so high

[General Lee]

35-40 liter's not sure there's 4.5 liters in a gallon, so 7 or 8 gallons

[mowgli]

a petrol nova hasn't done 50mpg for about 15 years, since the last old codger sold it to us lot

[nova_saloon]

this is very true. but they can do it! but yeh for some reason i caculated it wrong. never mind! but black stick just buy a 1.3 or 1.4 carb engine. its a straigh swap and it will be able to pull that 5th gear better.

[Stuart]

I got 44mpg when I was mega poor as a student from my 1.2 carb on a 5spd box... after getting some money that week it dropped to 17mpg lmao

[nova_saloon]

lol nioce

[Welsh Dan]

You'd probably save money by leaving 10 minutes earlier and maintaining 60mph on the motorway.

[Guderian]

I haven't driven a 5 spd Nov, but chopping and changing between other vauxhalls of this vintage with 4 or 5 speed boxes i found there was very little difference. We are talking the sort of small difference you could get by keeping the maintaining the tyre pressures 'spot on' and taking the mudflaps off!

[General Lee]

so in most peoples eyes it's not really worth it unless, upgrading engine.

[Riggy]

the 4th gear cog in a 4spd bpx and 5th gear in a 5 spd box , is the same wheel anyway , its the other gears that are different , so id say that on a motorway it would make no difference but around town maybe a little better :thumb:

[Welsh Dan]

the 4th gear cog in a 4spd bpx and 5th gear in a 5 spd box , is the same wheel anyway , its the other gears that are different , so id say that on a motorway it would make no difference but around town maybe a little better :thumb:

They have the same fourth, fifth is longer on the 5 speeds. I had both on my car when I fitted a 4 speed by mistake (lol) so I know that from experience.

Also:
http://www.xen0phobiak.f2s.com/novaload/gear_ratios.jpg

[Jack]

Fit a 20XE and F20. I used to get 35-40mpg from that even when driving round town lol

[Riggy]

Fit a 20XE and F20. I used to get 35-40mpg from that even when driving round town lol


and booting it lol

[mowgli]

Fit a 20XE and F20. I used to get 35-40mpg from that even when driving round town lol

yeah, but the town you live in has men walking in front of the horseless carriages with a red flag!

[Guderian]

Those stats/tables are interesting. A slightly 'lower' diff in the 5spd box to give 1st to 4th 'sprint' qualities (supposedly!) and a 'bolt on' 5th gear with a slightly silly ratio. This is very similar to early fwd cav. ratios-- and on those it has the effect of making 5th too 'high' to climb motorway hills with.

Don't get me wrong-either box is fine, but if looking for MPG gains i don't think that with a 1.2 it is going to really happen.

[Welsh Dan]

I'd stick a 5 speed cluster into the existing box and swap the pinions over -you don't even have to drop the gearbox off :).

[mowgli]

I'd stick a 5 speed cluster into the existing box and swap the pinions over -you don't even have to drop the gearbox off :).

but if you have the 3.74 diff, which it should, then it will struggle to pull top gear.

my 1600 has got the f13wr with the 3.74 diff & it needs to be doing 70+mph to go into top comfortably

[Welsh Dan]

Fair point, I hadn't thought of that bit, but the diff would only be about 6% longer so it could work. If it didn't you only waste an hour and a bottle of gearbox oil trying it on your own.