Re gearing a petrol 80.

[AndycruiserguyLomas]

This is one to ponder.

Recently I have taken to driving a petrol/lpg 80 after 100 000 miles in a 12v 80 auto. I also have a 24v manual which seems and in fact is higher geared.
The question is:
Has anyone any experience of changing the gearing (ie diff swaps) in 80's and to what degree of success they had.
The questions are:-

!. Will a petrol auto pull the higher gearing and be more economical?
2. Will a speedo unit out of a manual read correctly in an auto fitted with manual diffs?
3. Did anyone ever put auto diffs into a manual, if so how did they find it?

Thanks Andy

 

[Paul_Humphreys]

There are a few of us that run 4.88 diffs. Works fine if you put bigger tyres on. Maybe that is what you need to try if your running standard size tyres. Cheaper than changeing diffs if you need tyres anyway.

Paul

 

[AndycruiserguyLomas]

Hi Paul,Thanks for that info,however I dont really want to get into the "bigger tyres" scenario, predominantly due to cost.
I have all the diffs I need to do swaps. MY main reason for the post was to see if by putting 3.7 diffs into the place of 4.1's could I make a 4.5 petrol more economical.

Thanks Andy

 

[adrianr]

Not and expert but would presume that you would lose on performance, but on cruising better economy then possibly yes.

Seems most people go the other way and put auto diffs in manual after increasing tyre size.

Jon Wildsmith might be the fella to answer this one?

 

[Paul_Humphreys]

If you have the diffs Andy why just not swap them and see. If it is not right then swap them back.

As for bigger tyres, I mean going up to 285/75/16s, not a lot over size.

Paul

 

[Andrew Prince]

!. Will a petrol auto pull the higher gearing and be more economical?

I'm sure it will handle the higher gearing but chances are Mr T got his sums right when designing the gearing, so I would doubt you'll get an improvement in economy. The engine will slog harder under acceleration, which will use even more fuel - maybe you get some back cruising but you'll need to do a lot of miles cruising to offset the higher consumption every time you pull off.

The petrol is thirsty - if economy is important, look for a different engine/vehicle! Faffing with diff ratios isn't going to materially change things IMHO.

 

[AndycruiserguyLomas]

Yes Andrew, I tend to agree with your thinking about Mr T's selection of gear ratios to be well thought about and well tested.
However my post was posted to find out if anyone had tried my idea, if they had, their input would have been useful to save me wasting my time/spending my time wisely.
So if anyone has tried it, let me know.
So far running an 80 petrol/lpg has worked out slightly more than a 12v auto.
(the up side is the cars are half diesel car price and usually better lower mileage examples, a lot quieter and smoother)

Andy.

 

[Toby]

Andy,

I don't think you'll gain anything with higher than stock gearing unless you do all your driving at constant speed on empty motorways.

Some of us have re-geared to bring overall gearing back to near stock having fitted taller tyres - I certainly experienced reduced performance and economy on big tyres before regearing which has the same effect as what you're proposing. Some (or all?) of this could be down to extra lift (more drag) and higher rotating weight of course...

Let us know how you get on if you go ahead.

Best of luck,
Toby
1990 HDJ80
1994 HDJ80

 

[AndycruiserguyLomas]

Thanks for that Toby, I'm trying to see if anyone's tried it with a petrol 80. I would imagine fitting 315's having about the same effect.
We'll see, I've not decided yet. Anyway am running 285's, no noticeable difference to performance over the 275's that came off.

Thanks all for the input.

Andy.