Fuel Economy and diff ratio

[sandman77hx]

Hi all. The fuel economy of my HJ 253 tri-matic van is best described in the same way the transmission generally is....traumatic. Fuel is fed by a new "reconditioned" Quadrajet with rpm's of 2,750rpm at 100 kph. I'm running 65 series tyres on stock GTS rims so am assuming the diff ratio is 3.36. Fuel economy is 17 litres per 100 k's and can't work out why it's so high....or is this normal?? Put extractors on today so I hope to see some immediate improvement.

Can anyone provide detail around what sort of fuel economy I should be expecting and whether I'm right in guessing its a 3.36 diff?

Many thanks in advance.

[RAEME]

Shite!!!! Not that high, mate 3.08 is your friend.

[utevanwgn]

Check your dif ratio , 2750 seems too low for 3.36 and auto.
The 253 seems over geared and having to work too hard to make the power to run at 100k, Ie seems you have to put the boot into it too much.
Almost as bad as revving to 4000 to do 100k.
However Your not going to get 100k out of 10L either.
cheers

[HK1837]

Getting the quaddy set up right will help plus setting the dizzy right for the engine also.

But a 253 with a non overdrive non lockup converter auto in a 1500kg vehicle with probably a 3.08:1 diff won't be brilliant fuel wise. You could spend significant $ chasing 3 or 4L/100. Even if you do 10,000kM a year, saving 300L @ $1.50/L is only $450 a year.

[sandman77hx]

Thanks for the input all. This is my daily driver as I travel the country side knocking on farmer's doors looking for Mantiques. Last week did over a 1,000kM and expect to do around 3,000km average per month going forward. At the moment I'm happy to cruise at 80kph but the fuel economy remains terrible at above 15.

[Valencia]

Id be looking at a 2.78 ratio for long trips

[utevanwgn]

I thought a 2.78 would improve fuel economy when my Q was 253 powered so I changed it from a 3.08 and it decreased fuel economy .
So I measured manifold vacuum at 100k and then put the 3.08 back in and measured manifold vacuum again and found I had hire vacuum with the 3.08.
Make of this what you will but I took it to mean that my poor little 253 was lugging its guts out pushing a 2.78 and was much happier revving higher with the 3.08 and not working as hard.

[XX7SANDMN]

vs 5lt turbo 700 easy to buy easy to fit better fuel economy and cruise all day with a 2.78 or 2.60 and cheap as converstion if you can find a donor car

[HK1837]

^^That was going to be my suggestion once I saw the kM the car does. No need to change the diff though, the 4L60's overdrive is meant for a 3.08:1 rear axle.

[HZSM50]

My 308 sits @3200 rpm at 110kph that's with 4 speed and a 3.36 diff. I have toyed with changing the diff and maybe go to a T5.
But now I'm just used to it, not bad I get about 300km highway driving out of the gas tank its 80 litres.
I don't do the kM you do. Its hard to compare this to modern cars economy but as I was told when my Sandman was new, fuel cost about 25 cents a litre.