[yoda]

Hi All

I have a siera SJ50 with a gearing/economy issue id like to solve

im led to believe im running 3.9 diffs with a 1.4 transfer case

My issue is that 80km.h in 5th runs 3000rpm on 29" tyres netting shocking fuel ecomomy.

i need to achieve 100+ km.h at 3000rpm or shes not worth running..

What combination do you run and whats your figures to go with it

Cheers Yoda

[Gwagensteve]

Stock gearing provides about 3550 rpm at 100km/h. A stock 1.3 will run about 10l/100km.

Lowering cruise revs below that figure will make economy worse as you'll need a larger throttle opening to maintain the same speed (put simply)

Taller tyres will lower cruise rpm but economy will get worse.

Steve.

[want33s]

yoda wrote:

im led to believe im running 3.9 diffs with a 1.4 transfer case

You will need to confirm your diff ratios and your transfer ratio before anyone can offer any sound advice.

[yoda]

hi , yeh i know i have to confirm ratios in mine yet,
was initially interested finding out what they had standard

cheers

[Gwagensteve]

You're assumption is correct. Diffs are 3.9, transfer 1.409. Your gearing sounds correct for a standard car.

What economy are you getting?

Steve

[want33s]

Steve SJ50 could be NT or WT....

[Gwagensteve]

Fair call, but cruise rpm is similar in any case, but yes, diff gears could be 3.7.

Steve

[droverdave]

On a side note It might be worthwhile checking your speedo as well for compaing all of this.

My little drover is around 10km/h out as standard, an indicated 110km is about 100km/h on the gps. Sits around the 3500rpm mark as suggested above (stock everything). It's screaming its guts out at that speed though, 90-95km/hr suits me.

[yoda]

speedo is pretty accurate on the gps

on paper 3.5's a 1.4 transfer on my tyres all equates true with 3000 at 80, so the maths is correct

so now assuming i do indeed have 3.9 and a 1.4 tc ( and if you knew the car when i got it theres no reason to possibly think its been changed)
i need to work out best option to get better fuel out of it. as i said i dont know exact figures but i know its to high
Unfortunately shes unregistered and im not able to actually get a real figure.

Now it seems im running stock gear, im indeed interested to other peoples milage on std gear,

also as it revs its ring off 100 is sort of out of the question, but as a country car need to sit at 100 for safety

[SierraDan]

I used to have 31" tyres, 2.5 high range(5.14 low), 3.9 in the diffs and would get about 10l/100km. At 100km/h i was a bit under 4k if i remember correctly.

My personal opinion, g13 motors cruise nice at around the 3500-4000rpm mark.

[tanshi]

yoda what size tyres are you running. ?

your engine needs to rev higher than 3000 at 100 kms to get better fuel use.

[rick1970]

I went from 3.9 to 4.1 diff gears , std 1.3 LWB bar 235/75's, and noticed the fuel economy differance.....better with the 4.1's. its around 11L/100km atm.......

You need to drive where you engine makes its torque.....and with a 1.3 Sierra, that will be around 3500rpm.....

I do a fair bit of 100 country road stuff, and actually took notes of fuel usage with the 3.9 diffs and 235/75 tyres, was actually better when not using 5th gear....ie cruising in 4th and about 4000rpm...averaged about 1L/100km better.

Had the same experiance with old HR Holden years ago, warm 192 toyota 4 speed......better fuel economy with 3.55 gears than 3.08's.......and much better to drive.

[yoda]

running 29" tyres

[Zook101]

If you Guys are saying a std 1.3 Ltr should run @ 3500 RPM in 5th gear at 100Km, Im interested to know What yours idle at???

I've got a SWB 87 1.3lt Running 29'' tyres Like Yoda i Sit On 3000RPM @ 100 Km Ideling between 700RPM- 1000RPM

[tanshi]

and we have a winner. your running 29s on a car designed to run 26s with standard gearing.

im not sure how idle speed is relevant but i'll bite. 700-1000 is a good idle speed

[Zook101]

Yer Fair Enough comment There Tanshi!
So changing the gearing is the only option to get it to 3500RPM @ 100 KM Like Rick1970???

[tanshi]

nope there is one other option, that is going back to 26 inch tyres

[Zook101]

No Worries Thanks Tanshi

[droverdave]

I would suspet that with the 29" tyres your speedo would be incorrect?

26" to 29" is a fair increase, at least 10%.

[jdk81]

Have you considered ignition or carby are out of spec? Even fuel pump could be on its way out.

it seems a few sierras Ive worked with etc, often return fuel economy of around 12L/100km no matter what you do
Excluding slip streaming a truck for 400 km, or drive uphill into a headwind for 2 hours etc

[yoda]

yeh it needs a tune up, but at least ive sussed out a few answers to a few questions