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Vehicle: Suzuki sj70 swb

Post Posted: Fri Sep 06, 2019 10:09 pm 
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G'day, Im running 600 16 bar treads on lj rims under my sierra but was wondering if it is worth going to a 750 16? could you run 750 16s on lj rims or are they to narrow, and running 750 16s will i break running gear? I have a locker if that makes any difference and will hopefully be getting reduction gears soon.
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Post Posted: Fri Sep 06, 2019 10:30 pm 
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7.50 is too wide to be legal/happy on a 4.5” rim. It’s a good size but a little annoying to fit as it’s a fairly tall tyre. (~31.75” when new)

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Post Posted: Fri Sep 06, 2019 10:41 pm 
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Ok thanks gwagensteve what tyre size would you suggest on lj rims under a sierra or would you stick with 600 16s?
cheers Jack

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Post Posted: Fri Sep 06, 2019 11:01 pm 
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7.00 16 is pretty sweet.

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Post Posted: Sat Sep 07, 2019 9:14 am 
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I'll look at some of them for my next set, would 6.5 gears be ok? so long as the car can do 90 i'd be happy.
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Post Posted: Sat Sep 07, 2019 9:29 am 
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I don't know who is perpetuating stories that transfer reduction gears reduce top speed, but I read it all the time and it's fundamentally wrong - not your fault Jack - it's like "suspension lift fits bigger tyres" and "suspension lift adds flex" and "shocks for a 2" lift" - they are falsehoods that the 4WD community can't get their head around for some reason.

Top speed of a stock sierra in good condition is plenty - it's around 125km/h. At that speed the engine's torque can't overcome wind resistance. I think that peak speed figure is achieved at around 4500 rpm in 5th which is where torque and horsepower curves intersect, so as revs increase torque is falling and wind resistance is increasing by a square law and the car can't go faster.

(interesting - it takes about 100hp to make a car go 100mph, 500hp to make it go 200mph, and 1600hp to make it go 300mph.... - my numbers are loose but you get where I'm heading)

nobody fits transfer gears with stock tyres. Transfer gears increase high range reduction which offsets the larger diameter tyres everyone fits.

For 6.5 gears that's about 17%- or cruise revs return to stock with 30.4" tyres. (So 31's) with a 7.00 16 at ~30" revs will be very slightly increased at highway speeds which will help to offset the increased aerodynamic drag of the taller tyre/car. There will be no meaningful effect on top speed.

Running a 30" tyre with no transfer gears causes a marked reduction in top speed because the car is now too tall geared and doesn't produce enough torque at lower cruise RPM to maintain speed, so 5th gear becomes useless, and you're back to buzzing along in 4th on any incline.

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Post Posted: Sat Sep 07, 2019 9:35 am 
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Ok that explains it clearly how would you increase you high range revs so that you are doing 90kph in 5 gear at between 3600 to 4000 and have a decent low range?
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Post Posted: Sat Sep 07, 2019 10:01 am 
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That's easy - you need to swap to deeper diff gears like 4.57's or 5.12, but what's the requirement to be so deeply geared on road? if it's for towing or heavy loads, you're much better using 4th, which will give you almost exactly that outcome rather than loading up 5th, which isn't very strong.

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Post Posted: Sat Sep 07, 2019 10:08 am 
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partly for towing but i know you shouldn't tow in 5th, but mostly because i never use 5th except for down hills and i never cruse above 90 so it would be nice to have the gears closer together.
Is it hard to fit ring and pinon gears and will they fit in with an air locker? thanks for all the help.
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Post Posted: Sat Sep 07, 2019 10:34 am 
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The car should pull 90 in 5th just fine. With slight overgearing with reduction gears I"m sure it would be no problem with stock diff gears. Then, when towing, you can use 4th at 90 and when not towing you can use 5th.

Spend LOTS of time with a gearing calculator like the one on grimmjeeper to understand what you're working with.

Changing diff gears isn't really a DIY job unless you have the tools required and know how to use them. IT's easy to get wrong an destroy gears. Yes, the will fit with an airlocker, but it's an expensive job. some ratios are easier to get than others

options are 3.9(NT 1.3) 4.1 (jimny, WT maruti) 4.3 (jimny, some GV) 4.57(aftermarket) 4.65 (auto carby vitara) 4.88 (gv) 5.12 (Vitara and some GV) 5.38 (US 1.3 litre vitara) and 5.7 4X4art aftermarket

but diff gears won't correct low range gearing, you'll still need transfer gears for that, and as they correct high range of bigger tyres themselves, that's normally enough. Diff ratio changes aren't generally required until you're over 33" tyres.

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