Hi everyone,

Im running a stock A15 4 speed original diff i think (H165)I had the A15 4 speed conversion done in adelaide and drove my car from adelaide to brissy and it drove fine until i got to brissy the box went a week after i got back.

I had plans to find a five speed as it would be handy for all the long highway trips i do but ive learnt 5 speeds are rare and expensive. Would a higher ratio diff be the way to go in the sense it wouldnt give the gear box such a hard time at higher speeds.

If any of this makes sense all feedback is greatly appreciated thanks

Sunny (B310) came with the H150 diff. 1200 utes came with the H165.

Keep in mind that an A15 does not need overdrive, it can run at 4000 RPM all day long with no worries. But for better fuel economy and less noise, either fit the 5-speed OR a lower diff ratio. If you have the 3.900 ratio now, a 3.700 will lower the RPMs on the highway, at the expense of slightly slower acceleration. A14s here came with as low as 3.54 ratio, which were slow as a dog taking off, but got very good fuel economy.

Keeping the higher ratio (4.11 is superb) along with 5-speed overdrive is the most flexible combination, but costs a a lot for the 5-speed. A good 4-speed is only $50.

I run 4.11 with a 4-speed, and have gone on two 1000-mile trips cruising at 120kph.

A 3.9 diff ratio will drop the revs a tad and 3.7 ratio even better if you can find one. 60A 5 sp with the overdrive is even better and atleast 2 times more expensive than a 4 spd. Or if money is no problem, go the 5 spd and a 3.9 / 3.7. So you have to weigh up costs. 1st you have to buy a 60A 4 speed anyway. That may set you back say $300+ and installing say $150 (roughly not a debate people). A 60A 5spd will be roughly $800-$1000 plus installing it. So you have to ask yourself for double the price is it worth what you want to achieve and drop the rev and save fuel and engine life etc. A diff center if you can find one would set you back probably $200+ and then it has to be installed. It will drop your rev's a tad. So a 60A 4 spd and new diff will set you back probably $750+ VS say 60A 5spd $900-$1100 installed. The 5 spd is a better option for long trips or do both. 5 spd s arent that rare , just expensive !!

well said Nick

Oh sunnygirl, there are 5 spds that dont have overdrive too , just shorter gears so make sure its overdrive !! I can't tell personally by looking at it... Perhaps DD: or someone can explain the difference by numbers or the way it looks for you.

If it says 56 on it, it is not overdrive. All the other A-series 5-speeds are overdrive including 63A and 60-series. But they look a lot like the four-speeds.

I don't think a long highway run would kill a gear box after 1 week, so either it was on the way out it driving style possibly (hard shifting burnouts and other abusive driving styles )

Need to know car driving style and other associated mods. How did the gear box break? Symptoms noises?
Id just get another 4 speed see what happens.

After anges box breaking runs in his race car and ended up being pinion angle between.diff and box.

Make sure it is a 60A 4 or 5 spd box not a 56. The 60A will handle an A15 no probs

A 56a does too as long as your not launching like a drag racer every set of lights. Lasted years behind My a15 no issues ever

Well for the same price or a tad more, i'd go 60A and be alot more confident and do it right the 1st time