Help me figure out what diff I need to achieve a certain cruising speed

[MTD]

Hey guys.
For my own personal preferences, I want to be able to cruise in my 18is at 75mph (or 120kph), at 2500rpm (or lower obviously). I know it will loose a lot of power, but the car will also have some power mods to compensate. So at least what diff ratio do I need o achieve this?

I know there is some math I can do to find this, but I cant figure it out.

All help appreciated!

[roguetoaster]

You'd need a 2.93 differential with stock tire/wheel sizes to achieve sub-2500RPM at 75MPH. I would think that this would make the car fairly boring to drive, and possibly even dangerously slow.

Use the link and reference the chart below for gear ratios or load the premade sets that are lower down on the linked page.

http://www.endtuning.com/gearratios/

[MTD]

oh shit, that's way lower than i expected. Could i go down to 3.73 without losing much power, or is 3,91 the safe decision here? (the power mods i talked about would put me at 160-180 crank hp by the way)

Also, for fuel economy i am right in assuming the rpms i should cruise in are 2400/2500, right?

[roguetoaster]

I'd be fairly surprised if you'd make 180CHP with quickie mods. I realize I should've mentioned that there's the old debate of running at low RPM vs in the power band of the engine. As we know, engines are only efficient (powerful or economical can be two different points) at a select range of RPM.

Personally, I think that the M42 is most comfortable between 3-3500RPM, and cruising there is not harmful to fuel economy. This also means that where there is a small variation in speed that I'm near the power peak and have to apply less throttle to recover to cruising RPM. Conversely, it means that I am probably using more fuel all of the time vs say 2500 RPM.

As to rear end ratio, the 3.73 is too slow off the line for my liking, and the 4.1 was just fine, but as I had one that was fresh I've been running a 4.45 for just over a year, which I like very much. The achievable mileage for all of those won't be too different, with the 3.73 showing as much as 34MPG overall, and the 4.45 delivering a low of 26/high of 32. The 4.1 seemed to hover between 28 & 30 depending on a number of factors.

Essentially, it doesn't matter much to those of us in the US, as the price of a differential versus the price of fuel means it will be a fair time before the initial cost is returned. Overseas, it makes more sense to go 3.73, or even 3.46 if you want good economy, but the 3.91 isn't enough of a change to really bother. On the plus side, 3.73 or lower will at least allow you to actually hit 60MPH in second gear.

[MTD]

Oh they are not quicky mods. I am following tips from a friend that already had a 184hp 18is. Includes head porting, pistons, stroke, etc. In the next weeks i should do at least the head work + chip repro which should put me at 160hp.

You should note that i'm in Europe, in a country especially where fuel prices are insanely high. Every mpg counts. Furthermore, the highways where i cruise here tend to be more horizontal, so i mostly cruise with my foot down. I don't need to pin it to go up hill and then let go to go down hill. For this kind of cruising i should stay in the 2500 rpm range right?

[fresh_TD]

I have a 3.73 in mine, and like it a lot. I think is perfect for the car.

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[roguetoaster]

Oh they are not quicky mods. I am following tips from a friend that already had a 184hp 18is. Includes head porting, pistons, stroke, etc. In the next weeks i should do at least the head work + chip repro which should put me at 160hp.

You should note that i'm in Europe, in a country especially where fuel prices are insanely high. Every mpg counts. Furthermore, the highways where i cruise here tend to be more horizontal, so i mostly cruise with my foot down. I don't need to pin it to go up hill and then let go to go down hill. For this kind of cruising i should stay in the 2500 rpm range right?

Like fresh_TD said, the 3.73 is a good choice.

Curious if it's practical for you to lower your actual speed on the highway to somewhere in the realm of 100KPH? If so, a stock ratio will net great MPG.

It's somewhat left field, but do you have an oxygen sensor on your car? Although I can't confirm it, I'd suspect that cars equipped as such could get better MPG with their adaptive fuel tables. If you don't its only three things that need to be changed to have one.

[MTD]

I have a 3.73 in mine, and like it a lot. I think is perfect for the car.

Sent from my SAMSUNG-SM-N910A using Tapatalk

Nice! can you expand a bit more on how it feels? did you notice the power loss much?

Like fresh_TD said, the 3.73 is a good choice.

Curious if it's practical for you to lower your actual speed on the highway to somewhere in the realm of 100KPH? If so, a stock ratio will net great MPG.

It's somewhat left field, but do you have an oxygen sensor on your car? Although I can't confirm it, I'd suspect that cars equipped as such could get better MPG with their adaptive fuel tables. If you don't its only three things that need to be changed to have one.

Yeah, at the moment i do cruise at 90-95 kph, to stay in the 2400-2500 range. I wanted a lower diff to be able to cruise at higher speeds though.

And yes i do have a o2 sensor. This week though i turned it off to run some mpg tests without it though, i think it needs to be replaced anyway. And i also do not have a cat anymore, so it isn't needed right?

[fresh_TD]

Nice! can you expand a bit more on how it feels? did you notice the power loss much?

Power loss? No .... 1st gear is actually useful now. The 3.73 drops about 500rpm when compared to the stock 4.10. I strongly suggest not to go anything lower then 3.73 though.


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[MTD]

Thanks! 3.73 it is. Will start looking for one once i get the head work done

[roguetoaster]

Yeah, at the moment i do cruise at 90-95 kph, to stay in the 2400-2500 range. I wanted a lower diff to be able to cruise at higher speeds though.

And yes i do have a o2 sensor. This week though i turned it off to run some mpg tests without it though, i think it needs to be replaced anyway. And i also do not have a cat anymore, so it isn't needed right?

Makes more sense that you want to be able to cruise at higher speed at the same RPM. A 3.73 will improve your speed, but won't kill the fun of the M42.

The O2 sensor still matters for fuel mixture, and should be retained even if you no longer have a catalytic converter.

[MTD]

Very well then, but anyways i need to change it. They are universal right?

[roguetoaster]

Very well then, but anyways i need to change it. They are universal right?

You can get a universal (providing it has the correct number of wires, look at your sensor connector to double check) and splice it in, or just get an OE fitment unit. Price difference is probably around 40EUR.

[MTD]

annoyingly expensive either way ok, thanks for all the help @roguetoaster !!

[5Toes]

3.25LSD medium case in my car and its a riot to drive. dare I say... perfect. 1st gear is long enough to give you great traction off the line, second takes you to 70 indicated and 5th gear is useless unless youre above 70mph. I think the car is faster to 60 as well because of the better traction off line and eliminated 3rd gear change. I am happy to report better fuel economy as well. its fun dropping third on the highway and being able to rip it to 100..... I could go on and on