Gear ratios and speeds in gear for different gearboxes

[WISE9UY]

Hi All,

There is a thread running in parts for sale regarding a 3.07 final drive diff and HB Chris mentioned that the 5-speed OD Getrag 265 with a 3.45 diff will run 77mph at 3000rpm in 5th. So given that I am in the throws of deciding what combination to run as I have some good choices, I took my car out to take some readings. Here is what I get with my 4-speed running a 3.45 final drive.

Speeds at 3000rpm
1st: 20mph
2nd: 32mph
3rd: 48mph
4th: 67.5 mph

62mph/100kmh @ 2700 rpm

These readings are as indicated by the speedo and tachometer so are as close as I can tell.

The gear ratios are as follows:
1st 3.855 (13.3)
2nd 2.203 (7.6)
3rd 1.402 (4.84)4th 1 (3.45)

Anyone else want to put up their combination and revs to speed. I guess it can all be calculated too but this may be a handy reference.

 

[Ohio Jon]

This website has a RPM Gear ratio calculator.
http://www.bokchoys.com/differential/e46_gear_ratio_calculator.htm
It should help you determine what you want. It's made for the E46 ratios, but you can type in whatever you want.

 

[WISE9UY]

This website has a RPM Gear ratio calculator.
http://www.bokchoys.com/differential/e46_gear_ratio_calculator.htm
It should help you determine what you want. It's made for the E46 ratios, but you can type in whatever you want.

Great site! Thank-you for adding to the thread. It was good to enter the details of my car and see the speedo error!

 

[WISE9UY]

Interesting looking at the gear ratio and speed calculator results comparing 3.07, 3.45 and 3.64 final drives. The 3.07 is actually not so extremely low as I thought consider the torque of these M30's. As we all will know, you can effortlessly take off in 2nd and the reason I want the LSD is that first will spin too easily as it is.

Is anyone out there running the 3.07 with the 265 OD Gearbox and if so, how is it in the real world?

 

[HB Chris]

My results utilized a GPS for true speed as my diff is taller by 5%, the speedo indicated 70MPH, true speed 77MPH.

 

[sfdon]

I ran the 3.07 on my 5 speed OD
Hated it
Worthless on US freeways

5 th gear on a OD is .81
3.07 diff
Do the math- delta 3.45 ... 3.07
11%
Delta 1.0.... .81
19%

That 3000 rpm at 67.5 mph goes to 2100 rpm
The torque at 2100 rpm is ok
The hp at 2100 rpm is less than 75 hp on the m90 with a schrick cam


The 200hp csl got the 3.25 in Europe
For autobahn flying

 

[WISE9UY]

This is good feedback thank-you sfdon... this is good real world feedback that is hard to determine just looking at the numbers.

While the gear calculator site noted above by ohiojon, it calculates the 67.5mph as 2300rpm for the 3.07 I hear what you are saying with regards to power and torque. For me it will be a different setup again with potentially a turbo motor if I go that route. Just for comparison records, here are the mph values at 3000rpm using 16' tire size and the OD gearbox with the 3.07, 3.25, 3.45 and 3.64 final drive (mph values are rounded).

3.07 3.25 3.45 3.64
1st 19 18 17 16
2nd 33 31 29 28
3rd 52 49 46 44
4th 72 69 64 61
5th 89 84 79 75

Easy math to just double the above values to see max speed at 6000rpm which is the max continuous rpm roughly for these motors.

 

[Gary Knox]

Great post WISE9UY. I do, however, doubt that the mph at 6K would be double that at 3K! So, I'd call it 'theoretical speed at 6K'.

Not many of us would have enough horsepower to push the e9 through the air at that speed. I'm SURE my ~275hp with the current 3.07 would not get to 178 mph in 5th gear! Would it have gotten to 150 with the 3.64? Maybe, but I even doubt that.! My '89 Porsche 928 S4 with ~325 HP (tweaked a little) and a 2.54 rear axle would get to 140 in 4th gear, but going to 5th (at least on the tracks I was driving) hardly had any improvement - even slowed a bit if still going uphill! Maybe in 2-3 miles, it might have gotten to 155+.

With significant modification to the chassis and car weight (and probably a tweaked engine), Al Holbert got a factory/Weissach supplied 928 S4 to 170 at Bonneville when the 1987 model was introduced. IF the differential on that car was stock, it was a 2.20 with 1:1 drive on 5th gear.

 

[JFENG]

I have not driven a modern 3.5l high performance build in an E9 but did have an e34 535i with a B35. In my opinion that b35 came alive around 3000 rpm. I doubt a carbureted B35 will have more torque down low. To be honest, I have never felt our motors were torquey. The old XK 4.2 straight six w 10:1 CR and triple SU's . . . now that was a torquey motor!

I think SFDon is exactly right. The 3.07 LSD is only good with the old 4-spd or the non-OD 5-spd. Even then, if the car will do HPDE or track days I would give up 150mph capability to get up to 125mph more quickly. Id actually prefer a 3.45 and a 7000rpm redline.

FWIW, one of my old cars needs about 3500rpm to cruise at 80 mph, and I don't find it tiring even if I'm going at that rate for a couple hours. My wife, however does complain about having to wear ear plugs.

 

[Rek]

"Great post WISE9UY. I do, however, doubt that the mph at 6K would be double that at 3K! So, I'd call it 'theoretical speed at 6K. "

If it isn't then your clutch is slipping.

 

[sfdon]

I had that 3.07 in a 1983 745i turbo. First the factory m102 and then the m106 engines.
The m102 has max torque at 4000 rpm
280 ft/lbs
Nothing at 2000 rpm
Horrible turbo lag, pathetic in city driving
The m102 came only with the 3.07 and a 3 speed auto
Great on the fwy!

 

[Lotuss7]

Let's all not forget the third factor. Tire diameter.

 

[WISE9UY]

Hi All... Glad it is of use to you! Indeed tire diameter is of some gearing influence. For completeness of information, I used the tire size of 205/55/16, which I imagine most would run on the 16" rim. I realize that the 5th gear speeds make for impossible top ends and that would not be expected in 5th as it is commonly expected to make top speed in 4th and not 5th... unless you are making crazy power of course :-D

 

[sfdon]

Well stated.

 

[sreams]

In my opinion that b35 came alive around 3000 rpm. I doubt a carbureted B35 will have more torque down low.

My M30B35 with Weber DGEVs makes peak wheel torque of 185lbft at 3400rpm on the dyno. At 2500rpm, it still makes over 175lbft at the wheels. Torque is between 175-185lbft from 2400rpm to 4600rpm.