Hey everyone, first and foremost thanks for the participation. Incredible!
Why are GPS speedos not allowed in Germany?
Because our certification gods at TÜV say so. Has something to do with accuracy. For example, when I went from the original 195/70R14 to a 17" on my E28 I couldn't deviate more than 2% in wheel circumference when choosing tires. 215/45 it was. All done to keep the speedo exact.
The DZ version of this box has a very high 1st year ratio, over 5:1. The BZ version is better at 4:1. The easiest way to do this if you are fine with converting to an electric speedometer is to locate a 5 speed transmission from an e23/24/28, e34 (fitted with M30 engine) as these are all known to fit and have the same (approximately)top gear ratio as the 6 speed
Good noticing. I've since spoken to a friend who breaks a lot of E28s and we've tossed ideas. He mentioned that the smaller version of that box is very much enough for our coupes and yes, BZ works better. Have to check the mounting angle, though. Maybe the units from 116i etc. are physically smaller and have the correct angle. The E28 5spd Getrag (I believe 260) are now old and worn and expensive to rebuild, too.
Possible 6-speed solution (I bought an adapter plate and will someday, try to fit it):
The Mazda unit would make it unworthy for historic registration. Too much restomod.
Technically, gazzol is right. The 6-cyl BMW engine has a wide enough torque band to accommodate its original, 4-speed transmission. The benefit of installing a 5 (or 6) speed transmission is getting an overdrive top gear; not having more closely-spaced ratios.
But gyrospita's issue isn't technical; it's economic. He pointed out that: "I've read up on the Getrag 265/6 conversions. Good gearboxes are really pricey here in Germany." Still, it might be faster/cheaper/easier to pay up for a Getrag 5-speed with a mechanical tach drive than to figure out how to do a 6-speed conversion. Or just retain the 4-speed and install a higher ratio differential.
I cannot legally change the gearing ratio of the diff due to changes in emissions that would come from that. I've swapped an E28 to M52 and had to keep the original ratio of the donor car or stay within 7% of that. Luckily, 210 large case diffs came in 3,07 as well.
I didn't (yet) look at the gearing tables for each individual unit. The idea came from finding an offer for an adapter plate for GS6s to M30s. Ideally, I can combine a sporty short gearing with 1.00 in 5th and an overdrive in 6th to essentially combine the 265/5's and 265/6's advantages.
Best bet for a 6 speed would be one out of a late e34 m5 as the s38 based 3.8 would have the same bolt pattern as an m30. With the cost and complexity of it all though, a good 5 speed OD box would work just as well and has been done countless times before
IIRC that's the same gearbox as in manual E31 850is, so 5000€ is the going rate? And just to throw in some more numbers: A 256/6 is 3500€ from the Netherlands, a revised matching LSD is 2200€. So fiddling with a 300-500€ gearbox while retaining original gearing is motivating enough to find a solution for the speedo drive. Maybe a conversion parts package (swap set) can fall off that tree to be sold to all E3, E9, E12, E24 owners.
Pls throw in your recommendations for a technical solution or a speedo pickup and drive unit with and without GPS. Maybe one crazy concidence says that the old speedo drives can be retrofitted somewhere.
