1990 E350 with 7.3 and C-6

There is two way to lower cruise RPM. One is to install a taller axle ratio and the other is to swap in a tranny newer tranny than has OD. In your case since tranny needs a rebuild the logical path would be to swap in a E4OD which was used behind 7.3's in later models if you wish to lower cruise RPM.

----------------- TheSnoMan.com

The really-winds-up part is controlled by the differential.

As I recall, the C6 uses a 1:1 ratio at the transmission output, so to get more speed at less revs, you wouild need an overdrive that won't go into the C6, or new gears in the diff. If you get new gears in the diff, you may need to consider lower ratios for 1st and 2nd, or just go with the new performance as the standard.

I do not know what the ratio is in your diff, but assuming it is set to about 4.88, then if you went to a 4.1 or maybe a 3.73, then you would get lower engine speeds on the freeway. The trade off would be that you also get less of the runs-like-a-bat-out-of-hell characteristic.

Thanks Jeff Strickland!

Thank you SnoMan. How much trouble would it be to switch trannys? It surely isn't an exact fit replacement. Shorter driveshaft? Tranny mounts? And what about my gear indicator on the steering column? LOL!

Later models of that vehical came with it so it is a bolt in. Any parts you would need could be had from a bone yard.. You could make existing shifter work or swap steering colum. It may sound like a lot of work but if you plan to keep it is it well worth it for a few reason. One is the C6 has no abilty to lock converter ever so that adds a few hundred RPM to cruise and second it will give you OD which will lower RPM and noisy and improve MPG too. Look for a van in bone yard with one and it will have everything you need. try the link below.

----------------- TheSnoMan.com

I 'preciate it! Thank you.