opinions, swap tranny or change rear end ratio?

[Kubla]

Hi all

if you had to chose one, would you swap out your AT545 for a MT643 or change your 4.78 rear end for something like 4.11 to up your top speed to 70 mph both would cost about the same

[cschlessman]

Both!

Chuck

[mtg]

I have an AT542 also, and I would definitely swap the transmission rather than rear gears.
The AT doesn't have a lockup torque converter, so even though the MT has the same 1:1 top gear, locking the converter would effectively make the top gear around 10% taller, and dump less waste heat into the coolant from the trans oil.

What sort of pricing have you found to get the swap done?

[Kubla]

I have not priced out the actual swap, I have priced transmissions but still trying to make up my mind

[Destroymix]

I'd do the transmission, it would give you your highway rpm drop AND be a great strong upgrade.

[wauto]

Anybody consider a two speed rear end ?

[frank-id]

If both trans have the same final ratio, no speed change will happen. The rear gears change is the obvious choise for a higher road speed. But with a higher rear axle ratio, more energy or horsepower is required. Usually horsepower/torgue costs lots of money. Keep your present trans. Frank

[mtg]

I briefly looked into a 2 speed rear end, but from what I recall, the cost was no less than a transmission swap.

Frank-id: see my above post. Normally, you would be correct, but the AT542/545 transmissions cannot lock the torque converter, which significantly changes the picture.

[charles_m]

Looks like you've got a DT466 in a bus that is a bit shorter than usual (rated cap of 34 if your profile is right), am I correct?
If so you've got the HP to get to 70 easily, but you've likely not got the gearing.
The AT545 is unusual behind a DT466, but I bet they did it because your GVWR is lower than most full-size buses.

Will you do much mountainous driving?
Is that the price for a rebuilt transmission with warranty?

I'd almost always go for a tougher tranny with a locking converter in a full-size bus. Ive seen plenty of AT545s burn up, but I've never personally had a MT643 fail in a school bus. They are rated to a much higher GVWR than our buses will ever see.

But there is a lot to be said for keeping your RPM's lower when driving on the highways.

How many miles are on your current AT? If it's had a long, hard life, that would play into my decision. If it's young, healthy, and you're going to be doing lots of fast driving on the flats, get the gears. If the old ones in good shape, you might be able to sell them and put that toward a beefier trans, too. Once you get the new gears in, take all the money you save on fuel (plus the money you got for the old gears when you sold em), and save up for when your AT finally kicks the bucket on account of trying to turn your taller gears ;)
Then, you'll have the best of both worlds.

[Booyah45828]

Lock up converter or not, the mph gain won't be as great as most would think because at that speed you're likely well past converter stall speed, so it shouldn't be slipping much if at all. If you want to go faster you'll have to numerically lower your final drive ratio. Swapping from the at545 to mt643 won't do that.

[Kubla]

well the guy accepted my $600 offer on the MT643 so I will be picking it up next week, my tranny only has about 110,000 on it, slippage is in the 10% range and since most of my planned driving will be full speed interstate, worried about heat build up

also will be a back up incase the MT643 in the bluebird wanderlodge craps out