30's and 3.55

[5-90]

Should have mentioned it earlier...2000 with AW-4 and 3.55's. Right now it's got a 2" lift. Why do you think 30 or 31's work out better than 225/75/15's (28's)?

Essential mathematics - having the 30/31" tyres put the the overall drive ratio (transmission, final drive, tyre circumference) right about in the middle of the effective power band at various cruise speeds.

The OEM 28" tyres put it a little low.

So, even with the additional rotational weight and inertia, you were also much closer to optimal cruising crankshaft speed with the larger tyres.

I don't recall the formula offhand, but you should be able to find it in "racer math," and be able to calculate it for yourself against the torque output curve if your engine (I should have a copy of that somewhere if you can't find one.)

Automatics tend to like to cruise at slightly different speeds than manuals - except when the torque converter is locked up (has to do with mild torque multiplication in the converter,) but it's not a problem.

It's quite possible to cruise at too low of a crankshaft speed - which screws things up. This is why I think that pairing 3.07:1 axle gearing with the manual gearbox and the 6-242 was an irretrievably stupid idea. I think it has to do with marketing convincing people to cruise with their engine speed as low as possible - but as much as marketing has been corrupting our language, they cannot change physics. (No matter how hard they try...)

Since transmission ratios cannot be changed easily, the rolling circumference of the tyre (which is always lower than the actual circumference, but close enough that using the nominal size is doable) and final drive (axle) ratios are selected to complement each other for effective road vehicle design. The idea is to put cruising road speed as close to the peak torque output crankshaft speed as possible. Why? Because an engine is operating at its most efficient when generating peak torque - which, for us, typically means 2500-3000rpm at the crankshaft. Ask any pilot of piston-driven aircraft - part of type certification is (or at least, used to be) knowing the torque/power output curves of the engine, in order to calculate optimum cruising speeds and ranges...

 

[Porch951]

Everything said above makes sense, except that 30" tires put 4th gear cruising speed in town around 1800rpms and on the freeway around 2200....which is too low.

I was constantly trying to knock my AW4 out of 4th because it was bogging. The AW4 loves to get into 4th as quickly as possible, and manually shifting gets tiring. Of course i live at elevation so power is further decreased...

 

[cdawall]

Everything said above makes sense, except that 30" tires put 4th gear cruising speed in town around 1800rpms and on the freeway around 2200....which is too low.

I was constantly trying to knock my AW4 out of 4th because it was bogging. The AW4 loves to get into 4th as quickly as possible, and manually shifting gets tiring. Of course i live at elevation so power is further decreased...

I live pretty high up as well. 3.55's and lighter 285/75's are not that terribly bad. rev's around 1400RPM at freeway speed IIRC. If you are careful with the throttle it will go up hills without downshifting. Can keep a constant 18MPG out of it as well. I would be willing to bet with a good cam you could keep it in OD with 3.55's and 33's these are good little I-6's with some lowend to them

 

[Dest]

I barely notice my 31s, and I swap to them on weekends for wheeling then back down to normal size.

 

[5-90]

Everything said above makes sense, except that 30" tires put 4th gear cruising speed in town around 1800rpms and on the freeway around 2200....which is too low.

I was constantly trying to knock my AW4 out of 4th because it was bogging. The AW4 loves to get into 4th as quickly as possible, and manually shifting gets tiring. Of course i live at elevation so power is further decreased...

Automatic? You can lock out overdrive - but I didn't have any trouble with 31's on my 87 for years.

It's similar to what I've been doing in not using fifth gear with 3.07 and 31's - overdrive isn't useful until up around 90mph - while I don't have any trouble with that, the local constabulary tends to take exception to it for some odd reason...