E150 rear ratio

Hi,
I have a 1985 E150 van conversion (to a passenger van). Recently slightly larger tires (28.9" dia vs 27.1") were put on (due to an excessive number of blowouts) and this seems to have thrown the final
drive ratio off by quite a bit. Even taking into account the change in the speedometer and odometer, my mileage has gone down to 11-12 from 14-15 mpg.
Currently, when crusing on the freeway at 60-65, I'm pulling about 10" of vacuum which drops down below 3 when going up even the slightest incline. Going 70 on the flat is 7" and on any incline goes to 0.
In gear and at a standstill its 18" and 21" at idle and in neutral. High rpms in neutral will still pull 20-21" so i dont think i have a major exhaust restriction, and im not running lean so i dont think there are any major vacuum leaks.
This leads me to believe that the new ratio (equivalent to putting a 3.25 gearset in) is a bit too tall for the larger diameter tires and weak motor. As it is, the engine (5L V8, 2bbl 2150 feedback carb) and tranny (AOD, not AODE) are stock. The current gearset is a 9" 3.5. Am I on the right track here? Or should I be looking for something with the motor/tranny?
I believe that in order to return the final drive ratio to the way it was before putting on larger tires, i'll need a 3.72 gearset (actually 3.75, but those are hard to find!). Ive also considered a 3.89 or 4.11
If changing the gearset is reccomended, what ratio? I'd like a bit more acceleration and torque than the original stock set up (with the smaller tires and 3.5) but I also want to keep the gas mileage reasonable.
Would using a 3.89 or 4.11 make a big difference? (The van is a conversion, so its several hundred lbs heaver than the empty cargo van it was, but i don't tow and i do spend a lot of time on the freeway)
Oh yeah... what sorts of odd tools are needed for this operation should i try and do it? Aside from a heavy duty socket set / torque wrench and a dial gauge, that is. I've seen some people use a press (which i really dont have), but is that really necessary?
I know, its a lot of questions, but it helps to keep everything together.
thanks in advance!
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Yes, running larger overall diameter tires can feel like a switch to a numerically lower gearset. Your situation sounds a bit extreme, but then again a conversion van with a 302 is going to be a slug, and a carb'd induction won't help. The low vacuum readings tend to support this; you are forced to operate with more throttle opening because the larger tires are lugging the motor.
In my opinion, the best course of action would be to find a new tire shop, preferrably one smart enough to sell you an original dimension tire of the proper load rating (go with a LT tire, skip on passenger tires) for your vehicle. Maintaining proper inflation is of course crucial, moreso with such a heavy vehicle.
Even in a dropout center axle like a ford 9", a ring-and-pinion change is a major operation. Pinion depth, backlash, et al must be set within specs and a proper contact pattern verified or the gearset will last less than a week.
The brakes must be dissassembled, the retainers unbolted and the axles removed (good luck without a big slidehammer.) The driveshaft must be dropped to remove the centersection. While not always necessary, it is recommended to replace the bearings (carrier and axle) since the unit is already pulled apart. A press will be required for removal and installation. Removing the old ring gear and installing the new is possible with the application of a big hammer, a brass drift and profuse swearing, but the chances of mangling something in the process are significant; a press is again pretty much required. A break-in period must be observed following installation to avoid failure as well.
Expect to pay around $200 for the gear set. Add in the cost of an 'installtion kit' (pinion seal, collar, etc.), the possible cost of bearings, then the downtime on the vehicle and the possibility of having to farm out some of the work, and a simple tire change gets awfully appealing.
But then again, that's just me. Best of luck, Mark.
On Fri, 16 Jan 2004 02:15:43 -0700, tlw wrote:

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The tires on there are P235/75R15s, which seem to be one of the optional sizes for that vehicle -- Doesn't look like they did anything wrong there. The original size tire, a 205/75R15 does not seem to come in a LT rating (ARGH) Ive seen some 215/75R15s but those are still larger than the orignals. I'll do a bit more looking, but it seems that LT rated tires are more common with 16" rims and they're all well over 29" in diameter. (in other words, no improvement)

Yup. that part i know about. Sounds like fun :p

I think the axle and carrier bearings are in okay shape, no strange noises coming from them. I'm expecting to leave them alone. That leaves the pinion bearings and yoke. Whats the deal with those?

Yeah, I agree. The tires look appealing, but I've yet to find some with the proper weight rating -- i'll keep looking. Meanwhile, id like to keep my options open.
As for the way its looking right now, assuming I find new tires, thats gonna cost somewhere around $450-550 -- if theres a way to change the gearset for less than that, I'm all for it.

thanks! hopefully gas stays affordable in the meantime!
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If that is the case, then you may be stuck with a gear change.
This is assuming, of course, that all the standard stuff has been checked and eliminated: not overheating, fuel pressure ok, air filter doesn't look like a dryer lint trap, etc.
I don't intend to tell you it's a dumb idea to swap gears, but it would be an awful lot of money, work and time if something little is to blame. 1985--is this an electronic or feedback carb? Or is it all 'old-school?' I may have been premature to disregard the possibility of an engine trouble--hell, maybe all it needs is a carb rebuild and some tuning. My father had a fuelie 302 van once, and it was a dog even when runnning perfectly, so I was kinda biased on first thought.
And I don't see why the yoke couldn't be retained, and the pinion bearing, well I'd replace it, but if it's good, it's your gamble. I've seen it done before without any problem. I just don't like having to tear open rearends any more than necessary :0
Mark
On Fri, 16 Jan 2004 17:25:07 -0700, tlw wrote:

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Yeah.. got a new filter (fuel and air), temp seems okay, at least accoding to the temp gauge, and I checked the air intake to make sure there isnt a nest in it. I'll probably yank some plugs to see what they're doing in the morning... with the power valve open most of the time, i dont think they'll be very clean :p

Yeah, I'm trying to avoid it too. Its a feedback carb, not EFI. You can re-jet it and alter the idle mix with screws, as well as the choke and float ... all the fun carb stuff except main jet screws -- the computer takes care of that by changing the duty cycle on a solenoid that leans out the mix when its on.
As far as the feedback system goes, the (almost new) O2 sensor switches properly (except when lugging the motor). I did a carb rebuild prior to getting the new tires, and at that time, I managed to squeak a few tanks with 16-17.5 mpg.
Other factors in tuning (like dwell, advance curve, etc) also happen to be computer controlled. I think a new curve would help get a bit more power, but thats not exactly addressing the problem at hand. (not to mention just as expensive). Only thing i can change is the base timing, which ive tried at both stock (10 deg BTC) and slightly advanced (13 deg BTC) with no obvious improvement.
The 302 in this configuration is rated at something like 150hp peak... I think its just small for this application. (I'd love to see it in the taurus i used to have, though)

Oh nonono I meant removing the yoke and bearing from the pinion shaft. I'm not sure how tight the bearing and pinion splines are.
I dunno if ford used a crush collar or shims, but if its a crush collar, my understanding is that once the pinion nut comes off, its the point of no return -- a new crush collar needs to be installed or else the preload is off.
If I can get the yoke off with some prying, thats promising, but if it absolutely requires a press, that poses a problem. Ditto for the pinion bearing.
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If they're filty, maybe try leaning it out a bit--rich mixtures can eat power in a hurry.
On Sat, 17 Jan 2004 02:49:52 -0700, tlw wrote:

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what does 1 inch smaller have to do with blowouts?
Go back to your orignal size tire... youll save that much money in gas.
Ken

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On 16 Jan 2004 01:15:43 -0800, snipped-for-privacy@aura.dhs.org (tlw) wrote:

Having read a few of the other responses and your description of what it is doing, I wonder whether your trans is dropping out of OD as it should under load. OD is basically a cruiser ratio. If your throttle cable is not properly adjusted or the linkage is loose, it may hold OD too long. The little 5.0 really needs ot be able to rev a little for best torque and, it usually does not hurt fuel economy if any at all. In fact, we once had a carbed E150 Chateau with the 5.0 that got better mileage as well as better performance to just forget OD and let it rev at higher road speeds. Check the throttle cable adjustment to make sure it is tight enough. If it is not properly adjusted, it will result in low line pressure that can eventually result in trans failure as well as being agravating to drive.
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