Need some help with the math

[Smurf]

Good summary, Tele Man. But that's about where my train of thought gets derailed. What sort of impact each of those would actually have on engine RPM, and therefore MPG, is beyond my knowledge.

Anyone able to provide an easy method to figuring it out? If it helps, I currently do ~1900RPM at 65MPH, with the 225/60 16s and 3.73s.

[gone-ot]

I did something similar many, many, years (circa 1976) with my '72 Pinto with 1.6L engine, when I switched from stock 3.55:1 axle to 3.18:1 axle. That only lowered the engine speed about 300 rpm, from 2,810 rpm down 2,520 rpm, at 55 MPH. I would've needed a 2:50-2:53:1 axle, which wasn't available until 1977, to get under 2,000 rpm at 55 mph.

FE went from 27-28 mpg to 32-36 mpg -- but at the sametime I'd also added: (a) "tri-Y" header, (b) electronic ignition and (c) front spoiler -- so I can't say how much of that improvement was due soley to the lowered axle ratio.

Deja Vu: http://ecomodder.com/forum/156606-post3.html

[some_other_dave]

Please note that the calculated numbers for tire diameter and revs per mile are only approximations. Real-world tires often do not match those figures. Look up tires that you think might work on tirerack.com ; the manufacturer provides measured revs/mile figures to them. Of course, those also change as the tire wears....

-soD

[Smurf]

How would all this effect highway MPG? I realize in-town it would hurt, rotating the larger mass, but highway would improve.

If I went from 1900RPM @65MPH with 20MPG, what if I got the RPMs down to 1700RPM @65MPH, how would that effect MPG?

[gone-ot]

...here's my definite maybe answer: "probably" yes because you'd "probably" shift engine rpm closer into its 'best' BSFC area, which "probably" would lower its fuel consumption; however, it could "probably" mean that hills & high winds might "probably" become too much of a load and force you to either (a) apply more throttle (more fuel) or down shift (more rpm)--both of which will increase fuel consumption.

...don't you just love "weaseley-worded" answers?!?!

[some_other_dave]

One member here went from 15" wheels up to (much heavier!) 18" wheels and saw very little loss in the city, and a gain on the freeway. A green Scion xB, I think it was?

I think this was the thread I was trying to remember:
http://ecomodder.com/forum/showthrea...der-17440.html

I was off a bit; it was 15s up to 17s. And I misremembered the results, they showed pretty much zero change.

-soD

[hat_man]

You guys are so full of good information.

I am planning the axle swap for other reasons also. A mechanic friend and I are doing an axle flip in the rear and lowering springs in the front to drop the whole truck 4" in the rear and 2"-2.5" in the front. I have to dismantle the driveshaft and rear leafs to do this anyhow so it was a good opportunity to just swap in a new rearend with the taller gears and not have to try and just change the internals. This will effectively lower the stance of the truck and level it out or possibly leave a slight forward rake. The Ranger axles from '92 back to '84 are 1" shorter on both sides. By lowering the rear 4" and having 1"shorter half shafts it puts the widest part of the wheeltub lower and should allow me room for full rear skirts.

Because it is a somewhat "permanent" mod I was trying to figure out what kind of FE gain might be had. I can still get a '84-'92 rear with the 3:73's. I just saw it as an opportunity to go for a little FE boost. I could actually get a rear with 3:27's but I think that might be too much on hills and in town. I mean, I am driving a truck with about 110HP.

In my mind I was thinking that if the taller gears didn't hurt too much that someday I might try the 3:31's (either tires or gears) because they make a speedo gear for that and I could keep everything in sync. Dare to dream.

If you all have anymore thoughts I am all ears.