Alternative to a shorter diff?

Out of curiosity. what is the difference between going with a tire that has a smaller diameter vs a shorter differential?
It seems to me the the ultimate result ought to be basically the same, which brings up the question: Why doesn't anyone do it?

Nick

Probably because no one wants to go with a smaller wheel/tire? I know I wouldn't.

The speedo would be meseed up, so would the hanlding...and it would look strange

Nate I just wanna say I'm with you on the E65. That photo in your sig is stunning to say the least! And the car in person is absolutely awesome.

Originally posted by webguy330i
Nate I just wanna say I'm with you on the E65. That photo in your sig is stunning to say the least! And the car in person is absolutely awesome.

Remind me, if I'm ever single again, to NOT have Chris or Nate set me up on any blind dates. Jeez guys...

Originally posted by TD


Remind me, if I'm ever single again, to NOT have Chris or Nate set me up on any blind dates. Jeez guys...

I have impeccabe taste in cars and women

Originally posted by TD


Remind me, if I'm ever single again, to NOT have Chris or Nate set me up on any blind dates. Jeez guys...

Blind dates are only for suckas foo.

I just have to wonder how it would mess up the handling? I haven't really heard anyone give any real reason not to do it, other than convention. I was playing with tire pressure today to simulate a shorter diff, and frankly, running the car with 33PSI up front and 28PSI in back gives it a phenomenal improvement in overall engine feel (as opposed to my usual 38/33, which I find gives optimal cornering). It pulls noticably better in all gears. (The fact the the rear tires have almost no traction in turns can be a hoot, too. It's neat going through corners with the feeling that the tail is leading.)

Throwing the speedometer off a little bit is OK with me.

Input?

Do some calculations with one of the many gear ratio/speed/tiresize calculators on th 'Net.

I'd think offhand that you'd have to change tire sizes really dramatically to notice a real difference (as opposed to your 'ass dyno').

According to my calculations, switching to a 205/40 tire would increase the number of turns of the wheel per mile by a little over five percent.
Now... Unless I'm totally off here, with a 3.46 differential, that would be somewhere approximately equivalent to dropping in a 3.65 or so diff. That doesn't seem like all that small a change to me.

Nick

I think what might be easy to compare is rpm@mph.

Back when I was trying to figure out if I had a 3.15 or 3.46 diff, looking at a rpm@mph table was the most helpful.

I dunno how you could want anything shorter than a 3.46. I'm already way too busy rowing the gearbox to want to go shorter than that.

a) I love rowing.
b) I have an xi and that adds a LOT of weight and drivetrain resistance.

Mostly b), though.

WebCars! Tire/Wheel Compensation Report
The circumference of the P205/50 x 17 original tire is 78.7623 inches

The circumference of the P205/40 x 17 aftermarket tire is 73.6913 inches

Your speedometer reading is 106.% of your true speed.

The ratio of your old vs. new tires is 1.06.

The current differential ratio is: 3.46
To retain the same overall gear ratio, you will need a differential with a 3.26 ratio.

--------------------------------------------------------------------------------
Without correction, your speedometer error will be:

Indicated speed: 10 mph. Actual Speed: 9.35 mph.
Indicated speed: 15 mph. Actual Speed: 14.0 mph.
Indicated speed: 20 mph. Actual Speed: 18.7 mph.
Indicated speed: 25 mph. Actual Speed: 23.3 mph.
Indicated speed: 30 mph. Actual Speed: 28.0 mph.
Indicated speed: 35 mph. Actual Speed: 32.7 mph.
Indicated speed: 40 mph. Actual Speed: 37.4 mph.
Indicated speed: 45 mph. Actual Speed: 42.1 mph.
Indicated speed: 50 mph. Actual Speed: 46.7 mph.
Indicated speed: 55 mph. Actual Speed: 51.4 mph.
Indicated speed: 60 mph. Actual Speed: 56.1 mph.
Indicated speed: 65 mph. Actual Speed: 60.8 mph.
Indicated speed: 70 mph. Actual Speed: 65.4 mph.
Indicated speed: 75 mph. Actual Speed: 70.1 mph.
Indicated speed: 80 mph. Actual Speed: 74.8 mph.
Indicated speed: 85 mph. Actual Speed: 79.5 mph.
Indicated speed: 90 mph. Actual Speed: 84.2 mph.
Indicated speed: 95 mph. Actual Speed: 88.8 mph.
Indicated speed: 100 mph. Actual Speed: 93.5 mph.

That's a-plenty of speedo error! Not to mention that your car will be significantly lower to the ground (2cm).

Well, that's a big size difference alright. 1.6in D smaller. Your car's gonna look like a lowrider up on hydraulics.

Ho, hum...
Lower to the ground doesn't bother me. Seems like an asset with an xi.
As for looks... Well, that really doesn't bother me much. I'd like better acceleration and any improvement in overall balance would be a real bonus.

So far, I haven't heard anyone say that anything IMPORTANT would be affected, is that correct?

I'm not exactly sure how the odometer measures distance on E46s, but the assumption is that it will also run 6% fast.

Also, 205/40s are really low profile, and could beat the crap out of your wheels.

Decisions, decisions... It'll be a few more miles until I can change my tires, anyhow. Probably just after I get my free oil change at 11.7K.

Nick

such as the engine's flywheel, the clutch and the wheels.

those would have a rather dramatic improvement on acceleration and braking, wouldn't you think?

will be very curious, once DINAN gets its act together and ships its stage 2 clutch/flywheel what the power improvement and/or acceleration improvement will be.

will be G-tech-ing before and after for sure.

(for E46 cars, the stage 2 clutch/LTW...already know what it does for other cars).

I guess I missed the post on this, but Nate, why do you have the new 7er on your sig instead of your car?? just curious.. :thumb:

Absolutely, blackdawg...
However, I'm too cheap to do that. :P I'm going to have to replace the tires ANYHOW, so I'd rather go with that.
I'd love an LTW clutch. Maybe when I wear this one out, I'll go for that.
Nick

Originally posted by Nick325xiT


So far, I haven't heard anyone say that anything IMPORTANT would be affected, is that correct?

Your ABS might be, especially as the tire wears down.

Originally posted by elbert


Your ABS might be, especially as the tire wears down.

As long as all 4 tires are the same size, ASC/DSC/ABS stuff shouldn't care.

Originally posted by Kaz
I'm not exactly sure how the odometer measures distance on E46s, but the assumption is that it will also run 6% fast.

Also, 205/40s are really low profile, and could beat the crap out of your wheels.

pretty sure the pick up is done at the diff so it would not affect the odometer.

Originally posted by CCs328Ci
I guess I missed the post on this, but Nate, why do you have the new 7er on your sig instead of your car?? just curious.. :thumb:

I have been involved in many debates over the 7, and I put it in my sig to make it clear that I like the car