4.10 gears...not what I expected.

[Paul Workman]

Originally Posted by WVZR-1

Some 'real' information I believe. Dominic, I'll use your ratio and tires as an example. If you had 3.73 gears and your 315/35R17 tires and were 'happy' you need to be concerned about not your losses but rather what you need to do to compensate for the increased tire that you added or you just disregard it and don't worry.

I'll use that for a start. I picked a MPH # because one is need to start from so I chose 60 MPH and 4th gear. I always use 1:1 regardless of the transmission. A 3.73 @ 60 MPH I got 2960 RPM by this:

60 X 3.73 X 336/25.4 (the 25.4 I used from a dimension I measured with my BFG 315/35R17's many years ago) I did NOT use the advertised diameter. That skews the results which BTW I believe the charts earlier in this thread are based off advertised numbers and not real dimensions that we can measure. Those #'s are close but NOT correct.

Now when you add the tire that you mention measuring 26.25 (in this thread) I get an RPM value of 2864:

60 X 3.73 X 336/26.25

Now to get your 'performance' back you need to add gear and since we know the various ratios available for the D44-HD it's easier to experiment using available ratios substituted into the formula.

60 X 3.90 X 336/26.25 = 2995 so you've got it all back with that gear change + very few RPMs.

If a person had 4.10's with 315/35R17 and was 'happy' then these #'s if they made the same tire change would be required.

60 X 4.10 X 336/25.4 = 3254 RPM (315's)

60 X 4.10 X 336/26.25 = 3148 RPM (26.25 measured)

The add required using available ratios:

60 X 4.30 X336/26.25 = 3302 so you've got it all back again + a few more RPMs.

These were very straight forward and it's much easier using 'known' available ratios to calculate recovery.

With your particular tire choice the 'loss' if you wanted to only be concerned with the loss is 3%. 25.4/26.25 = .96761 so the 3.45 becomes 3.34 and the 3.73 would become 3.61 or so.

Done in this fashion also to calculate the new ratio 25.4 (original tire) X 3.73 (axle ratio)/ 26.25 (new tire) = 3.6092

You could confirm those numbers I'd think by checking RPMs with an accurate tach and not the cluster component.

Every tire needs to be measured at the pressures you will intend to use and dimensions will vary for various brands.

I believe my math will stand the test. I hope or I've made a much larger A$$ of myself. There are calculators out there that likely will confirm these numbers or very close. The one I posted way back in this thread should work I believe.

This is where I got the dimensions that I used for your 19's.

_________________

Yikes!

"If you can't explain it simply, you don't understand it well enough." - Albert Einstein

(This discussion has veered off-topic, but since we're here now...)

Ratios.

No matter how you slice it, the root to questions regarding the effect on differential gears, speed, rpm, or any other equivalents resides in the ratio of one tire size to the other. Period.

Depending on the question, there may be a direct ratio relationship, or an inverse (i.e, 1/ratio) relationship, but regardless, that ratio between tire sizes permeates all calculations regarding differences in gear ratios, or distance covered per tire rotation, or rpm, or you name it.

For example: If speed over the ground and engine rpm are to remain constant, then the differential reduction ratio (ring/pinion) must be changed by the same ratio as that between the two tires. Otherwise, either speed over the ground will change for a given motor RPM, or RPM will change for a given speed.

Or, for an opposite example, to find the equivalent differential reduction ratio value resulting from a change of tire size, the inverse (1/RATIO) x differential reduction gear ratio will produce the effective equivalent rear end gear ratio (as well as the ratio correction required for the new speedo gear.

And, so it goes... (Well, almost. Practical applications typically have dynamic variables which skew results. Variables like Hog points out and many other practical bits and pieces get in to increase the ambiguity factor. But, as long as we can account for some of of the small dribbs and drabbs, and the ambiguity doesn't exceed say 5%, it should suffice for purpose of discussion, I should think.)

Hog: Good points as well.

But back to the OP's question: What are the real trade-offs between say the 4.10s vs. stock 3.45s in actual driving or racing situations??




WV: I must be staring right at it but do not see: where is that "336" factor you use in your calc's derived from?