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Old 07-22-2015, 10:51 AM   #1 (permalink)
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Tires profiles and sizes - the pros and cons

Well I know this is an often beat dead horse here in these forums and after searching and reading I have drawn some conclusions, and yet there still(for me) are some unanswered things about the compromises that are made with tires.

When looking at tires the questions that remain for me personally are related to the trade offs of wider/taller tires(aero drag and rolling resistance) vs smaller, narrower tire for lighter weight, less rolling resistance, and aero drag. Also the factor of vehicle height as it also increases the aero drag. Has anyone come up with a magic formula for determining the rolling resistance, aero drag of the wider tires and the increase or decrease in the vehicle aero drag by raising or lowering the vehicle.

At this point I am probably going to stick with a stock size tire, but there were at least 4 sizes offered on the vehicle I am buying tires for. The default size appears to be the best compromise but is not the smallest that was offered, and far from the largest. However at some point you can clearly go too large and there is a trade off. Anyone know at one point is the break even point on tire size and profile? Thanks.

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Last edited by Longsnowsm; 07-22-2015 at 04:33 PM..
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Old 07-22-2015, 04:00 PM   #2 (permalink)
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your rambling encourages NOT reading......
But from the first few paragraphs I can say this:
you seem to be weighting everything equally. not so.
Taller tires barely weigh more than the stock tire.
The issue w/ 'extra wieght' is when a taller wheel/rim is added (14" to 16") then you are moving the extra weight farther out.

But the solution is much simpler than a long rambling post.
Using a scangauge, take an mpg reading at 50, 55, 60, 65, 70, for aprox 5-8 miles on a flat freeway at a cruise controlled setting.
also record the rpms.
Now you have a baseline to measure everything by.
A 5% taller tire will spIN 5% LOWER mph AT A GIVEN rpm.

Trucks are normally geared for mpg on the stock tire. so going TOO tall can be counter productive.
But by having the baseline chart, you already know where the sweet spot is on the rpm side.

And smaller diameter(than stock) tires should be edited out of your rambling. YOu are wasting reading time on a complete non-issue.
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Old 07-22-2015, 04:28 PM   #3 (permalink)
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Sorry...

Sorry for the rambling... Just been trying to sort out vehicle height, rpm, weight, and adjustments for all the above with the various tire sizes, types etc. I will clean up the original post and summarize my questions. Thanks for the feedback.
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Old 07-22-2015, 04:42 PM   #4 (permalink)
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I will also interject this:
most of the discussions on taller tires deal with CARS.
Trucks tend to be a little different. Mainly because most don't have the stock size anymore and that confuses the conversation.
but by doing a BASELINE at the various speeds and rpm, you can fing the sweet spot in the rpms.

Most CARS benefit from a taller tire (5-10%) than stock. There are many threads that support this.
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Old 07-22-2015, 05:02 PM   #5 (permalink)
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RPM baseline

Thanks for the feedback. I will work on developing that RPM baseline over the weekend as I will be doing a long road trip and then look to see how that affects my tire shopping from there. Thanks for the input on the increase in size recommendation as it relates to cars. I agree, the truck situation is a bit more tricky. I will check to see how that recommendation for cars might apply to a truck.

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Old 07-22-2015, 08:47 PM   #6 (permalink)
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The variables are to varied. Wide vs narrow would have to factor in not only rolling resistance but wind resistance at the speeds you would normally travel. It might make sense to have wider tires for slower driving, but the increased wind resistance from the wider profile might be detrimental at higher speeds.

Probably the main factor is keeping engine RPM and BSFC in the sweet spot. For most cars, that probably means having taller "gearing" with larger diameter tires at the expense of acceleration. Ideally a change to the gearing would be made instead of changing the stock tire size.
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Old 07-23-2015, 02:24 AM   #7 (permalink)
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My car i believe would benefit from one profile higher. ie going from a 55 to a 60 as i always seem to be looking for gear change around 55mph car pulls like a train at this speed in 5th. 5th gear is geared to 22mph per 1k rpm. Max torque is developed from 2k rpm up to 4k rpm. If i go for a 60 profile i think from a tyre calulation site my speedo will be 3% slower. Do 50 mph on speedo means im travelling at 51.5 mph not a huge difference but hopefully pay divdends when i come to replace tyres in a couple of months.
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Old 07-23-2015, 02:24 AM   #8 (permalink)
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Ps 60 profile tyres are cheaper than 55 profile.
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Old 07-23-2015, 02:33 AM   #9 (permalink)
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Quote:
Originally Posted by redpoint5 View Post
It might make sense to have wider tires for slower driving
Why? (From an MPG point of view)
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Old 07-23-2015, 10:43 AM   #10 (permalink)
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Question

Quote:
Originally Posted by redpoint5 View Post
Probably the main factor is keeping engine RPM and BSFC in the sweet spot.
What is BSFC ?

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