Gas Milage Vs. Tire Width

[winstontj]

Not trying to create another "MYTHBUSTERS" thread here but I wanna know how much will my fuel milage go down if I go up from standard 185/75/R13 on E21 13" turbines up to a 14" or 15" x 6", 7" or 7.5" wheel? I'm thinking about going with a 6" up front and a 7" or 7.5" in the rear. I know that I shouldn't care... is't like "if you have to ask then you can't afford it" but I want to know.

I did a project in college with a Ford Explorer trying to make it get 30mpg. Skinny "pizza pie cutter" tires, exhaust, intake, and MILD head polishing was all it really needed. We did lower it about 1/2" as well.

Back on subject. Contrary to Mr. Ingraham's Mythbuster's post I do get 30mpg - maybe not all city but combined I get 31.5mpg exactly for the last three tanks on 87 fuel. SO I wanna bump up to fatter tires and go EFI and I'm wondering if they will negate each other or will the fatter tires really make that much difference?

[esty]

i believe this is akin to splitting hairs

[winstontj]

i believe this is akin to splitting hairs

WELL I WANNA KNOW!!!

To me it's a substancial change to go from a 32/36 to an ITB EFI setup, just as is going from 5" or 5.5" 185/75R13's to whatever you guys run - 205/40r15 maybe?

Is it really splitting hairs?

[Mike87]

For what it is worth, when I went from my 32/36 to 318EFI my mileage went from about 24 to 27 with 205/60-13 tires.

I think the width has some affect, but the stickyness or grip of the tire also has an affect. The stickier the tire, the worse mileage. So if you run a wide sticky tire your mileage will suffer.

Just my opinion.

[Napes]

Might want to remember that the sq. inches of tire patch (on the same car) will be the same no matter what size or width of tires you use. The patch gets wider with wider wheels/tires, but shorter, front-to-rear, to make up for the fact that the contact patch area stays the same.

Bob Napier

[SteveJ]

Which is why we run higher pressures.

Can you say "running on marbles"?

[winstontj]

Might want to remember that the sq. inches of tire patch (on the same car) will be the same no matter what size or width of tires you use. The patch gets wider with wider wheels/tires, but shorter, front-to-rear, to make up for the fact that the contact patch area stays the same.

Bob Napier

From what I know and have experience with is all-terrain tires and rolling resistance. When I did my Sr. project with the Ford Explorer the biggest difference to be had was more with decreasing rolling resistance than anything else. A "sticky" tire may have a higher rolling resistance but I don't know. I do know that a knobby tire is hard to roll while a smooth highway only tire is easy to roll. combine that with a skinny little tire that's easy to roll and it makes a difference.

For my car I will be going from a nice Michelin all season highway tread up to an aggressive Z rated 2 or 3 season tread. Also width affects the aerodynamics as well... I'll look up more when I have a minute but maybe it's less than I thought - or it will be less noticable than I thought.

[c.d.iesel]

rolling resistance

wind resistance

weight

ride height

drag cf

................doo your thing and you tell us the results

wider and 'larger' wheels/tires reduce the original

crisp light steering and precise suspension feedback.

Be functional or fashionposer

[winstontj]

rolling resistance

wind resistance

weight

ride height

drag cf

................doo your thing and you tell us the results

wider and 'larger' wheels/tires reduce the original

crisp light steering and precise suspension feedback.

Be functional or fashionposer

I'm gonna do one thing at a time just so I'll know. Last three fillups I've gotten 31.5mpg combined hwy/city and that's about 200 miles between fillups. The addition of the 5-spd bumped it up from about 28 to 30+ and the 3.45 rear end doesn't hurt. I will say that the 345 rear end sux in 5th and trying to climb a hill. I'll go up to fatter tires - maybe bottlecaps or even some 14" or 15" turbines if I can find them... then I'll re-post my milage.... then I'll bolt on all kinds of EFI crap and repost again.

My hope is that I can stay at ~30 with the fatter tires, and go back to where I am now, or maybe up to ~32-33mpg with the addition of efi.

ITB EFI stroker motor in the werks.... keeping my fingers (and toes) crossed that I can stay up over 20mpg!

[hpsenicka]

Might want to remember that the sq. inches of tire patch (on the same car) will be the same no matter what size or width of tires you use. The patch gets wider with wider wheels/tires, but shorter, front-to-rear, to make up for the fact that the contact patch area stays the same.

Bob Napier

Are you sure?? I have always understood that one of the principal reasons for using wider tires is to increase the contact patch area!

As for rolling resistance, it is likely more dependent on the degree of sidewall and tread flex/squirm.... causing the energy to be dissipated as heat. Increasing tire pressures will reduce sidewall flex and tread squirm, and will reduce rolling resistance.

[Beaner7102]

Might want to remember that the sq. inches of tire patch (on the same car) will be the same no matter what size or width of tires you use. The patch gets wider with wider wheels/tires, but shorter, front-to-rear, to make up for the fact that the contact patch area stays the same.

Bob Napier

Are you sure?? I have always understood that one of the principal reasons for using wider tires is to increase the contact patch area!

As for rolling resistance, it is likely more dependent on the degree of sidewall and tread flex/squirm.... causing the energy to be dissipated as heat. Increasing tire pressures will reduce sidewall flex and tread squirm, and will reduce rolling resistance.

Increasing tyre pressure ALSO reduces the contact patch, thefore reducing rolling resistance. Beaner7102

[JohnW]

Accurate gas mileage is difficult to pinpoint with 2002s. My odometer shows 10 miles when I've only covered between 9-9.5. Adjusting for this, I'm getting something between 21-24 mpg on a regular basis. (Oh yeah, 185/60/14, Weber 38/38, pertronix.) I'm really surprised to hear about non-tii 2002s getting 30+ mpg. But, then again, my Miata gets a consistent 33 mpg around town and it's only supposed to get high 20s at best. (And it's even got plus-1 wheels/tires!) Go figure.

[wazzu70]

There is too many variables to take into account to give you an answer. It all depends on how well your carb was setup and how well your EFI is tuned to tell whether you will see a difference. You cant just take on a so/so tuned carb and slap some 318i fuel injection stuff on and expect miracel results. It all matters how well everying it tuned as a system and how you drive it. As far as the tires making a difference. I really dont think its going to make much of a difference at all. There really isnt that much mroe resistance to make a difference if you are comparing the exact same tire in two slightly different tread widths. Also the amount of pressure in them ect has an effect on the amount of contact patch you will recieve. If you are that concerned about it, dont get the larger tires, and dont mod your motor.

Also the wider rear tires help under accel mostly because you have weight transfer to the rear which increases the contact patch. your increased width has more of an effect when the pattern is squished out to make an even larger print. Same goes for in a corner, the lateral load transfer to the outer wheel helps get it more grip (however keeping the car more level will give increased grip as two tires planted are more than one thats planted with some extra oomph).

In a nutshell a larger width tire will have more contact patch touching the ground, but not enough to give a hoot about unless you go a much more radical width.

[Slavs]

The skinnier tires should get better mileage since they are taller as well.

For example a 205/60 x 13 tire will dcrease your mileage by at least 3% just due to its smaller diameter. That is not taking into account the tire compund, contact patch, etc.

I have never heard of an 02 getting 30mpg. The factory tuned the 1502 for optimum mileage, and it averaged about 26mpg. The 1602 gets 22-28mpg in fully stock trim and at 60-65mph with 4.10 rear end and a 4 speed. The 318i 1.8 got 28mpg on a good day, and that is with a 5 speed overdrive. I'd love to improve mileage if any of you have any ideas. The only way I can think of getting optimum mileage from a 2 liter is to run the 8.5/1 compression pistons with the single barrel solex or fuel incection the cam being the stock 264 profile.

Slavs