Need Help! Bigger Tires?

[DragBean]

Yes aardvarcus. That is correct. I was originally thinking r13 meant an outside d of 26 but no.

The tires I'm replacing on the front drive train are 175/70r13 meaning fresh tires have an od of 22.5" if the math is 13+2×3.23×12×.60×175/1000
The tires are currently about 21.35 inches with load on them.

Does this mean mile odometer is counting miles short? Does the odometer compensate for changing tire sizes?

Also the current tread style seems pretty good (see pic) but the tires are only rated for 44 psi max. I'd have to get a similar tread and width to not negatively effect my mpg but if i get something with a higher psi rating and go a little bigger i should improve mpg a noticeable amount I'm thinking.

@fbov - so all the weight from the car gets transfer to the rims where the center bore contacts the axle? The lug bolts don't take the force?

Also there is the possibility of just using the dinged rim and getting new tires (it's not too bad, but I think it is probably permanently off balanced and may leak air) What do you think? The tread is so worn down that I'm losing almost an 1 & 1/4 from the new size (does that sound right?)

I think the max outside tire diameter I want to go with is 23" (compared to current 21 3/8")

[dfeldt91]

I have 14" stock wheels and just bought a set of 15" wheels. Both are wrapped with 195/60 tires for a 1" diameter increase which equates to a 4.1% decrease in RPMs. I get lug bolts Monday so I'm hoping that the larger size will give me a slight increase in fuel economy.

If you drive in your torque band then the size increase shouldn't cause any consequences except less fuel being used. Any larger of a tire and it will probably have a negative impact especially when outside the torque band. A narrower tire will give less resistance and a little better fuel efficiency which might come in handy with the larger wheel and tire size.

[DragBean]

Toyota 4A-F and 7A-FE engines

According to this chart, my engines torque output is pretty much flat after 2500 rpms.

Since I don't travel really less than 60 on the highway I'm guessing this means that I should aim for a tire size that gives me 2500 rpms at 60 mph? And then any speed beyond that is good as far as torque is concerned.

The one issue I have is that the torque output is at full load... We're hardly ever driving there. However, I guess this makes sure that all the parts can handle the increased torque load from bigger tires.

Anyways my car has no tachometer on it, so I looked up my gear ratio for the three gear A131 Auto Tranny to deduce my rpms.

1993-1997 Toyota Corolla transmissions and axles

Right now I'm going 3330 RPMS at 60 mph with my worn tires at 21 3/8 OD and with new tires at 22.5" OD that are the same it would be 3180 RPMS. Still high.

So if I were going to go 2500 rpms at 60 mph, that would be a 28.22 OD on the tire.

I think I'll go for 2800 rpms just to be on the safe side, so that yields a 25" OD.

That doesn't sound too drastic. A 10% change from stock conditions, 15% change from my current worn tire conditions.

[dfeldt91]

Darn, that is quite high. In my Golf MK3 with stock 195/60/14's I am at 3000 at 75 and about 2400 at 55 with it increasing 200 RPM per 5 MPH higher.

I'm looking forward to my 195/60/15's to lower that.

I think you will quickly see problems with a four inch larger tire. When turning the tire will more than likely rub the wells. I put 33 inch tires on my car that had 28 inch tires stock and it rubbed bad! Looked cool though! LOL

I am going up one inch in diameter with my swap and might even push it another half inch or so larger when I get new tires. That is about as large as I could go without narrower tires since it will start to rub when turning if any larger.

[DragBean]

I've spent the whole day learning about rims and tires.
I'm realizing there are a lot of variables.
I found a good website to buy tires. You can basically get exactly what you want for a good deal plus it's easy to see if a particular tire size exists.

Cheap Tires Online | Find the Best Deals on Tires | SimpleTire.com

I think overall the best and most cost effective thing to do for a non-hardcore ecomodder* is to up the diameter on the drivetrain wheels*, keep the non-drivetrain wheels the same (or even go smaller D to help reduce weight and radial inertia) and reduce the width on all tires to the smallest size your existing rim can take (which will probably only be a cm or two so your grip will be hardly effected much but the weight and drag will be lightly effected for the good).

If you are hardcore I would reduce the tire width by an appreciable amount which basically means getting new rims.

You can become hardcore by turning your engine off while on nice coasters and then using your e-brake to slow down so you don't lose break boost. And this should help to wear your tires more evenly if you have a FW.

You can then put the tires on yourself and then get someone with a Cosco card to balance them for like $3 a tire.

[CapriRacer]

Plus you have to be careful that things don't rub - very bad things can happen!! 7% sounds like a potential problem.

[ByDesign]

So many good points in this thread. Good conversation!

The key thing to remember in all of this is balance. There are good and bad effects of making changes. For instance, most cars are under-geared (IMHO) so that they "feel" peppy. Going to a larger tire diameter helps that, but only moderately.

Also, as mentioned by aardvarcus, it's the total rolling size, not wheel or tire. I just changed out 18" rims with 40's (summer sport tires) for 16" rims with narrower 70's for winter. Overall diameter is virtually the same, snow traction is immensely better.

Yes, narrower tires are better (usually) for rolling resistance and aerodynamics, but worse for traction (except in snow) and usually worse for ride and much more susceptible to damage in a big pot hole. Also will decrease braking capability.

The down-sides to larger diameter tires: 1) it lifts the car causing a increase in Cd, more drag, (all else held constant). Greater ground clearance may help with parking lots, but not with aerodynamics. 2) can interfere with internal clearances (just make sure they don't rub anywhere). 3) will change some of the vehicle dynamics (like turning capability) if you're into pushing the limits ....

Changes in wheel size is not compensated by the speedo or the odo. Most cars indicate higher than you are actually going, so check that with a stop watch and mile markers on the highway. Exactly 60 mph will take 60 seconds. (Most cars read 2-5 mph more than your actual speed.) Odo's, on the other hand, are pretty close to exact with the stock spec tires. Increasing size will show slightly fewer miles, but that's not worth the hassle unless you are really crazy about exactness -- in which case you should use a GPS to measure miles instead of the odo. (As a side note, your fuel econ measurements will be difficult to compare if you're using the odo because of the error you'll introduce with the change.)

Finally, I strongly suggest you don't mix and match sizes. Unless the car was made specifically for it, or you specifically want some other effect of size (think 60's muscle cars and dragsters) I'd keep them the same. Makes lots of things less complicated like rotating tires for best life and uniform wear, flat changes, spare tires, etc.. Increasing tire size will change the vehicle dynamics anyway, but making them different sizes may change more than you want.

Great discussion by everyone. Thanks!

[DragBean]

Originally Posted by ByDesign

So many good points in this thread. Good conversation!

The key thing to remember in all of this is balance. There are good and bad effects of making changes. For instance, most cars are under-geared (IMHO) so that they "feel" peppy. Going to a larger tire diameter helps that, but only moderately.

Also, as mentioned by aardvarcus, it's the total rolling size, not wheel or tire. I just changed out 18" rims with 40's (summer sport tires) for 16" rims with narrower 70's for winter. Overall diameter is virtually the same, snow traction is immensely better.

Yes, narrower tires are better (usually) for rolling resistance and aerodynamics, but worse for traction (except in snow) and usually worse for ride and much more susceptible to damage in a big pot hole. Also will decrease braking capability.

The down-sides to larger diameter tires: 1) it lifts the car causing a increase in Cd, more drag, (all else held constant). Greater ground clearance may help with parking lots, but not with aerodynamics. 2) can interfere with internal clearances (just make sure they don't rub anywhere). 3) will change some of the vehicle dynamics (like turning capability) if you're into pushing the limits ....

Changes in wheel size is not compensated by the speedo or the odo. Most cars indicate higher than you are actually going, so check that with a stop watch and mile markers on the highway. Exactly 60 mph will take 60 seconds. (Most cars read 2-5 mph more than your actual speed.) Odo's, on the other hand, are pretty close to exact with the stock spec tires. Increasing size will show slightly fewer miles, but that's not worth the hassle unless you are really crazy about exactness -- in which case you should use a GPS to measure miles instead of the odo. (As a side note, your fuel econ measurements will be difficult to compare if you're using the odo because of the error you'll introduce with the change.)

Finally, I strongly suggest you don't mix and match sizes. Unless the car was made specifically for it, or you specifically want some other effect of size (think 60's muscle cars and dragsters) I'd keep them the same. Makes lots of things less complicated like rotating tires for best life and uniform wear, flat changes, spare tires, etc.. Increasing tire size will change the vehicle dynamics anyway, but making them different sizes may change more than you want.

Great discussion by everyone. Thanks!

Good Points Design

In my case I'm in the process of doing re-doing the front air-dam because a huge rock went through it, so I'll be able to compensate for lifting the car 1.5".

I think there are definitely some benefits to be had for people more than just your casual ecomodder that can be achieved as easily as just buying new tires that are different on the front and back. The big thing being you don't want big tires on your non-drive train wheels because they are more weight and have way more radial inertia.

In terms of wear, I plan on using the e-brake handle more often to wear the tires a bit more evenly.

I'm glad you bring up the point of getting a flat. I think if you put a donut with a big tire on the drivetrain you'd proably destroy the diffy after 50 miles. Might be OK for going 5 miles at 30 mph though. I think the solution here for getting back home on the freeway is to take your back tires off and put them on the front and then go with the big and small guy on the front. (And be the slowest guy on the freeway, which you're probably really good at anyways if you are a dedicated ecommodder

You have a point with narrowing the tires and going over potholes. But I gets a bigger OD, then the taller walls will have more shock absorption than not having taller walls (as long as it is still a passenger class tire) so that should compensate for a slightly smaller width.

[dfeldt91]

I don't think non drive tire size(diameter) plays too big of a role since the engine is not required to power those wheels specifically. If you have smaller non drive tires they will wear out quicker as they are turning more per given distance with both sizes of tires being completely equal in construction and materials. It also turns the bearings for the hubs more which wears those.

If you get a half percentage point better gas mileage but have to buy wheel bearings or tires sooner than you really haven't made any headway and are actually going to lose money instead of saving.

Remember: I am not schooled for this kind of stuff so I could be completely wrong. Just my thinking on the subject...


Of course you could get the smaller tires and try it out. Although I think the savings are going to be pretty slim if any and could be inside the margin of error(wind, temperature, etc differences when testing).

[CapriRacer]

A couple of thoughts:

First is that tires deflect and the revolutions per mile is NOT what one calculates using the outside (freestanding) diameter. It's about 3% lower than that. Be sure you remember that when you do your calculations.

And, Yes! your car does NOT automatically compensate for the changing tire diameter. As a general rule, the speedometer is driven off the output shaft of the transmission. So when you change tires, you need to be aware that your speedometer and odometer are in error.

Having different tires front to rear MAY cause some emergency handling issues. The problem is that you will NOT know that until you are in such a situation - and then, it is too late. Frankly, I would make a complete changeover, rather than piecemeal.