**HELP** 914 tire size question's
 

just need some help with a problem/question...

i know the standard tire size for the 914 is something like 165r15.. which i understand to be just a basic tire.. but i've see 914's with other tires... the one i bought has 195 50 15's on fuch's

first question.. what does this do to the mph situation...

second question.. would 195 65 15's work and again what would it do to the mph situation..

if someone with experience in the area could educate me on what is the BEST choice.. i would greatly appreciate the help..

eric

I went thru this - as I remember the rolling diameter of the stock tire is 25.4". A tire store should have a chart which gives the rolling diameter of their tires.

I put 205/65s on mine to maintain stock rolling diameter, which makes the speedo read "correct"

Smaller diameter tires will increase the rpm required to maintain freeway speeds. A 195/50 will probably result in 4000 rpm @ 80 MPH, where as the stock diameter tire would be somewhere around 3400. Yea, I know what the speed limit is, but.........,

195/65s have the stock circumference (same as 165-15s), so the speedo will read correctly. But steering response and handling is not as good as 195/60s.

205/60s may not fit (they usually rub the fender lip).

195/60s ride a bit harsh compared to the stock 165-15s, but if the ride with 195/50s doesn't bother you much, then 195/60s are the way to go because they give good handling and steering response, and their circumference is within 1 - 2% of stock so the speedo will only be off by that small %. I would like 195/60s, but the roads up here are kinda rough, so I have 195/65s on my Fuchs.

Thanks.... to all..

I will be using 195 65 15 on steel rims...

I've been running 205/65/15 for a couple of years with no problems. Clearance is tight, but it fits and looks good.
Kent

205s fit on stock 914 wheels under the fenders of most, but not all, 914s. When there is a problem, it is usually only on one side and it is always with a rear fender. 195s should fit under stock fenders on all 914s, as long as 914 wheels are used.

Bug wheels generally will not allow anything wider than 165-width tires to fit.

Depending on what size you think the 165HR15 tire really is (165/78-15; 165/80-15; 165/82-15), 195/65-15 tires are practically the same size or a little smaller than the original tires. Tire Rack used to have a conversion chart showing different sizes with the same (or almost the same) overall diameter, but I can't find it any more... :(

Anyway, 196/65-15 is close to the original diameter. So is 195/70-15. So are 205/60-15, and 205/65-15. As I said, part of it depends on what aspect ratio you think the unlabeled tires were.

I have used 195/60-15 in the past, and liked it. I have run 195/50-15 for a while now. The handling is great, the feedback is great, but the ride is pretty harsh. I will probably try 195/65-15 for my street tires next time.

--DD

Because I have nothing to do and no life, let's discuss why you would run a lower profile (aspect ratio) tire.

A high aspect ratio tire does a couple of things:
1. The sidewalls flex and create heat, deterioriating the sidewalls.
2. The sidewalls flex and give a "squirrelly" feeling in a loaded (cornering) condition.
3. The sidewalls flex to give a "smoother" ride
4. At high speed, the tire will "crown" giving a smaller contact patch.

Lower aspect ratio tires do the opposite:
1. The sidewall does not flex as much, generating much less heat and the probablility of a sidewall blowout.
2. The "squirrelly" feeling in a loaded condition goes away as the rim is not shifting between the road contact patch and bead.
3. No sidewall flex, harsher ride
4. the contact patch of the tire stays flatter, giving more contact with the road surface.

There's more, but this has occupied a few moments of an otherwise uneventful day.:D

And I can't find the rolling diameter chart either.

I believe teh conversion is generally something like this:
the 1st number is the 'width' of the tread in contact with the road.
the 2nd Number is a % of the 1st and = the height of sidewall.
The 3rd of course is rim diameter. In MM I believe.
So 205/60/15 which is what I run on my 2liter fuchs's's is (rear end)
205 x .60 = 123mm sidewall.
and if 195/65/15 is very close to stock diameter = 117mm sidewall, on the same rim- thats a 6mm difference. I doubt it effects the speedo reading much...my car is within 1mph and I run 195/60's in the front.
I could be a little off, but I remember reading that somewhere.
_Bob O

you guys are awesome... again.. thank you sooooo much...

I knew what the numbers meant. I never thought to use them this effectively, thank you.

I like the round black ones:D

Remember that the numbers are more of a general guideline than absolute measurements. Some manufacturers seem to use the overal carcass width for the "width" number, some use the tread from shoulder to shoulder, and some seem to pull numbers out of a hat. (Hoosiers, in particular, run very wide for their size.)

Many tire companies will have a "revs/mile" figure for the various sizes available of a given model of tire. That will be much more accurate than fiddling the numbers. However, running the numbers will at least get you close, probably close enough for anything any of us will ever be doing.

--DD

I couldn't find the conversion chart on Tirerack's site anymore either, but if you select any make of tire and click on "Specs", they do have a chart of all the sizes that the tire is available in as well as their diameter and revs/mile

Andrew

FYI:

Tiresize difference calculator on Miata.net

Sang 76/914

how off will it make the speedo if you run 195/50's?
i'm having the same dillema trying to decide between 196/65's, etc

Run the numbers and see.

195mm * 0.50 * 2 [one sidewall on each side of the tire] + ( 15 * 25.4 ) == 576mm diameter for the 195/50-15.

195 * 0.60 * 2 + ( 15 * 25.4 ) == 615mm diameter for the 195/65-15.

The 195/65s are 6.77% taller than the 195/50s. Assuming the 195/65s give you a correct speedo and odo reading, then 195/50s will read about 7% fast.

--DD

I just put 205/65s on mine on Mahle's and they have more clearance than the 165s did on after market wheels. I took them to a local chain and they agreed to test fit the tires before purchase.

with 195/60 15's i figure my speedo is of by about 10%. i never know how fast or slow i'm really going. i was paced by a rx-7 recently and that was the difference he confirmed

kevin

kevin

Thanks for letting me keep up with you, Kevin.

My teener should be ready for the next trip. :)

- Roger / '76 914, '84 RX-7 daily driver

Hey sanglee007, Thnaks for the link! It is just what I have been needing.