I have 165/70's on my Midget and have determined that the speedo is off
by 5 mph at 60mph. That is indicated 60 mph is in reality 65. This was
determined by following a Motorcycle with a calibrated speedo at (my) 60
mph. As well as measuring marked mies along the interstate for several
hundred miles. It turns out that the odometer is off by about 3miles in
100. So I drive an indicated 65 (except on the SureKill ;-) ) and still
get passed like I was standding still.
Larry Macy
78 Midget
>robert weeks wrote:
>> While perusing my dog eared copy of Robert Bentley's "The Complete Official
>> Sprite / Midget 1967-1974" I noticed that the specified tire size for
>> radials on these cars are 145SR-13. The PO had mounted 155SR-13. I know
>> that this throws the speedo off, but in which direction? Am I going faster
>> than the speedo says or slower? Im guessing slower, but math was never my
>> strongest area
>
> Well, assuming the world is perfect, you are going slightly
>faster.
>
> The rule is supposed to be that tires with no aspect ratio
>markings have an 80 aspect ratio.
>
> If the world was perfect, that would mean your slightly-wider
>tires have a slight-taller profile as well. This means that each
>turn of the tire goes a little farther than your car would
>normally go. Therefore, same RPM, same speedo reading as before,
>but you are going a little faster.
>
> But in reality, tire diameters between makers and models
>and so on are rather loose. You may find your tire diameter
>is more, less, or the same as the original. All in all,
>it's not likely to be very far off at all for such a small
>difference.
>
>> Are there other advantages/disadvantages to running larger tires? Better
>> gas milage? Worse gas milage? Smoother ride?
>
> Well, if your tires were large enough to make a difference,
>think of it as having a lower ratio rear end (or a very mild
>overdrive). So in theory you get slightly better mileage
>and lower revs-at-speed but at the expense of acceleration.
>
>> I just checked British Wire Wheel and they
>> don't even carry them, the 155's are the smallest they stock.
>
> I still have 145s on my Midget, but hardly any midgets
>I see still do. I wouldn't worry. Many people I know are
>running 185s without modification, with 165s and 155s
>being the norm.
>
>--
>Trevor Boicey
>Ottawa, Canada
>tboicey@brit.ca
>http://www.brit.ca/~tboicey/
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