So after talking to someone tonight about the proper way to calculate the mileage discrepancy from running oversize tires.

 

Example used is running 205/70/14 tires on a 'Roo. 12% bigger than stock, IIRC. Either way, the number used in the example is 12% larger than stock.

 

One point of view is that the way to calculate is using this percentage of error. As in, the speedometer will always be in error by 12%. At 10mph the speedo would read 11.2mph, and at 70, it would read 78.4. To calculate gas mileage, you would multiply your odometer reading by 1.12.

 

The other point of view is that the speedometer error stays constant, no matter what the speed. As in, a 6mph error will always be a 6mph error. That the rollout of the car will stay the same. As in, where one rotation of the car's wheel used to take up x feet of road, now it takes y feet of road. Once you establish the error in MPH, you calculate accordingly based on mileage. Percentage has nothing to do with it, we're dealing with hard numbers only.

 

 

So which point of view is correct? Who's going to actually be more accurate as to how many miles their car traveled?

It will be much more accurate if you go by percentage. Your speedo will be twice as far off at 50mph than at 25mph.

yep %%% is right, cause 0 is the same either way...

 

I had a GPS in my car, it's only about 5 mph off at 30, but almost 20 @ 80...

I need to make a little printout for my brat.... rolling along at 17 MPH feels pretty fast in neighborhoods... dunno how fast I'm going, but it would be nice to know.

 

My tires are 28" or so if anyone knows how far off that is.

ok... I'm the mystery man. I'll state my case since we all think I'm nuts. Basically it goes like this: The spedo only "sees" RPM of the output of the tranny... so with stock tires lets say the car thinks that 1 axle rpm= 3 feet. the tire rolls once circumfrence stays the same, 1 rpm of the tire=3 feet.... Now 1 rpm of the tire = 5 feet.... spedo still sees rpms only so it still thinks that the cars only going 3 feet... but its really goin 5... the tires never change.. no matter how fast you spin them the error should remain the same. RPM x cir of the tire=MPH.

Although,,,, a tire with a rugged tread pattern can actually decrease MPG.

 

So any Highway increase in milage you recieve may be offset but a Rugged tire pattern such as an MT type tire

ok... I'm the mystery man. I'll state my case since we all think I'm nuts. Basically it goes like this: The spedo only "sees" RPM of the output of the tranny... so with stock tires lets say the car thinks that 1 axle rpm= 3 feet. the tire rolls once circumfrence stays the same, 1 rpm of the tire=3 feet.... Now 1 rpm of the tire = 5 feet.... spedo still sees rpms only so it still thinks that the cars only going 3 feet... but its really goin 5... the tires never change.. no matter how fast you spin them the error should remain the same. RPM x cir of the tire=MPH.

 

So, are you saying that the error should be calculated as a % difference or is it always, say, 10mph?

helpful site on caculation MPH differance betwine tire sizes.

http://www.net-comber.com/tirecalc.html

Might help figure MPG, provideing your engine doesn't have more drag on it due to the tire size and weight.:cool:

It's a percentage; not a flat number.

It's a percentage; not a flat number.

But why?

in KingBobDole's example above the tyre circumfrence changed from

3 feet / revolution to 5 ft/rev,

so,

the speedo thinks 1 mile equals 333 revolutions,

but,

333 revolutions actually equals (333 x 5) 1.666 miles,

so,

the speedo thinks 1 Mph equals 333 Rev / hour,

but,

at 333 Rev / hour you're really doing 1.666 Mph,

so,

at 100 Mph on the speedo you would be going 166.6 Mph!!!

because,

there is some error intoduced for EACH revolution,

so,

the more Rev / Hour you're doing the bigger the error!!!

166.6Mph in tha Brumby = some scary S__t!!

I don't know all the hows or whys but I know for a FACT that it's all wase a %. I haven't had anything with stock tires on it in a long time, so I use GPS for a speedo and the amount of error allwase gets worse the faster I go.

the fact is that V = w*r, where V is how fast you're going, w is the angular velocity of the wheel/tire and r is the radius. so, if you increase the radius of your tire, you increase your speed with the same angular velocity. So if you do it out with some ratios you get a percentage.

That's my physics major in action there

http://www.intercotire.com/site62.php

 

Punch in numbers, hit calculate.

ok then, at what point does it peak if it is a percentage. or does it continue to rise?because i have gone by stand alone speed traps at twenty miles an hour on the speedo and it says 25.i have also gone by a stand alone speed trap at 60 on the speedo and it said 65 .now you tell me why that is..........hence ,my speedo is off by 5 mph.i have done this several times to prove to myself that i can guage my mph by realizing the 5mph increase with tire size.once again , please,telll me why that is.

Thats just because your speedo was only accurate to within +/-10% from factory - and that error isn't consistant with speed, it could be all over the place. Error from tyres is conssistant %.

Ok guys, not to have you rehash this, but I'm still corn-fused. See for my brain to work I think that the spedo sees RPMs and Vehicle speed=tire circumfrence * tire RPMS.... I dont see how its variable at all.... Mostly because I did electric R/C car carpet racing and since its one speed and the engine has a max RPM you had to calculate its "roll-out" which was dependant on tire size. Personally I'm hoping that you guys (the majority it seems) are right, if my car is 12% off, I can do more like calculate gas milage and my wagon probably still has the same top speed. I just dont see how the error can increase, especially that much. I just dont get it.

HAVE FUN WITH THESE LINKS;)

http://4wheeldrive.about.com/od/calculators/index_r.htm

It is more that the diameter/circumference of the tire that you have to worry about. The concensus is correct - larger wheels generally equal less fuel mileage.

 

Generally as you increase the diameter of the wheel - you will tend to increase the width of the wheel as well. The additional weight of the larger tire/wheel combination along with the increased width generally will pull down acceleration performance and fuel economy. The plus side is generally much increased grip and improved braking performance.

 

As you increase the amount of unsprung weight (amount of weight not directly supported by the suspension) - you increase the amount of work done by the suspension. They have to work harder to keep the wheels and tires in contact with the road.

 

There is also the performance hit in acceleration with heavier/wider wheels. Not a problem if the car has ample power (Dodge Viper, Covette Z06, etc.) But in something with a little less power - the increase in the moment of inertia from the larger, heavier wheel will pull down acceleration numbers. This also goes toward lightwieght alloys - even though they may be very similar to weight of the OEM setup - the distribution of the mass of the wheel will still generate greater moments of inertia.

 

Granted - there are always exceptions - but pick at random, wheel/tire combinations that are +2 or larger and place them on most cars - their overall acceleration will suffer while gaining better braking and cornering. For some - that lose in acceleration is a small price to pay for better braking/cornering.

 

How much will it affect the performance - it varies according to the width and weight of the wheels. Usually it is on the order of every increase in static weight by 100lbs, will drop 1/4 times ETs a tenth. For every 10lbs in extra unsprung weight, ETs will drop a 10th.

this one's the best:

http://www.miata.net/garage/tirecalc.html

there is some error intoduced for EACH revolution,

so,

the more Rev / Hour you're doing the bigger the error!!!

166.6Mph in tha Brumby = some scary S__t!!

providing the little engine could turn the mas of the tire and rim at that speed without flying apart