Speedo calculation

[Bex]

I know this has been posted here already, but I've been searching and can't find the answer. So sorry for repeat question. I have increased my tire size from original 205/75 to 225/75. So what percentage faster am I going now, over what my speedometer is reading? Thanks.
(And just to advise - I took a 'long' trip today - long for Irish standards, only maybe 100 miles* - and boy, do new tires make a difference. Filled up before I left, travelled 121 miles, gas tank indicates only about 1/4 gone. Tires pressure at 28psi (rather than before where tires wouldn't hold pressure, so each tire was different, etc.) and new tires made a world of difference. Maybe will 'increase' my gas mileage too. Am doing an experiment (using .26 for liter to gallon) to see exactly what my car will do. Hope to surprise you all....

*Longest distance from northern most to southern most bit of Ireland is only about 500 miles. Almost commuting distance in New York, but considered full day's journey here...)

[reburg99]

I use my GPS to check the difference between the spedo and actual. I'm assuming the GPS is more acurate.

[Paladian]

Originally Posted by reburg99

I use my GPS to check the difference between the spedo and actual. I'm assuming the GPS is more acurate.

i do the same. on my 31x10.5 i have about 10km/h differance. when dash says 80 i am doing 90 on gps. hard to remember to drive 10 under all the time

[deeznutts77]

try this
Tire Size Calculator - tire & wheel plus sizing

[ack]

you can also try the following link to guesstimate speed:

Ack's Generic Gear Ratio/Speed Calulator

This method assumes that you have a tachometer - which allows you get an idea of your actual speed from your tach reading.

The problem with this way of calculating speed is that you might be off by an inch or so in your tire diameter from wear, actual factory diameter being different than the markings on the tire, inflation pressure or a combination of the above.

The best way to get an accurate idea of true speed - as others have stated - is to buy/borrow a GPS unit.

I hope that this helps!

[Bex]

Thanks for your replies!! It turns out that speedo will be 4.35% too slow...hope I can keep my eyes on the road while I'm using the calculator!

[fuzzy1]

Fuzzy shall now throw a stick in the works!
The calculator Deez posted has 1 flaw: it assumes the speedo was accurate with the stock tire size... & it more than likely wasn't. It seems pretty common that people GPS & find their actual speed is lower than the speedo reads on stock tires. I recall one guy who said with his 235/75's the speedo read closer to actual than with stock tires when compared to GPS...
Now to recalculate...

Fuzz

[kick-fix]

fuzzy is right. 3 ways
1: factory is high reading in spots.
2: factory purposely set it high ,so you dont get tickets.
3: after 20 years, that magnet inside can get weak, and speedo reads low.

the speedo is most accurate in the center of its range and or near 50mph.

GPS: yes, try that:

or friend who has stop watch and find mile makers,(kilo markers in Ireland?)

before GPS, we drive 60mph, , count sections for 1mile mark. (1minute or 60 seconds)
write down seconds.
go home do math. (when new car will show more than 60 seconds, for most cars)
(lawyers are everywhere, so engineer added , error. slowing the endless court battles)
dont be surpised if 30,50,80 all have diff errors. nature of beast.

[HotTommy]

One more word on this topic. I use the stop watch timed one mile stretch at an indicated speed of 60 MPH. If the speedometer is right on and you maintain the speed consistently, the time should be 60 seconds. If the time is off by only five seconds or so, you can just add one MPH for each second under 60 seconds or subtract one MPH for each second over 60. Beyond about five seconds, you need to do the math (speed = distance / time) to get an accurate speed.

Example: Indicated speed = 60 MPH, Seconds to travel one mile = 57, Actual speed = 63 MPH.

[Bex]

Originally Posted by kick-fix

fuzzy is right. 3 ways
1: factory is high reading in spots.
2: factory purposely set it high ,so you dont get tickets.
3: after 20 years, that magnet inside can get weak, and speedo reads low.

Well, this is like beating a dead horse (or Tracker), no? I have read that factory speedo's are approximately off by 5mph, in agreement with #2 above. (Am also reading about not using GPS for mileage...something about no reading in tunnels or something. But as I don't have GPS, am not studying this.
Anyway, I have just made a trip in my Tracker, of 200 miles, and filled up right after trip to check mileage. So many variables come into this! Weight (had 200 extra pounds in back of car), road (hilly in many places), wind (constant between 40-60mph winds during 100 miles of trip), stop and go (approximately 25% of trip), etc. Tried to keep car at 300 rpms or less, although hit a lovely 'motorway' with everyone flying past at 75 or 80, so did cruise at 65 mph (3500 rpms on tach) for about 20 minutes. Bad for experiment!!
Reading about how larger tires will actually make your mpg become worse in local driving, as it takes more power to overcome inertia of the car. Will slightly improve mpg in straight highway driving, as tire rotates further distance with each rpm. No mile (or kilo markers here). Filled up at end of trip: went 199.8 miles, took 28.27L to fill up (using new, awful calculation, means 7.35 gallons) = terrible 27mpg. Now the tire size calculator advises that new 235/75/15 tires (over 205/75/15 stock) means speedo is 4.350% slower than actual speed. Is it right then to assume that mileage itself is 4.350% less (odometer indicates 199.8 miles..have I really traveled 208.5 miles? Having thought all these years that I was getting such great mileage, I am grossly disappointed at the 27 or 28.39 mpg that I am now calculating.