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How accurate is your speedo? + Maths & regear questions/answers
Hi, in a bid to find out how fast I'm going and check real MPG I need to start at the beginning and work from there so bear with me, this might be long one!;
I am under the impression that a 1:4.875 + 31" tyre combo is 'standard' or at least the most issued combination? If not please correct me.
If this is the case then the standard speedo in KMs should be pretty accurate so I need to know whether the speedo conversion plates fitted on import have the numbers in the right place (you never know with cheapo face plates!)?
If it is then we are all able to have a good place to start when regearing or changing tyre sizes to correct our speed with regard to what the speedo says.
Now comes the maths for regearing or tyre size changing (effectively the same thing but in different places).
I have two regears on my system as not only are my tyres 33" but my diff ratio is 1:4.556 instead of 1:4.875.
I'll start with gear ratio increase.
Stock Ratio divided my Current Ratio x 100 = 7% difference
so take my speed and add 7% for the correct speed, ie my speedo says 50MPH but I'm really doing 53.5MPH.
Please check my maths.
Now, the same with the tyres
Newer tyre size of 33" divided by Stock size of 31" = 6.5% difference
so again add 6.45% to my already modified speed and I get 56.95MPH.
Again, please check my maths.
If that's correct my speedo is as follows
Speedo -> Actual Speed
10 -> 11.39
20 -> 22.78
30 -> 34.17
40 -> 45.56
50 -> 56.95
60 -> 68.34
70 -> 91.12
This can now be used to work out the correct MPG.
Using the figures previously worked out plus some extras and a few other simple calculations I get the following information for MPG
Reported KMs = 424 -> Actual KMs = 482.96
Actual KMs to Miles = 300.1
Litres used = 59
Litres to Gallons = 12.98 Gallons
so
300.1 divided by 12.98 = 23.12 MPG
Cost per Mile if you want it is worked out as;
Cost per fill divided by mileage for that fill
In my case £82.54 divided by 300.1 = £0.28 p/m
(to check it you can use the figures to work out that it was £1.40/L to buy)
Lots of information there, most likely not in a very easily read format but I'll tidy it up a bit as I go.
My formulae are all in an easy to use Excel file so once the maths have been proven I'll upload it and everyone can use it and you only need 7 things;
Stock Ratio, New Ratio, Stock Tyre Size, New Tyre Size, Reported KMs, Your Fuel Receipt with Amnount of fuel bought and cost of transaction.
This will give you Actual KMs, Actual Miles, Gallons, MPG, £/M and £/L
Also, usual things apply, if I'm wrong then sorry, but that's why I've asked for help and secondly, real world changes in tyre size need to be accountaed for, this is a straight text book setup, if I measure the actual circumference of my tyres (which I will do for accuracy) the results will be slightly different.
Cheers,
Ray.
I am under the impression that a 1:4.875 + 31" tyre combo is 'standard' or at least the most issued combination? If not please correct me.
If this is the case then the standard speedo in KMs should be pretty accurate so I need to know whether the speedo conversion plates fitted on import have the numbers in the right place (you never know with cheapo face plates!)?
If it is then we are all able to have a good place to start when regearing or changing tyre sizes to correct our speed with regard to what the speedo says.
Now comes the maths for regearing or tyre size changing (effectively the same thing but in different places).
I have two regears on my system as not only are my tyres 33" but my diff ratio is 1:4.556 instead of 1:4.875.
I'll start with gear ratio increase.
Stock Ratio divided my Current Ratio x 100 = 7% difference
so take my speed and add 7% for the correct speed, ie my speedo says 50MPH but I'm really doing 53.5MPH.
Please check my maths.
Now, the same with the tyres
Newer tyre size of 33" divided by Stock size of 31" = 6.5% difference
so again add 6.45% to my already modified speed and I get 56.95MPH.
Again, please check my maths.
If that's correct my speedo is as follows
Speedo -> Actual Speed
10 -> 11.39
20 -> 22.78
30 -> 34.17
40 -> 45.56
50 -> 56.95
60 -> 68.34
70 -> 91.12
This can now be used to work out the correct MPG.
Using the figures previously worked out plus some extras and a few other simple calculations I get the following information for MPG
Reported KMs = 424 -> Actual KMs = 482.96
Actual KMs to Miles = 300.1
Litres used = 59
Litres to Gallons = 12.98 Gallons
so
300.1 divided by 12.98 = 23.12 MPG
Cost per Mile if you want it is worked out as;
Cost per fill divided by mileage for that fill
In my case £82.54 divided by 300.1 = £0.28 p/m
(to check it you can use the figures to work out that it was £1.40/L to buy)
Lots of information there, most likely not in a very easily read format but I'll tidy it up a bit as I go.
My formulae are all in an easy to use Excel file so once the maths have been proven I'll upload it and everyone can use it and you only need 7 things;
Stock Ratio, New Ratio, Stock Tyre Size, New Tyre Size, Reported KMs, Your Fuel Receipt with Amnount of fuel bought and cost of transaction.
This will give you Actual KMs, Actual Miles, Gallons, MPG, £/M and £/L
Also, usual things apply, if I'm wrong then sorry, but that's why I've asked for help and secondly, real world changes in tyre size need to be accountaed for, this is a straight text book setup, if I measure the actual circumference of my tyres (which I will do for accuracy) the results will be slightly different.
Cheers,
Ray.
never noticed it was a pre facelift - your'e right - it would have to be a magic box to work on a cable drive speedo.
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