Mathematician Required

[TonyR]

So having jumped straight in and bought a set of 20" alloys with 275/40 /20's for our T6, i then proceeded to look at all the comments on here about wheel/tyre sizes. Gawd there are sooooo many). I understand that 35 is the better Aspect Ratio with regards to matching the original circumference. But comfort is important so anything that makes for a slightly softer ride all the better.
My question is about loosing MPG with bigger wheels. Has anyone done a test (i assume someone has) using satnav or GPS device to see what the 'real' miles covered were over a long run? rather than going by the odometer reading which i understand can be under by a significant amount and therefore making it appear to be worse MPG than reality.

 

[Dieseldonkey]

This works if you brim your tank at each fill ups.

Take your mileage between fill ups. Multiply this number by 1.0525. This will give you the distance, compensating for the larger circumference of your particular tyre size (275/40r20) compared to a standard rolling radius. (215/60r17). The bigger tyre will rotate fewer times, over the same distance as a standard tyre will.

Divide the litres of fuel added by 4.55 to give you the number of gallons used. Divide your mileage by gallons to get your mpg.

Remember your standard rolling radius probably wont be 100% accurate though, in terms of giving a calibrated distance.

 

[TonyR]

This works if you brim your tank at each fill ups.

Take your mileage between fill ups. Multiply this number by 1.0525. This will give you the distance, compensating for the larger circumference of your particular tyre size (275/40r20) compared to a standard rolling radius. (215/60r17). The bigger tyre will rotate fewer times, over the same distance as a standard tyre will.

Multiply the litres of fuel added by 4.55 to give you the number of gallons used. Divide your adjusted mileage by gallons to get your mpg.

Remember your standard rolling radius probably wont be 100% accurate though, in terms of giving a calibrated distance.

Thank you for that
Have you done this or do you know anyone who has done this? just intrigued to know how much this does affect MPG's in real terms.

 

[Dieseldonkey]

I run a slightly oversized tyre in the summer months myself, however the difference in circumference is only 1.77%. So never bothered to do the calculations for my own van I'm afraid. Plus I run a diesel air heater and an auxiliary coolant heater in winter so it would difficult to do a long term comparison on my fuel consumption, as a percentage of my diesel goes on generating heat.

Sorry Greta.

 

[huw169]

Speedo and therefore odo will always run higher even on standard tyres. It is calculated on the theoretical diameter of the correct tyre with full tread. Stick a 2ton van on it and the tyre compresses at the road contact point. That is the real rolling diameter. As it’s smaller, that’s why all speedos over read a little.

You can use (pi x (radius squared)) if you want to want to work out your before and after rolling circumference.

 

[DXX]

This works if you brim your tank at each fill ups.

Take your mileage between fill ups. Multiply this number by 1.0525. This will give you the distance, compensating for the larger circumference of your particular tyre size (275/40r20) compared to a standard rolling radius. (215/60r17). The bigger tyre will rotate fewer times, over the same distance as a standard tyre will.

Multiply the litres of fuel added by 4.55 to give you the number of gallons used. Divide your adjusted mileage by gallons to get your mpg.

Remember your standard rolling radius probably wont be 100% accurate though, in terms of giving a calibrated distance.

I think you mean divide NOT multiply the # of litres filled by 4.55 to calculate the # of gallons used.

 

[Dieseldonkey]

I think you mean divide NOT multiply the # of litres filled by 4.55 to calculate the # of gallons used.

Oops. Correct @DXX . Now edited.

 

[Ezz]

My speedo always read about 10 percent over a gps, since fitting 20’s it’s bang on with a gps, so the mfd mpg must be 10 percent out..

 

[DXX]

The GPS is reading distance over ground not the distance on the tarmac, it does not register distance travelled over gradients.

 

[Ezz]

Sat nav does surely?

 

[Dieseldonkey]

Theres a directive in relation to speedometers. UN/ECE regulation 39. A degree of over reading is built in to the design. Also means manufacturers are playing it safe. Think of the litigation against them if your speedo displayed a lower speed and folk were getting booked left right and centre.

The GPS is reading distance over ground not the distance on the tarmac, it does not register distance travelled over gradients.

Similar to cosine error. Test it. Set cruise control on the flat and see what happens to your GPS speed when you reach a steep hill. Vectors.