Thread: New tyres, different size, more fuel?

Originally Posted by PhilipA

I often woder if people add the 5% increase in rolling diameter to the Kilometers on the odometer that they travel when they calculate fuel consumption.

All things like rolling diameter being equal, economy should IMPROVE slightly by having a higher gearing as the engine is turning slower for a given speed.

I found with my RRC that economy was exactly the same, as near as could be calculated with bigger tyres, after I added the 5% because the car travelled 5% further.
Regards Philip A

Yes, I have done the calculations since I changed my tyres. Ihad 235/70R16 Hwy terrain (OD 735mm) and now I have 255/70R16 All terrain (OD763 mm), which is about 4% increase. Before the change, I noticed the speedo was "spot on" compared with the speed displayed on a GPS. Now, after the change, when the speedo shows 90kph the real speed (GPS) is 93. Anyway, I made some calculations and found for example that when the odo. have recorded 690.7 km, the real distance travelled is 717.0 km. I also calculated the consumption for both of the values and the results were 11.29 L/100km and10.88L/100km respectively.

Now, I have been recording the consumption in my car since I bought it about a year ago and I noticed an increase in the consumption since the tyres change of about 0.7L/100km in average being the best two values(before and after the change) 9.26 and 10.04 L/100km. Obviously varies according to the conditions, ah and its a 2003 D2a Td5 auto.

Hope this information will help you.

Luis.

Something wrong there Luis!!!!!

No D2 is accurate on stock tyres...

In fact, every D2 I have ever tested is out by 8-10% under on stock rubber...

Its taken going from factory to 245-75-16 to get most of them to read 1-2% over...

Don't know Tombie, it's been like that since I bought it. I was expecting the same (speedo 8 - 10% above the real speed on standard tyres) as it was my previous D1, but as I say, I've compared that for a year with different GPS units and that's what it is. The numbers I posted are based on the whole year worth of data, I have recorded every single fill and even take notes about conditions, mods etc. As far as know everything looks and feels fine, and I thought it was normal on D2s after a friend of mine who had a 2004 V8 auto confirmed it to me,he had 265/75R16 and his speedo was something like 8 - 10 kph under than it should be (he got a couple of fines because of that). Also a few of the threads are about the same. http://www.aulro.com/afvb/discovery-...ger-tyres.html

My D2a with stock 235/70's was near perfect compared with a number of GPS's. With 245/70 the speedo was about 2% under actual.

Regards Gordo Sent from my iPhone using Forum Runner

I've got stock 235 michelin hp tyres on mine and if I'm doing 100k by the speedo the gps reads 102k so pretty close to spot on on mine

Originally Posted by Gaudough

With 245/70 the speedo was about 2% under actual.

I found the same with my 245/70 bfg AT's. just got some 265/76 MT MTZs and the speedo reads 90 when I'm doing 100 by the GPS. Although the D2 speedo seems to be non linear - 50 = 50. 55=60

According to my info D2a standard is 235/70 (D = 28.95 inches)
I have 255/65 (D = 29.05 fitted and the speedo is pretty much spot on at least between 50 and 110

As I mentioned before, mine was spot on with the standard size, now with 255/70R16 (D=30.04 in) is around 3 kph under the real speed: Speedo = 90, Real Speed (GPS) = 93.