Gear ratios

[schweps]

Hi everybody,
Recently i've changed my rims and tyres.
Original rims were 15x7 Et10 with 265/70R15 tyres on.
New off-road rims are 16x8 Et-30 with 235/80R16 Cooper STT tyres on.
My vehicle is a 3.0 V6 with 6G72 engine, 5 speed manual gear box V5MT1.
Model code is V43WGNXEL6 and trans axle is stated like "V5MT1 4875"
After this bigger tyres i was expecting some fuel economy at highways but disappointed.
So i'm involved with gear ratios. I found the following gearbox ratios from the service manuals.

1st gear 3,918:1
2nd gear 2,261:1
3rd gear 1,395:1
4th gear 1:1
5th gear 0,829:1
Ring/Pinion 4,875

With a gps i did some velocity readings at exact rpm's. The values inside ( ) are speedometer readings to compare.

@2000rpm
1st gear 14,3kmph (15kmph)
2nd gear 24,5kmph
3rd gear 40,2kmph (40,5kmph)
4th gear 55,9kmph (57kmph)
5th gear 67kmph (66kmph)

@3000rpm
2nd gear 37,8kmph
3rd gear 61kmph
4th gear 85,4kmph
5th gear 103kmph(~100kmph)

Then i measured the rolling radius, directly from the tyres as 38,5cm. Diameter is calculated as 30,31". Overall diameter
of this tyres are given 31,7", i don't know what is the cause of this difference. May be the wide rims and low pressure.
Could somebody tell me what should be the pressure.

According to the rolling radius, the circumference is calculated as 2,419m

For 5th gear @3000rpm, the input shaft rev. of the diff. is 3000rpm/0,829=3618,81rpm
Rev. of the axle (so the tyre as well) 3618,81/4,875=742,3rpm
742,3rpm x 2,419m = 1795,67mpm
The speed is 1795,67 x 60/1000 = 107,74kmph

For 4th gear @3000rpm, 3000/1=3000 3000/4,875=615,38 615,8x2,419=1489,62
1489,62x60/1000=89,37kmph

For 3rd gear @3000rpm, 64,02kmph
For 2nd gear @3000rpm, 39,5kmph

Calculated velocities for 2000rpm are;
1st gear 15,19kmph
2nd gear 26,33kmph
3rd gear 42,68kmph
4th gear 59,53kmph
5th gear 71,82kmph

Do you think the calculation is correct and differences are ok or some modifications on the gears might have been done?
cheers.

[DamTriton]

Looking at the tyres alone (all that is going to matter) you have gone from a calculated 755mm diameter to about 775mm diameter, about 2.5% difference, or about the thickness of your tyre tread difference in diameter.

No, it will not make a significant difference to your fuel consumption.

[schweps]

Thanks for your reply damkia,
Happy new year to you guys and gals.