Gear Ratios... motorway driving

[mowgli]

if the engine is in good condition, then at a steady 75mph, it should get close to 45 mpg, gm actually designed lean bits into the ecu so at 56 & 75 it runs really economically, they have been fiddling fuel mileage official figures for years

[Torroni]

if the engine is in good condition, then at a steady 75mph, it should get close to 45 mpg, gm actually designed lean bits into the ecu so at 56 & 75 it runs really economically, they have been fiddling fuel mileage official figures for years

I've never had anywhere near this sort of fuel economy. A full tank of petrol usually lasts around 200-225 miles maximum, which is a combination of motorway and 'around town' driving. It just feels like there is no need to run at 4000rpm if the engine can handle a slightly bigger 5th gear.

Will there be a problem with the way the ECU is set-up if i change the final drive?

[Welsh Dan]

Changing the gearbox wont affect the ECU.

[Adam]

If you change the final drive it will effect all gears tho, whereas fitting a WR 5th gear will allow low engine rpm on motorway, but still keep the other gears the same.

I'd change 5th.

[Chris Astley]

The last gte i had did between 240-270 to a tank full, maybe get your car setup on a rolling road and serviced if it hasnt been done recently!

Chris

[ade]

isnt 4000revs a bit high for 70mph? I know when I drove in my 1.4 Nova 70 mph was about 3200revs (i think). Its the same in my seat and the volvo s40 i drive (2ltr).

[Chris Astley]

What size tyres you using? with a f13cr box at 4000rpm using 195/50R15 for example you would be doing 78mph.

If you fitted the WR 5th gear you would be doing 97mph which i dont think the engine would pull it very well if you have standard horsepower.

Chris

[Torroni]

What size tyres you using? with a f13cr box at 4000rpm using 195/50R15 for example you would be doing 78mph.

If you fitted the WR 5th gear you would be doing 97mph which i dont think the engine would pull it very well if you have standard horsepower.

Chris

Hi Chris...

Tyres are 15" and speedo is accurate as I've checked this with my in-car GPS? How did you calculate the numbers above or does my engine need serious re-tuning??

[Stuart]

erm as a quick one here ( i cba to read the whole thread) but if you want to drop the engine speed for a given road speed then either fit bigger wheels, OR fit a lower number FD like a 3.74 etc.

but im fairly sure the FD in the F13 in gte/gsi's was a 3.74 anyway