Diff questions

[oze30]

If i was to swap my auto diff over for a 3.74, how would that effect fuel consumption/accel? i have a 325 touring and presently getting 180ish miles to the tank. or 300km for 65l.. erm.. NOT GOOD!!!

[kam-325i]

High Ratios = Better accelerattion. Higher engine speed at motorway speed. Decrease in MPG
Lower Ratios = Not so good acceleration. Lower engine speed at motorway sppeds. increase in MPG

[Simon]

High Ratios = Better accelerattion. Higher engine speed at motorway speed. Decrease in MPG
Lower Ratios = Not so good acceleration. Lower engine speed at motorway sppeds. increase in MPG

I think you got the MPG's the wrong way around.

[oze30]

ok.. so a higher ratio is one that is numerically lower?

[Simon]

ok.. so a higher ratio is one that is numerically lower?

High ratio would be numerically higher.

[oze30]

so what is the stock ratio on a 325 auto touring?

[Simon]

so what is the stock ratio on a 325 auto touring?

I 'think' it's a 3.91, what speed are you doing at 4000rpm in fifth?

[liam012]

if we contrast say 3.25 3.64 and 4.1 for example

3.25 - worst acceleration - best mpg (eg 80mph@3000rpm)
3.64 - slightly better accelereation - worse mpg (eg 70mph@3000rpm)
4.1 - best acceleration - worst mpg (eg 60mph@3000rpm)

ok this is as far as i know how the ratios affect things - the increase or decrease in mpg is caused by runnning the lower rev at the high speed - so your car that uses 3.25ratio diff only needs 3000 rpm revolutions of the engine to maintain 80mph if you swapped to a 4.1ratio diff it might take 3900 revolutions and theree goes your fuel and mpg
i was just using figures above as pure fictional example ,

if i am wrong can someone harshly correct me!! as i have this understanding off diffs forged into my brain and remeber being very confused about it when i first asked about diffs so if i have it wrong let us know! cheers

[Andy335Touring]

if we contrast say 3.25 3.64 and 4.1 for example

3.25 - worst acceleration - best mpg (eg 80mph@3000rpm)
3.64 - slightly better accelereation - worse mpg (eg 70mph@3000rpm)
4.1 - best acceleration - worst mpg (eg 60mph@3000rpm)

ok this is as far as i know how the ratios affect things - the increase or decrease in mpg is caused by runnning the lower rev at the high speed - so your car that uses 3.25ratio diff only needs 3000 rpm revolutions of the engine to maintain 80mph if you swapped to a 4.1ratio diff it might take 3900 revolutions and theree goes your fuel and mpg
i was just using figures above as pure fictional example ,

if i am wrong can someone harshly correct me!! as i have this understanding off diffs forged into my brain and remeber being very confused about it when i first asked about diffs so if i have it wrong let us know! cheers

Looks good to me

[oze30]

simon..

oze30 wrote:
so what is the stock ratio on a 325 auto touring?


I 'think' it's a 3.91, what speed are you doing at 4000rpm in fifth?

[Simon13]

not sure on the fuel consumption part!

both my 325's can do about 35mpg at 60mph, ones auto with a 3.64 the other manual and 3.91.

At 4000 in fifth my 3.91 car will be doing 90
@ 4k in the 3.64 it's around 100 i think

[liam012]

eerm i wouldn't have a clue why that is!#

i think then youre into territory of torque and size of gears affecting mpg?
as i said above was only rough figures but i stand by the fact that if you had a car with say 3.25diff and swapped only the diff to a 4.1 then acceleration would increase speed at 3000 rev would decrease and mpg would decrease - if youre talking about different cars with different gearboxs and probably someway different engine spec then its way way out of my knowledge!!

[astondg]

High Ratios = Better accelerattion. Higher engine speed at motorway speed. Decrease in MPG
Lower Ratios = Not so good acceleration. Lower engine speed at motorway sppeds. increase in MPG

Looks ok to me. Decrease MPG means less Miles Per Gallon of petrol (worse petrol consumption) which is what you get with a High ratio (like a 4.1). Increase MPG means more Miles Per Gallon of petrol (better petrol consumption) which is what you get with a Low ratio (like 3.25).

Aston

[Simon]

High Ratios = Better accelerattion. Higher engine speed at motorway speed. Decrease in MPG
Lower Ratios = Not so good acceleration. Lower engine speed at motorway sppeds. increase in MPG

I think you got the MPG's the wrong way around.

Looks ok to me. Decrease MPG means less Miles Per Gallon of petrol (worse petrol consumption) which is what you get with a High ratio (like a 4.1). Increase MPG means more Miles Per Gallon of petrol (better petrol consumption) which is what you get with a Low ratio (like 3.25).

Aston

Yeah, my fault, I read it wrong, like decrease as in getting better, and increase as getting worse

[astondg]

that's ok, if we edit our posts no-one will know .

Aston