11 replies to this topic

[racingbob]

have a 3.44 diff in the car with 165 60 5" 12 tyres
just put on some 048r lower profile 55
there 3/4 inch different in height
car seems quicker accelerate in 3rd realy goes
engine now done 1700 miles since built
feels like engine now looser and revs free
but i reckon must be the tyre change any thoughts

[George929]

yes it will accelerate better with smaller tyres/ wheels. but it takes it from your top end.

[stretch tech]

my thirty has a 3.44 diff, sorry for thread steal, but what difference in acceleration is there from a 3.1 diff ?

cheers

Rhys

[racingbob]

yes it will accelerate better with smaller tyres/ wheels. but it takes it from your top end.

will do a check on revs and speed see what it says
before the engine showed 68 at 4000 revs
not worried about top speed i dont go more than 70 75mph
boy that mg metro cam a beauty

[racingbob]

looks like a 2" difference in cicumference

between 165 60 12 and 165 55 12 (048r)

about a 3% difference in speedo as far as i can make out

i know not much but do notice the performance in third

[blacktulip]

personally i prefer the 3.1FD as i didnt think there was much difference in acceleration but did notice an increase in mpg. that was in my old mini i had.

[Cooperman]

On a twisty road there is a lot to be said for the 3.44 FD with standard gear ratios. The mph/1000 rpm with 165/60 x 12 tyres will be around 16.8 mph/1000 (call it 17/1000), so 5000 rpm in top will give 85 mph. With 55% profile, that will drop the mph/1000 to around 15.5 mph/1000 rpm so acceleration will be better, but the revs at the same cruise will be higher.
My 'S' on 10" wheels has a 3.9:1 final drive, so I get only 14 mph/1000 rpm in top but acceleration is fantastic from 0 to 80 mph.

[racingbob]

On a twisty road there is a lot to be said for the 3.44 FD with standard gear ratios. The mph/1000 rpm with 165/60 x 12 tyres will be around 16.8 mph/1000 (call it 17/1000), so 5000 rpm in top will give 85 mph. With 55% profile, that will drop the mph/1000 to around 15.5 mph/1000 rpm so acceleration will be better, but the revs at the same cruise will be higher.
My 'S' on 10" wheels has a 3.9:1 final drive, so I get only 14 mph/1000 rpm in top but acceleration is fantastic from 0 to 80 mph.

so basically your saying its turned it into almost a 3.76

its only a 3% difference in circumference from around 62 to 60

so dont think its gone as far as that more like a 3.6 will check speedo next time out

and compare the revs and speed to how it was

as i said before dont mind as dont go high speed no need to

[Cooperman]

In simple terms, if the basic diameter is the same (12"), then the conversion ratio from 60 profile to 55 profile is to multiply by 55 and divide by 60. Your speed at the same revs, therefore, will be 55/60ths of that with a 60 profile tyre of the same wheel diameter and tyre width. Simples!

[racingbob]

In simple terms, if the basic diameter is the same (12"), then the conversion ratio from 60 profile to 55 profile is to multiply by 55 and divide by 60. Your speed at the same revs, therefore, will be 55/60ths of that with a 60 profile tyre of the same wheel diameter and tyre width. Simples!

thanks for that
no wonder its so bloody quick
check speedo/revs asap
so speedo should show if true 62 before showed 70 so may show 64 now

[Ethel]

It could also be that the new tyres have less rolling resistance. The difference may be less clear cut in the real world as it depends when you change gear. If you write first off as being too low to avoid spinning the tyres at full throttle, holding on to 2nd & 3rd that bit longer with taller tyres/final drive could offset the advantage.

[racingbob]

just looked on tyre checker

it will only be about 2.5 mph less

so speedo before was showing 70

now should be showing just over 67