Average Maximum Speed at most UK circuits (saloons)

[Al Weyman]

Thinking of changing the diff ratio in my black car as at the moment it is good for 183mph (theoretically ) with the 18" wheels and a 3.27 diff and I think thats just a tad over the top . I was reconning on gearing it to max out at about 150mph and gain on the acceleration but wondered what you guys work to as its impractical with the type of diff in my car to do changes between circuits I will have to pick a ratio (3.73?) and stick with it.

[Peter Mallett]

Dunno. Is it important?

[Tim Falce]

You say 18 inch wheels but what profile tyres do you have and what's the rev limit on your engine? PM me your email address and I'll send you a calculator for working out speed and revs for differing tyre axle ratio combinations.

[Peter Mallett]

Actually being slightly more serious. There's Mallory and Thruxton which AFAIK produce the fastest lap times. Two circuits more diverse you couldn't find. At Mallory you need lots of low down grunt to get out of the hairpin but after that its all high gear flat stuff.

Thruxton is pretty much flat all round but you need less grunt out of the chicane because its on a flat surface. It has longer straights so you need a lower ratio. Mallory has short straights so a higher ratio suits it better.

Cadwell is twisty and only one long straight so its point and squirt.

Depending on the circuit Oulton is a high ratio except for the hill so unless you hit it at a reasonable speed you lose out going up it.

Sliverstone is low ratio simply because you just don't come to a stop except for the hairpin on the Int'l circuit.

Brands Indy is low ratio because its quite fast and there's no real stopping in the track.

[Al Weyman]

I have been using this calculator from Richmond Gears who's product I use http://www.richmondgear.com/101032.html The reason it is relevant is because I wanted to get an idea what for example your car Peter as thats a good base point, reasonably powerful but not in the Mosler league would be hitting at the end of the Snett or Thruxton straight.

For example using the Richmond calculator I think ideally as a diff for all tracks something in the region of 3.5 like I have in the yellow car or 3.6 maybe but they are not available with the 7.65inch ring gear GM axle unfortunately so its either a 3.42 (bit high and hardly worth the expence and hassle over a 3.27) or the 3.73 which will give me 150mph at 7300rpm or if I really wanted to red line it to the limit (I was hoping to keep the revs more conservative on this unit) of 7800 rpm, 160mph. Realistically I am of a mind to think 150mph should be more than enough and score points with better accelleration with the possible exception of Thruxton as I have hit 155mph there before in my yellow car when it had different aerodynamics and a 6.6 litre engine.

BTW I have 25.7" diameter wheels/tyres on the back (275x35x18 Toyo 888's)

[AU N EGL]

Al also think where your shift points are on each track. Some times a change in Diff ratios may seem good, but if the shift point is in the middle of a corner, then may be not.

[Al Weyman]

To be honest I will have to pick a ratio and live with it as its a pain to change a crown wheel and pinion in a GM axle as I am sure you know, its not like a Ford where you can have a 3rd member built up and slot it in in about half an hour and we can't use a different make or a fast change job. I just about got away with the 3.27 when I ran the 16inch rims but its a tad tall now so I think I will have to order the 3.73 and live with it at the couple of seriously fast straights in the UK (Snetterton & Thruxton) every where else I think it will pay dividends.

[COLIN STUBBS]

3.75:1 always used to be the common race ratio for Capris when we ran one and seemed to suit most tracks. Although it was obvious at certain tracks when the guys had higher diffs fitted.
And dont forget higher gear is the lower numbered diff in case of any confusion here..
Iv had the standard 4.25 in my 216gti and never really use 5th gear.
Finally found a 4.43 and fitting it today!! Good luck Al

[AU N EGL]

a 3.73 is a good ratio. Many of the Camero guys have changed to that one.

I use a 3.42 and have looked at a 3.73 and a differnt transmission ratio. Numbers look good, but that is only the numbers.

Some one sent this link to me and it has helpped play with differnt ratios and tire sizes http://users.erols.com/dmapes/GEARCLC.HTM

[Al Weyman]

Thanks guys, using the calculator I still will hit 65mph in 1st and 154mph in top at 7500rpm with the close ratio Muncie 4 speed and a 3.73 rear ratio. Might make it easier to drive around the paddock as well!

[AU N EGL]

Originally Posted by Al Weyman

Thanks guys, using the calculator I still will hit 65mph in 1st and 154mph in top at 7500rpm with the close ratio Muncie 4 speed and a 3.73 rear ratio. Might make it easier to drive around the paddock as well!

I like that 60-65 mph in first gear idea. and 154 is more then enough for most ciruits.

[Al Weyman]

More than enough for my bottle as well! (cockney phrase sorry Au)