Lowering the gearing on a car by reducing wheel size?

[Electrics not for me]

So, i have bought a mostly finished kit car with a nicely fettled Rover V8 engine, an ancient 3.9 litre former RR engine putting out a modest but useful 250 bhp. I've taken it out a few times and for whatever reason the gearing seems overly long for a car that's unpleasantly noisy to use above 100 mph. For me this car is all about quiet back roads and track days and car meets up to 90 ish mph. (Tracks may be a little different).
Rather than toy with gearbox or back axle can i reduce the gearing by fitting smaller wheels? It has 17" wheels and i'm after a set of lightweight 15s which will reduce unsprung weight too. If i drop to 15s and keep the aspect ratio the same will that lower the gearing? I realise the speedo will need adjusting, if i've got this right.

[Evanivitch]

Yes, it will lower the gearing.

And as you acknowledged, make your speed inaccurate (it will over read) and probably impact your suspension geometry and will effect your ride height.

[GreenV8S]

It'll lower the gearing, as you suggest. It will throw the speedo out accordingly.

Note that the RV8 is usually a torquey engine rather than a high revver so you might not get much performance benefit from the closer gears, and it might make cruising rather noisy and inefficient.

[PaulKemp]

Use one of the readily available gear speed calculators online to see what the change in wheel/tyre size change will do to the speed in gear. If your 17” wheels are running tyres with a 40 profile then changing to 15” with a 60 profile may not change the rolling radius

[Electrics not for me]

PaulKemp said:

Use one of the readily available gear speed calculators online to see what the change in wheel/tyre size change will do to the speed in gear. If your 17” wheels are running tyres with a 40 profile then changing to 15” with a 60 profile may not change the rolling radius

I am keeping the profile the same.

[Electrics not for me]

Well i've found some much lighter 15 inch wheels and fitted 205/60 x 15 tyres. A quick calculator session gives me a 6.1 % lower overall gearing figure. It was on 215/55 x 17.
Must say, how much is placebo i don't know but it feels better and the acceleration definitely seems sharper. I know the rims were about 2kgs lighter each if i remember correctly which has to help i suppose.
Thanks anyway for the replies.

[300bhp/ton]

Electrics not for me said:

Well i've found some much lighter 15 inch wheels and fitted 205/60 x 15 tyres. A quick calculator session gives me a 6.1 % lower overall gearing figure. It was on 215/55 x 17.
Must say, how much is placebo i don't know but it feels better and the acceleration definitely seems sharper. I know the rims were about 2kgs lighter each if i remember correctly which has to help i suppose.
Thanks anyway for the replies.

What diff/axle are you running? Swapping the crown wheel & pinion will likely make a much starker difference. I’m sure the tyres have helped. But terminal speed in gear won’t likely have change much.

[300bhp/ton]

GreenV8S said:

It'll lower the gearing, as you suggest. It will throw the speedo out accordingly.

Note that the RV8 is usually a torquey engine rather than a high revver so you might not get much performance benefit from the closer gears, and it might make cruising rather noisy and inefficient.

I’d disagree about the revs and torque. Sure an RV8 won’t be. 7000rpm plus screamer. But that doesn’t mean they won’t make the best power at higher revs. A factory standard Thor V8 from a p38 Range Rover makes peak power at the red line. Which means you do need to rev it and drive it hard to get the most from it.

They are only “torquey” when you compare to period 2.0 litre or smaller displacement 4 pots.

[SystemOfAFrown]

300bhp/ton said:

I’d disagree about the revs and torque. Sure an RV8 won’t be. 7000rpm plus screamer. But that doesn’t mean they won’t make the best power at higher revs. A factory standard Thor V8 from a p38 Range Rover makes peak power at the red line. Which means you do need to rev it and drive it hard to get the most from it.

They are only “torquey” when you compare to period 2.0 litre or smaller displacement 4 pots.

Both the 4.0L and the 4.6L make peak power at 4750 RPM and peak torque at 3000 RPM. Pretty sure the red line is a fair bit higher, around 5600?

[300bhp/ton]

SystemOfAFrown said:

Both the 4.0L and the 4.6L make peak power at 4750 RPM and peak torque at 3000 RPM. Pretty sure the red line is a fair bit higher, around 5600?

Is that the GEMS ones?

Tornado motorsport had this on their site under their performance chip for the Thor, but the page is gone now.

"The Bosch engine is mapped in such a way that it produces maximum power when it hits the Rev Limiter, which is set at 5400 RPM."

[camel_landy]

The RV8 IS a torquey engine... You can split as many hairs as you like but it's a torquey engine.

It's also worth noting the OP mentioned:

• Doesn't want to mess about with the axle or gearbox.
• It's a RaRo 3.9, therefore it's probably going to be an early 90s engine from a late(ish) classic.

The OP also suggests they've found a solution they're happy with and the case has been closed.

M

[300bhp/ton]

camel_landy said:

The RV8 IS a torquey engine... You can split as many hairs as you like but it's a torquey engine.

It's also worth noting the OP mentioned:

• Doesn't want to mess about with the axle or gearbox.
• It's a RaRo 3.9, therefore it's probably going to be an early 90s engine from a late(ish) classic.

The OP also suggests they've found a solution they're happy with and the case has been closed.

M

Thing is. That just isn’t true. Had a cammed 3.5 in my TR7 V8. Wonderful engine until the oil pump fail. Really came “on cam” at 3500rpm and screamed to over 6000rpm with ease. 230hp on the dyno and 219ft-Lb.

Got a serp 3.9 in there now. Also changed gearing via rear crown wheel & pinion. I clocked the car at 144mph with a 3.08:1 rear. Impressive for a 1976 Triumph. Currently running very short gearing and red line in top is only 116mph. But it absolutely screams on the local B & C roads or at events.

Edited by 300bhp/ton on Wednesday 17th August 20:14

Edited by 300bhp/ton on Wednesday 17th August 20:15

[stevieturbo]

camel_landy said:

The RV8 IS a torquey engine... You can split as many hairs as you like but it's a torquey engine.

Depends what you compare it to really

A lot of boring mundane diesels make more torque these days...and quite a few more power too.

[GreenV8S]

300bhp/ton said:

Thing is. That just isn’t true. Had a cammed 3.5 in my TR7 V8. Wonderful engine until the oil pump fail.

Thing is, the OP is asking about 'an ancient 3.9 litre former RR engine putting out a modest but useful 250 bhp' not an engine you put a leary cam into.

In factory spec it's a torqey engine. You can turn any engine into a screamer with enough time and money but the RV8 isn't a high revving engine as standard and would take a lot of work to turn it into one.

[camel_landy]

GreenV8S said:

300bhp/ton said:

Thing is. That just isn’t true. Had a cammed 3.5 in my TR7 V8. Wonderful engine until the oil pump fail.

Thing is, the OP is asking about 'an ancient 3.9 litre former RR engine putting out a modest but useful 250 bhp' not an engine you put a leary cam into.

In factory spec it's a torqey engine. You can turn any engine into a screamer with enough time and money but the RV8 isn't a high revving engine as standard and would take a lot of work to turn it into one.

It's probably worth pointing out that we're not saying you can't tune up the RV8 but by default it's a torquey engine.

I'd also add that the kit car in question is probably a Westfield (judging by the OP's garage), so even in standard trim, the RV8 makes it an 'interesting' car. (In my Westie days, I remember there was a chap who ran his RV8 with NOS!!!)

M

[300bhp/ton]

GreenV8S said:

300bhp/ton said:

Thing is. That just isn’t true. Had a cammed 3.5 in my TR7 V8. Wonderful engine until the oil pump fail.

Thing is, the OP is asking about 'an ancient 3.9 litre former RR engine putting out a modest but useful 250 bhp' not an engine you put a leary cam into.

In factory spec it's a torqey engine. You can turn any engine into a screamer with enough time and money but the RV8 isn't a high revving engine as standard and would take a lot of work to turn it into one.

High revving engines can still be torquey, and a low revving one isn't necessarily a torque monster. In the case of the RV8, they make more low end grunt than 4 pot petrols they usually replace. Giving the impression of lots of torque. But the reality is a 2.5 litre TDI Land Rover engine is more "torquey". The V8 will still do its best "work" in the mid to high rpm rev range (regardless of how high those numbers actually are). Hence the fact that GEMS ones make Peak power fairly high in their rev range and the latter Thors even higher.

An old 3.5 RV8 strangled on twin carbs won't rev well, esp if it is of low CR nature. But that isn't what the op has. They have a 3.9 (don't know if it is a serp or vee belt). But if it is making 250hp, then it has been relatively heavily modded and likely runs a cam way more aggressive than stock, given from the factory the 3.9 would be rated at 182bhp.

[camel_landy]

300bhp/ton said:

...But the reality is a 2.5 litre TDI Land Rover engine is more "torquey"...

No it isn't:

• 2.5 TDi approx 270Nm @ 1800rpm
• 3.9 V8 approx 320Nm @ 2500rpm

M

[300bhp/ton]

camel_landy said:

300bhp/ton said:

...But the reality is a 2.5 litre TDI Land Rover engine is more "torquey"...

No it isn't:

• 2.5 TDi approx 270Nm @ 1800rpm
• 3.9 V8 approx 320Nm @ 2500rpm

M

So the V8 has PEAK torque at higher rpm...

[camel_landy]

300bhp/ton said:

camel_landy said:

300bhp/ton said:

...But the reality is a 2.5 litre TDI Land Rover engine is more "torquey"...

No it isn't:

• 2.5 TDi approx 270Nm @ 1800rpm
• 3.9 V8 approx 320Nm @ 2500rpm

M

So the V8 has PEAK torque at higher rpm...

<sigh>

How much torque does the 3.9 V8 put out at 1800rpm?

(IIRC it's still more than the TDi but can't find the chart I used to have to confirm).

M