Right this weekend I'm visiting friends in Surrey, now google maps is suggesting 3 roots:

http://maps.google.co.uk/maps?f=d&source=s_d&saddr=Plymouth,+Devon,+UK&daddr=Surrey,+United+Kingdom&hl=en&geocode=&mra=ls&sll=51.262251,-0.467252&sspn=1.278731,1.771545&g=Surrey,+United+Kingdom&ie=UTF8&z=7

I'm more inclined to take the M4 rout since it'll be an easier drive and I can just stay in the middle lane and not use my mirrors (jokes).

However which will be do less damage to my petrol tank? The physically shorter rout with more hills and gear changes or the longer rout of constant speed? Is this a catch 22 situation or is the M5/M4 the obvious choice?

Speed will make more difference than hills.

I can get 70mpg on A roads if I try, but only 50 on the motorway*.

* I could obviously improve this by driving at 60mph, but that's just silly 😛

the one that allows you to drive at 50mph, steadily, with no stopping, roundabouts, and not too many gear chnges.

i find b roads more efficient, as i get stuck behind stuff to chug along at 50 happily listening to teh radio, and getting 60mpg

if im on a clear motorway, before i know it im doing 85 and getting 40mpg.

It is constant speed that is the best way.
if you drive at a steady 70 on the motorway then you will fair better interms of mpg than driveing on a hilly route where you have to use more of the accelerater pedal in order to get up the hills.
As for your route it will depend on the difference in the distance as to which is cheaper.

Sit behind a lorry on the motorway. I can get 30% better MPG doing that – I've got a thirsty car and often find myself on empty on the motorway.

You don't have to get close either – the optimum point is the back end of their turbulence which is a perfectly safe distance.

I always take M4/M5 when going the other way.

Come off at J10 of the M4, down the A322, keeping an ear on the traffic reports. If the M25 is bad, stay on it down towards Guildford, if it's clear than M3/M25.

So basically if I can be patient enough drive the rout with the motorways and sit at 50.

However the B roads will more likely force me to drive at 50 where's the motorway will more than likely cause me drive faster?

Correct, although I don't find the stop/start of a/b roads makes that much difference if you anticipate slowing and roll rather than heavy braking, and don't accelerate too hard.

as someone who used to drive to guildford bi-weekly i'd go 303 without question. go up to taunton and take the a358 though, to miss out the widdly bits from exeter – yeovil.

I get much better mpg on the motorway with cruise control on than I do anywhere else

most efficient way is flat out, the whole way.

less time spent burning fuel. ergo more efficient.

pure science fact that…

Constant speed at the "trade-off" point for your car (slower is better but only to a point where it becomes stupid). Hills and B roads are killers for economy, at least if you try to keep your average speed up.

I do Totnes to Central London and back several times a year and the A38/A303/M3 route is much less harsh on petrol costs compared with the motorway nearly all the way option. It's shorter, keeps your average speed down and takes about the same time. I also find that the A303 option prevents my missus from filling up the car with crap from the Bristol Ikea, which can make the motorway route more expensive as well 😉

I can get 70mpg on A roads if I try, but only 50 on the motorway*.

As an owner of a car that gives you a comprehensive MPG breakdown at all times, I can say that the best for MPG is wide open A roads with few hills. As they get hillier/more windy they drop to being less economical than a 70mph motorway run.

Having said that I once got 78mpg from my sister's house to my Mum's, cos it was on empty A roads basically up a short steep hill then a very long gentle coast down the other side. So certain hills are better than others 🙂

I went from home (SE London) to Plymouth uni and so did this trip pretty often. I ALWAYS found the 303/M3 route better in every way than M5/M4. Only reason I'd do the latter was if it snowed heavily and that was a clearer route.

depends on the value of your time as well surely?

If its on a weekday, would you be better off working some overtime then driving faster or finishing on the dot and driving slower?

eg, if costs £10 extra to do the journey in the fast lane, but takes an hour off the journey, and your boss pays you £10 after tax then youd break even and be in your boss' good books (assuming he wants to pay you overtime).