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MG MGA - Revs versus speed
| Hi guys, well the good weather yesterday allowed me my first proper run around the new forest, in my new MGA wel my old new to me MGA. Being a complete novice on MGAs, don't take offence but it reminded me of driving my old Frogeye Sprite in a nice way off course, low to the ground, windy, cold, the feeling you are going faster than you really are, speaking of speed, what should I expect to get out of my 1500, basic engine, no 5 speed box or overdrive etc. I was watching the revs, approx figures, at 3,000 I was doing about 60mph assuming both gauges are fairly accurate. Wasn't sure just how hard to push it, that seemed a good cruising speed, should I get more ? Cheers Jack. |
| Jack New Forest |
| We will soon get you hooked on the MGA Jack. You are beginning to bite. This is what my my 5-speed and 3.9 diff gives me: 1000 rpm = 23.5 mph 1275 rpm = 30 mph 1700 rpm = 40 mph 2000 rpm = 47 mph 2125 rpm = 50 mph 2550 rpm = 60 mph 3000 rpm = 70.5 mph 3400 rpm = 80 mph 3800 rpm = 90 mph 4000 rpm = 94 mph 4250 rpm = 100 mph Steve |
| Steve Gyles |
| Jack if your car has the standard 4.33:1 diff and relatively standard size tyres then your should be getting around 17.5mph per 1000rpm. So your 3000 rpm will be 52.5mph and will be doing 4000rpm at 70mph. Steves car with its 3.9 diff (from an MGB) and his overdriven 5th gear is geared very much higher than most of our cars, which makes it rev very much less. His engine is I believe, a 1798 mgb unit which has more torque than most too and will pull this high gear easily and this makes the car much quieter on motorways and more economic too. I think that your 1500 would struggle to pull the 3.9 diff but it would probably be ok with a 5-speed gearbox conversion if you should ever consider this. Colyn |
| Colyn Firth |
| Jack Just to say my standard gearing delivers about 'the book' 17.5 that Colyn quotes. I do very little motorway driving so this suits me. In the distant past I used to do big motorway drives and cruise around a real 80mph, at 4500rpm, for hours on end but this was pre 70 limit and I wouldn't do it now. On accuracy of gauges speedos can be well out. Originals were typically recording +10%. I always thought the rev counter was a lot better but when playing with timing last summer I compared it to my timing light's electronic revcounter and that showed the rev counter was about 10% over, and that is an untouched completeley standard mechanical set up. As well as checking revs this way of course you can check your speedo with a smart phone app. My modern golf reads about 6% over. Paul |
| Paul Dean |
| There is a GPS speedometer app for phones. I found my TD was a bit optomistic with the OEM diff by about 3 mph. With the 4.3 MGA gearset, it's showing about 3 mph low. |
| MAndrus |
| Thanks guys, I'm loving my MGA a bit slow, but we can sort that, Steve, amazing figures, my friend Peter Morgan has the five speed box, looks the way to go ! Lots of good info here tonight, I can see only after a few drives with new girlfriend, why you folks are so passionate. Looking forward to the future with this girl. So it's five speed box or MGBGT engine with overdrive, BUT and its a big BUT I would like to keep the car original, but I guess I can't do that and have more speed ! Or can I, That's your challenge guys ! Cheers great forum. Jack. |
| Jack New Forest |
| Jack unfortunately fitting the MGB overdrive gearbox is not straightforward, you would have to modify the gearbox mountings which involves some metal welding and fabrication and the also I believe, the tunnel to get it to fit into the MGA. There are now a couple of 5-speed gearbox conversions available for the MGA, one is the High Gear Engineering conversion (like mine and Steves) which uses the Ford T9 (Sierra) gearbox and the other is a very recent development that uses a Mazda MX3 gearbox. I dont recall the name of this kit but it was advertised recently in Safety Fast I think. Both these kits come with bespoke bell housings and all the parts to make them fit the car. One thing is for sure, fitting a 5-speed is not a cheap option. I think that you will find that most of us who have done this 5-speed gearbox conversion probably plan to keep their cars forever. Colyn |
| Colyn Firth |
| Be aware that both your tacho and speedo are likely to be inaccurate. My tacho reads 2500 at 60kph and 4000 at 100kph by a GPS. |
| A Bowie |
| Maybe it's another advantage of the 1800V T9 conversion. My speedo is spot on with my sat nav, having been recalibrated after the conversion. That said, anyone can have their speedo recalibrated if they are unhappy with the errors. I was once told that all manufacturers deliberately overread their speedos. Otherwise, particularly if one was underreading, they would risk being sued by drivers for being booked for speeding while following their instrument read outs. I guess the electronic tachometer can count the impulses ok and is also very accurate? Jack, for your long term planning if you are thinking the 5-speed route, consider carefully the differential ratio. I think the standard ratio would be best around the New Forest. The 3.9, on the other hand, might suit your frequent French journies. I found the 3.9, when I lived in Lancashire, was great for numerous M6 journies, but I feel it is a tadge too high now for general driving around Hampshire. Steve |
| Steve Gyles |
| Yes, i agree with Steve. I run the 5 speed with a 4.1 and find it gives the best compromise of revs/speed and liveliness when needed. regards Colin |
| Colin Manley |
| Even nowadays where they have the technology to get it spot on, speedos read high! Good for giving the right impression ("doesn't feel like 70!" - it's not, it's only 65) and also it gives some margin with the speed cameras. My Golf reads 74 against a gps of 70. But getting back to MGA's, 60 odd years must take it's toll with speedos and rev counters. Add to that the potential for axles and wheel/tyre sizes to have been changed and I would be surprised if there was much precision in their operation. Mine isn't bad over 40 but true 30 is actually 24 on the speedo - I have to watch that one! For some reason the error isn't linear. I had my Frog speedo recalibrated by the chap in Redditch as that was a real mess after axle and wheel changes. It was reading 20% high and cyclists were passing me in 30mph limits! He did a good job and it is now spot on but of course fuel consumption figures have deteriorated by about 20% too. |
| Graeme Williams |
| If I had a standard 4-speed box and I wanted to see how the car went with higher gearing I would first try fitting a 3.9 diff and see how the car coped with it. Its a relatively easy job especially compared to swapping the gearbox and many owners, particularly over in the USA, do this and are very happy with it. I would like to try a 4.1 diff on my car sometime but I must admit that I havent seen any advertised for sale for many years. Moss appear to list new ones (at a price) and I wondered if anyone had ever fitted one from them? Colyn |
| Colyn Firth |
| Neil Don't forget to clamp off the MMO hose when doing CO measurements, as the MMO drips will screw up your readings. Don't run for too long without the feed to minimise vane wear. Dominic |
| dominic clancy |
| Hi Steve who did your recalibration and cost please. |
| J Hughes |
| Richard Jenkins (woodykeys67@msn.com) in Redditch charged £65 for a recalibration and overhaul of my Frog speedo (and return postage). This was setting it to non-standard diff ratio and wheel sizes. Good bloke, excellent job. I think he has done a few now, particularly on the Sprite section. |
| Graeme Williams |
| Also,2000 rpm,0 mph......clutch gone. |
| M Blencowe |
| JH It was a number of years ago when I last had the speedo calibrated - about 10 years. Prices have gone up a bit since then. I went to Speedy Cables http://www.speedycables.com/ They have a page (calibration) that tells you how to take relevant measurements. The overhaul and calibration looks like £75. Their work was spot on for me. There are other companies with good reputations. Steve |
| Steve Gyles |
| Easiest and cheapest way to get your speedo closet reality ( but always reading high!) is to monitor your speed with GPS and on the dial..work out the ratio between the two speeds and buy a small speedo cable gearbox to fit on the car gearbox output to the speedo cable. There are a few companies making these little wonders and I have used them on my mga ( post Sierra conversion ) and on my 74 jeep. |
| Neil Ferguson |
| I make corrector gearboxes that sit on the back of the speedo-plug and play. Regards Declan  |
| Declan Burns |
The inside
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| Declan Burns |
| Beautiful piece of engineering Declan, it reminds me of the kind of work my Dad used to put together when he was making parts for his working model steam engines. It is a great diy solution for anyone fitting a 5-speed gearbox or changing diff ratios. Colyn |
| Colyn Firth |
| Steve Many thanks for the link. John. |
| J Hughes |
| Here's one I built recently for a Delahaye. Regards Declan  |
| Declan Burns |
This thread was discussed between 01/03/2016 and 04/03/2016
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